Evaluation of household appliance

Abstract

A method (200) for grading household appliances (105), comprising the steps of: determining (205) life cycle parameters of a plurality of household appliances (105); sorting the appliances (105) into a directed graph (400) using the life cycle parameters as a sorting criterion such that there is a Pareto improvement between successive appliances (105) along a path through the graph (400); determining (215) a lifecycle ranking for each appliance (105) as a function of the position of the appliance (105) in the path in the map (400); and determining (225) a rank of the appliance (105) based on the lifecycle ranking of the appliance (105).

Description

TECHNICAL FIELD

[0001] The present invention relates to the evaluation of household appliances. More specifically, the present invention relates to the quality evaluation of second-hand household appliances. BACKGROUND

[0002] In households, a wide variety of appliances can be found. New household appliances can be of high quality, such that the appliance can be used for many years. However, due to new improvements or environmental changes in the household, it can be desirable to replace one household appliance for another. Since the old appliance can still function properly, it can be resold or sold separately.

[0003] Potential buyers of second-hand household appliances have difficulties determining their quality. Especially when the buyer has no way of manipulating the appliance before purchase, as is common on internet trading platforms, the impression from pictures or the seller’s description can be deceptive. SUMMARY

[0004] It is an object of the present invention to provide an improved technique for grading second-hand household appliances. The invention solves the object by the subject matter of the independent claims enclosed. The dependent claims describe preferred embodiments.

[0005] According to a first aspect of the present invention, a method for grading household appliances comprises the steps of determining life cycle parameters of a plurality of household appliances, ranking the appliances into a directed graph using the life cycle parameters as ranking criteria such that there is a Pareto improvement between consecutive appliances along a path through the graph, determining a life cycle rank for each appliance according to its position in the path through the graph, and determining a grade for the appliance based on its life cycle rank.

[0006] By using usage or quality information of a plurality of household appliances, it is possible to grade individual appliances. Usage data can already be available if the appliances are connected to a network. The proposed method can form a general and practical way to provide a reasonable grade indicating the usability or quality of a second-hand appliance.

[0007] It is especially suggested to apply the concept of “Pareto improvement” to rank the appliances in an order reflecting their quality or usability. Preferably, the parameters are indicative of how much functionality can still be extracted from a second-hand household appliance. It is especially proposed to assign a grade to the household appliance, which can be normalized on a predetermined scale, e.g. 0-100, where 0 indicates an appliance that hardly functions and 100 indicates an appliance in brand new condition. The score can be used to compare two second-hand household appliances. Based on the score, it can be determined when a household appliance needs to be replaced. Furthermore, an operational value can be determined indicating when a household appliance should be discarded. The appliances can be resold or sold based on the associated grade.

[0008] It is especially preferred that a ranking is performed for a large number of household appliances, for example 1000 or more. The appliances can be comparable in their technical specifications and are especially designed for the same purpose. That is, coffee machines can only be compared with coffee machines of preferably the same type, etc. The method can make use of data already collected from networked household appliances. Such appliances are becoming more and more common, so there can already be a considerable amount of historical data available.

[0009] The mentioned Pareto improvement stems from a principle used in welfare economics and formalizes the concept of being "better in every possible way". A first appliance is considered to be better than a second appliance by a Pareto improvement if every parameter of the first appliance is at least as good as the corresponding parameter of the second appliance and at least one parameter of the first appliance is better than the corresponding parameter of the second appliance.

[0010] In other words, the first appliance is better than the second appliance in at least one parameter and is not worse than the second appliance in all other parameters. This criterion allows for application to an arbitrary number of parameters. In this way, a complex comparison between different household appliances can be made possible. The criterion also allows for a quick and efficient ranking of a large number of household appliances according to their quality or availability.

[0011] Preferably, the parameters of the appliances are chosen or mapped such that low values are considered to be advantageous. For this, the determined parameters can be transformed appropriately. This can include taking the absolute value, a negative value or the inverse of the determined value. Furthermore, preferably, all parameters are positive. The parameters can be normalized to a predetermined scale. Different parameters can use different scales. An example scale can range from 0 to 1, 1 to 10 or 1 to 100. It is noted that alternative methods are possible in which all parameters are negative and / or parameters with a high value are considered to be advantageous.

[0012] In some preferred embodiments, another ranking according to an inverse Pareto criterion is determined for each household appliance; and the appliances are ranked based on a combination of the two rankings. The combination can especially include an arithmetic mean.

[0013] The inverse Pareto criterion can be determined by taking the inverse of each parameter and ordering the appliances such that there is a Pareto deterioration between consecutive appliances. If a first appliance is considered to be better than a second appliance by a Pareto improvement, the second appliance can be considered to be worse by a Pareto deterioration. A first appliance considered to be worse than a second appliance by a Pareto deterioration is not as good as the second appliance in at least one parameter and is at most as good as the second appliance in all other parameters.

[0014] In this way, a second reverse ranking can be assigned to each household appliance. The second ranking can help to mitigate the effect of one parameter that is significantly different from the average value having a large impact on the ranking position of the household appliance in the graph. The second ranking can help to deal with outliers, measurement errors and similar errors.

[0015] The proposed method can be applied to different types of household appliances. It has been found that the most important parameters for ranking household appliances include their age and the amount of work they have done. Therefore, it is preferred that the life cycle parameter comprises at least the age of the household appliance and the number of processes performed by the household appliance.

[0016] The proposed method is well suited to add more parameters. This can make the ranking more representative of the equipment quality, but improvements due to other parameters than age and number of processes are expected to be small.

[0017] Different types of household appliances can use different processes. For example, for a coffee machine, one process can be the preparation of one hot beverage. For a washing machine, one process can comprise one washing program. Similarly, a dishwasher can count its processes by the programs executed for cleaning dishes.

[0018] Preferably, the household appliance comprises a kitchen machine, a laundry care machine or a power tool. Each of these types of household appliances can have one or more parameters that can be used to evaluate the appliance. In the following, preferred (but optional) parameters for evaluating a household appliance are given: All types of household appliances: • the age of the appliance; and • the number of uses of the appliance, e.g. the number of appliance-specific processes performed.

[0019] Coffee machine: • the age of the appliance, • the number of beverages prepared, • the number of days at least one beverage was prepared, • the number of days the appliance was not connected to the internet, • the longest number of days the appliance was not connected, • the number of times and average duration of Wi-Fi signal loss, • the number of times the water filter was flushed, • the average number of uses between water filter flushes, • the number of times the milk system was flushed, • the average number of uses between milk system flushes, • the number of cleaning programs executed (including cleaning during combined cleaning and descaling runs, • average number of uses between cleaning programs, • number of descale runs (including descale during combined clean and descale runs), • average number of uses between descale program runs, • number of descale overdue reminders, • number of times the device could not make a beverage due to descale overdue, and • number of error messages and number of errors by type.

[0020] Dishwasher: • age of the appliance, • number of times a dishwashing program was executed, • average number of days after a salt low warning that the user needs to refill the salt, • average value of the water turbidity sensor, • number of times a cleaning program was executed, • number of times the filter was clogged, and • number of errors.

[0021] Washing machine and dryer: • age of the appliance, • number of uses of the appliance, • amplitude of the vibrations during different types of spin cycles, • power consumption of the motor, and • number of errors.

[0022] Drill and similar rotary tools (e.g. power screwdriver, impact drill, hammer drill, angle grinder, etc.): • total usage time, • number of uses of the trigger, • load on the device during use (e.g. based on input power histogram), • number of times the device overheated, • number of times the kickback control was activated, and • number of times the device was turned off due to detecting a drop.

[0023] Battery (e.g. for power tools, vacuum cleaner, small kitchen appliances): • number of times it was charged, and • remaining life of the battery.

[0024] According to some preferred embodiments, similar determinations can be performed on usage parameters of a household appliance. The usage parameters can be indicative of the degree to which the appliance was maintained or problems encountered in its so far usage life.

[0025] More specifically, the method can comprise the steps of determining usage parameters for a plurality of household appliances (the plurality of household appliances preferably being the same as the plurality of household appliances for which the life cycle parameters are determined), ordering the appliances into a directed graph using the usage parameters as an ordering criterion such that there is a Pareto improvement between successive appliances along a path through the graph, and determining a usage rank for each appliance according to its position in the path through the graph. The ranking can be determined based on the assigned life cycle rank and additionally based on the determined usage rank. The ranking of the appliances can be determined based on a weighted average of the life cycle rank and the usage rank.

[0026] In a simple embodiment, an average between the usage rank and the life cycle rank can be determined. Preferably, however, the life cycle rank has a greater weight in determining the ranking. Good results have been achieved with a weighted average of the ranking, in which the life cycle parameter accounts for approximately 80% and the usage parameter accounts for approximately 20%.

[0027] To determine the ranking from the directed graph, it is proposed to first select a predetermined number. This number can indicate the lowest possible rank to be generated. In one embodiment, this number can be one. Next, an iterative process can be initiated. In this process, each appliance that forms the start of a path can be ranked with the selected number. The ranked appliances can be removed from the graph, and the number can be increased. Typically, this number is increased by one. The process can be repeated until all appliances have been ranked.

[0028] It is noted that with the given method, several appliances can have the same rank. For appliances with the same rank, the associated parameters are not necessarily the same.

[0029] Preferably, the ranking is used to determine a ranking such that the ranking is normalized to a predetermined scale, for example 0-100. This corresponds to a percentage scale, which is intuitive for a user. Thus, 100 can be assigned to a household appliance that is in a very good condition, and 0 to 1 can be assigned to a household appliance that is in a worst condition.

[0030] It can be required that the appliance is still operational in order to be ranked in the described manner. Unresolved errors that prevent it from being operational can exclude the appliance from being ranked.

[0031] The above-described process can order the household appliances in a sequence that reflects their availability or quality. However, the ordering can not be linear, such that the distance in ranking between a first appliance and a second appliance can only weakly indicate the degree to which one appliance is superior to the other appliance.

[0032] To introduce an improved way of grading appliances, the ranked appliances can be further processed. This can include selecting the appliance with the lowest rank and setting its grade to a predetermined number; iteratively selecting the appliance with the next higher rank; determining a distance between the selected appliance and the next lower ranked appliance; and setting the rank of the selected appliance to the determined distance plus the rank of the next lower ranked appliance.

[0033] After determining the rank of an appliance, its grade can be determined. In one embodiment, the grade can be the rank; in another embodiment, the grade can be the inverse of the rank. An optional technique for determining the grade is described below, which has a more linear relationship to the grade on a predetermined scale.

[0034] The predetermined number can indicate the worst (i.e. lowest) possible grade. According to the example given above, the predetermined number can be zero. This means that the distance value is always positive. Thus, for a given distance metric, the further the parameter set of a domestic appliance is from the worst possible appliance, the higher the grade it obtains. This can allow appliances to be better distributed across the scale. It can be easier to compare the quality of two or more domestic appliances.

[0035] The distance between the parameter sets of two domestic appliances can be determined in a number of ways. According to some embodiments, the Mahalanobis distance can be employed. According to some further embodiments, the distance can be determined along the direction of the first principal component of the selected appliance relative to the previous appliance. In this case, it is preferred to perform a principal component analysis on the appliance data. Preferably, this is done before determining the distance between appliances or their associated parameter sets.

[0036] Whether or not a linearization technique is used, it is preferred to map the resulting grade to a predetermined scale, such as 0-100.

[0037] The determined grade of the second-hand domestic appliance can be provided to the user of the appliance. As the determined grade can be considered as an assessment result and can affect the legal or monetary value attributed to the appliance, it is advisable to protect the grade from tampering or forgery. The method can include the step of digitally signing an electronic document containing the determined grade. The electronic document can be used as a certificate reflecting the availability or quality of the second-hand domestic appliance.

[0038] Preferably, the electronic document also includes an identification of the appliance. This can include a serial number of the appliance. In addition, it can also include an indication of the manufacturer or model of the appliance and / or a picture of the appliance. It is further proposed that the electronic document includes a timestamp indicating when the appliance was graded.

[0039] The electronic document can be provided to a person associated with the appliance. Especially for networked appliances, the person in charge of the appliance (or owning the appliance) can be such a person. Optionally, a request to issue the document to a different person can be accepted.

[0040] According to some embodiments of the present application, a person associated with a household appliance can be allowed to access online the current grade of his or her appliance. This can help the user to estimate the value of the appliance and whether it is time to replace the appliance.

[0041] Preferably, parameters of the household appliance are collected intermittently or continuously over the lifetime of the household appliance. As long as the household appliance is online, i.e. connected to a central repository for collecting its parameters, the collected data about the usage period can be uploaded in time. The amount of data uploaded is typically not an issue, so extensive information can be collected by the respective service.

[0042] If the household appliance is not networked or if the network access is interrupted, the collected information can be stored locally. Once communication with the remote service can be established again, the information upload can be triggered. In some cases, the collected information can be read out from the memory of the appliance during a service. The on-board memory of the appliance can be limited, so the stored data can not be comprehensive enough.

[0043] According to another aspect of the present application, a device for grading household appliances is presented. The device comprises an interface for retrieving life cycle parameters of a plurality of household appliances; and processing means. In this context, the processing means are adapted to: - order the appliances into a directed graph with the life cycle parameters as ordering criteria, such that there is a Pareto improvement between consecutive appliances along a path through the graph; - determine a life cycle rank for each appliance depending on its position in the path through the graph; and - determine a grade for the appliance based on its life cycle rank.

[0044] The processing means can be adapted to fully or partially execute the method disclosed herein. The processing means can be of electronic nature and can comprise a microcomputer or microcontroller, an ASIC or similar device. The method can be implemented as a computer program product with a set of program code instructions and can be stored on a computer readable medium. Features or advantages of the method can apply to the respective device or system and vice versa.

[0045] The device preferably runs as a service on a server or cloud infrastructure. The device can be connected to or identical to the devices used so far for collecting usage data from household appliances.

[0046] According to another aspect of the present invention, a system is proposed comprising a device as disclosed herein and a plurality of household appliances. The appliances are preferably technically comparable. That is, the appliances should serve a similar or the same purpose. Furthermore, the models of the appliances should be similar or the same. BRIEF DESCRIPTION OF DRAWINGS

[0047] Non-limiting embodiments of the present invention will now be discussed in more detail with reference to the drawings, in which: Figure 1 A system is shown; Figure 2 A flowchart of a method is shown; Figure 3 A dataflow diagram is shown; and Figure 4 An example of grading household appliances is shown. DETAILED DESCRIPTION

[0048] Figure 1 One embodiment of a system 100 is shown, which comprises at least one household appliance 105 and one external device 110. It is to be understood that the presently proposed invention is preferably executed on a large number of appliances 105. Figure 1 The household appliance 105 shown in Fig. 1 is for example a washing machine, but other household appliances 105 are equally possible. The household appliance 105 comprises a controller 115, which is adapted to determine parameters of the appliance 105 and its operation and to forward the parameters to the device 110.

[0049] The device 110 comprises a processing means 120, a communication interface 125 and a memory 130. Information can be retrieved from a plurality of household appliances 105 via the communication interface 125 and stored in the memory 130 for subsequent processing.

[0050] It is proposed to evaluate the household appliances 105 based on the retrieved information. A user 135 associated with the appliances 105 can request at the device 110 to grade the appliances 105. This can for example be done by using a mobile device 140, in particular a smartphone. Other user interfaces are possible as well.

[0051] The device 110 can determine the grades of the appliances 105 and provide an electronic document 145, which the user 135 can present, print or otherwise use. In Figure 1 In the exemplary embodiment shown, the electronic document 145 comprises the determined grades 150, a timestamp 155 indicating the time of completion of the evaluation, an identification 160 of the relevant appliances 105 and an encryption signature 165 certifying the authenticity of the document 145 and ensuring that the document 145 is protected against forgery or tampering.

[0052] Referring to Figure 2 , there is provided a flowchart of one embodiment of a method 200 for ranking household appliances 105. The method 200 can be performed primarily on the device 110.

[0053] In step 205, parameters of a plurality of household appliances 105 can be determined. In practice, a batch of household appliances 105 can provide usage and / or lifecycle information over their lifetime of use. In one embodiment, such information can be uploaded to the device 110 immediately upon the start or completion of a work sequence, or upon the occurrence of a predetermined event. Such events can include, for example, the occurrence of an error or interruption in a running process. Appliances 105 that are not connected to the device 110 can store retrieved information locally, which can subsequently be forwarded from the local to the device 110. The device 110 can store the retrieved information in the memory 130.

[0054] In step 210, the information collected for the plurality of household appliances 105 is processed. As will be explained in more detail below with reference to Figure 4 , each appliance 105 is associated with a set of parameters that can be considered a vector.

[0055] From the vector information, a directed graph can be created, which includes a node for each household appliance 105. Associated with the node is the vector of parameter information. The appliances 105 are ordered in the directed graph such that there is a Pareto improvement between a pair of consecutive household appliances 105.

[0056] In step 215, a ranking can be assigned to the household appliances 105 in the directed graph. Roughly speaking, the appliances 105 can be arranged in order from the "best" appliance 105 to the "worst" appliance. This order is represented as a path in Figure 4 , and the graph can include more than one path.

[0057] The ranking can include the following steps: a vector can be considered "non-dominated" if there is no vector that is greater than it. Such a vector can be the starting point of a path through the directed graph. It is assumed that all coordinates of a given vector are negatively correlated with the quantity to be described. The quantity can be quality in the given context. Positively correlated coordinates should be multiplied by -1. In other words, a vector can be considered "good" if it contains values for all parameters that are small; it can be considered "bad" if it contains values that are large.

[0058] From the top of the graph, i.e. from the starting point, a predetermined lowest rank can be assigned to each non-dominated vector. For example, the lowest rank can be 1. Thereafter, all ranked vectors can be removed from the graph, the rank can be incremented, and the process can be repeated until all vectors or household appliances 105, respectively, have been ranked.

[0059] A vector with one element significantly larger than the average can get a very low rank. To mitigate this effect, a second inverse rank can be determined for each appliance 105. To this end, all vectors in the directed graph can be multiplied by -1. Thereafter, the above-described process of iteratively assigning ranks can be repeated, but this time starting from the end of the paths through the directed graph. The final rank of a domestic appliance 105 can be a linear combination of the two determined ranks. In one embodiment, the final rank can be the arithmetic mean between the two ranks.

[0060] In step 220, the determined ranks can be linearized such that the rank distance between two appliances 105 better corresponds to the difference in quality or availability. The domestic appliances 105 can be traversed in order according to their ranks. The first domestic appliance divided by rank can be associated with a zero level (or another predetermined number).

[0061] For each subsequent domestic appliance 105, a score can be determined that comprises the sum of two elements. The first element is the rank of the domestic appliance 105 that is one level lower than the considered domestic appliance 105. The second element is the distance between the appliance 105 in question and the appliance 105 that is one rank lower. The sum of these two elements is then assigned as a level to the domestic appliance 105.

[0062] To determine the distance, the Mahalanobis distance measure can be used. Other distance measures are also possible, for example the Euclidean distance. Preferably, however, the distance is determined in the direction of the first principal component of this appliance 105 relative to the previous appliance 105. To speed up the calculation, a principal component analysis (PCA) can be performed on the appliances 105 in the directed graph before linearization.

[0063] In step 225, the levels assigned to the appliances 105 can be normalized to a predetermined scale. In the case where the linearization step 220 is not performed, the levels can be determined based on the ranks. In one embodiment, the levels can be the determined ranks. Normalization can be performed thereafter.

[0064] In step 230, an electronic document 145 can be provided that comprises the determined levels 150. More information can be included (see Figure 1 ).

[0065] Figure 3 An exemplary data flow diagram 300 is shown. Abstractly, the summary score 305 is determined based on relative scores 310, which in turn stem from absolute scores 315, which are based on aggregated data 320. Life cycle information is shown in the upper section, usage information 330 in the lower section.

[0066] The life cycle information 325 can indicate how often and / or how long the appliance 105 has been used. The usage information 330 can indicate how well the household appliance 105 has been maintained.

[0067] Exemplary life cycle information, which can be comprised by the aggregated data 320, can accordingly include the number of performed cycles, the production date at the last cycle run or the current date. The number of usage times can be converted into the number of cycles as an absolute score 315. The production date and the date of the last cycle can be combined to yield an absolute score 315 reflecting the age of the appliance 105 between its manufacturing date and the current date.

[0068] As a relative score 310, the number of cycles can be copied from the absolute score 315. The age of the absolute score 315 from the manufacturing date to the current date can be converted into the age in months as a relative score 310. The elements of the relative score 310 can be combined into a first summary score 335.

[0069] With respect to the usage information 330, a similar approach can be performed. Exemplary parameters, which can be collected as aggregated data 320, can include events when a fault condition was detected, e.g. salt deficiency in the dishwasher 105, operating parameters such as turbidity of the wash water in the dishwasher, or other values. The aggregated data 320 can be converted into a respective absolute score 315. In the example given above, the average reaction time between the detection of the salt deficiency and the taking of remedial measures can thus be determined. From the turbidity readings, an average turbidity can be determined.

[0070] From these intermediate results, a relative score 310 can be determined. The reaction time for replenishing salt can be converted into the number of days in which the salt deficiency state existed. The average turbidity can be converted into an average turbidity value. Other parameters can be treated accordingly.

[0071] From the elements of the relative score 310 of the usage information 330, a second summary score 340 can be determined. The steps 335 and 340 can correspond to the steps 215 of the method 200, respectively.

[0072] The summary scores 335, 340 can be combined into a third summary score 345 based on predetermined weights 350. Preferably, the life cycle score accounts for about 80% of the third summary score 345 and the usage score accounts for about 20% of the third summary score 345. The determined score 345 can be rendered on the electronic document 145. Preferably, the document 145 also comprises other information shown. Optionally, elements of the relative score 310 can be included.

[0073] Figure 4An example of ranking home appliances 105 is shown. In the upper section a directed graph 400 is shown, in the lower section a table 405 is shown. The directed graph 400 is the result of ordering a set of predetermined exemplary appliances 105 in a Pareto improvement relationship. The directed graph is acyclic, which means that it has no loops.

[0074] Each home appliance 105 is represented by a vector with three dimensions, which relate to three parameters. Preferably, the parameters are independent from each other. Each parameter is negatively correlated with a quantity that indicates quality. In other words, for all parameters, the lower the numerical value, the higher the quality of the corresponding appliance 105. Example parameters can represent the age of use, the number of program cycles completed, and the number of errors encountered.

[0075] In the directed graph 400, the vectors are connected in such a way that the arrow between a vector and a subsequent vector indicates a Pareto improvement. In other words, in terms of Pareto improvement, the arrow jumps from an appliance that is considered "worse" to an appliance that is considered "better".

[0076] The graph 400 can have the following properties: - If vector A has a better value than vector B on one parameter, while the values in all other parameters are identical, then vector A should get the same or better rank than vector B.

[0077] - If vector A and vector B are close to each other in a given distance metric, then their ranks should also be close to each other. If vector A is closer to vector B than to vector C, then the Pareto rank of vector A is closer to the rank of vector B than to the rank of vector C.

[0078] The rank determined for a vector can be called a Pareto rank. This rank is useful for comparing appliances 105, because the difference between two Pareto scores satisfies the following properties of a distance metric: If d(x,y) is defined as the difference between two Pareto ranks x and y, then d can satisfy the following criteria: - Non-negativity: d(x,y) >= 0 - Identity: d(x,y) = 0 if and only if x == y - Symmetry: d(x,y) = d(y,x) - Triangle inequality: d(x,y) + d(y,z) >= d(x,z).

[0079] The paths through the directed graph 400 start from the vectors (7, 4, 1) and (2, 2, 2). These vectors are thus mathematically "non-dominated". Non-dominated vectors can be assigned a predetermined rank, which in the present exemplary case is "1". This is the lowest rank that will be assigned to a vector in the graph 400. After this step, all ranked vectors can be removed from the directed graph 400.

[0080] The predetermined rank can then be increased, in particular incremented, and the process can be repeated. The iteration stops when all vectors have been assigned a rank. In the directed graph 400, the exemplary ranks are shown outside the ellipses around the vector elements.

[0081] Figure 4 The table 405 in the lower section shows the vectors of the directed graph 400 ordered by the associated rank. The rank of a vector, and thus of the associated household appliance 105, can be determined directly after its associated rank, i.e. the rank can be taken as the rank. In some embodiments, a linearization and / or mapping of the ranks can be performed to determine the ranks.

[0082] The vector with the lowest rank can be assigned a predetermined rank, e.g. 0. The vector with the next higher rank can be assigned a rank that is the sum of the previously determined lower rank vector plus the distance of the current vector to that lower rank vector. The process can be repeated until all vectors have been assigned an appropriate rank.

[0083] Afterwards, the determined ranks can be normalized to a predetermined scale, e.g. between 0 and 100.

[0084] List of reference signs

Claims

1. A method (200) for grading household appliances (105), the method comprising the steps of: - determining (205) life cycle parameters for a plurality of household appliances (105); - ordering (210) the appliances (105) into a directed graph (400) using the life cycle parameters as ordering criteria; - such that there is a Pareto improvement between successive appliances (105) along a path through the graph (400); - determining (215) a life cycle rank for each appliance (105) according to its position in the path through the graph (400); and - determining (225) a grade for the appliance (105) based on its life cycle rank. A first appliance (105) is considered to be superior to a second appliance (105) by a Pareto improvement if each parameter of the first appliance (105) is at least as good as the corresponding parameter of the second appliance (105) and at least one parameter of the first appliance (105) is superior to the corresponding parameter of the second appliance (105).

2. The method (200) of claim 1, wherein The parameters of the appliances (105) are adjusted such that low values are considered to be advantageous.

3. The method (200) of claim 1 or 2, wherein, A further rank according to the inverse Pareto criterion is determined for each household appliance (105); and the appliances (105) are graded based on a combination of the two ranks.

4. The method (200) according to any of the preceding claims, wherein The life cycle parameters comprise at least a service life of the household appliance (105) and a number of processes performed with the household appliance (105).

5. The method (200) according to any of the preceding claims, wherein The household appliances (105) comprise kitchen machines, laundry care machines or power tools.

6. The method (200) according to any of the preceding claims, wherein The method (200) further comprises the steps of:

7. The method (200) according to any of the preceding claims, wherein - determining usage parameters for a plurality of household appliances (105); - ordering the appliances (105) into a directed graph (400) using the usage parameters as ordering criteria; - such that there is a Pareto improvement between successive appliances (105) along a path through the graph (400); and - determining a usage rank for each appliance (105) according to its position in the path through the graph (400); - wherein the grade for the appliance (105) is determined additionally based on its usage rank. The rank of an appliance (105) is determined by selecting a predetermined number; then iteratively:

8. The method (200) according to any of the preceding claims, wherein - ranking each appliance (105) constituting a starting point of a path with that number; - removing the ranked appliances (105) from the graph (400); and - increasing the number; until all appliances (105) are ranked. The grade of an appliance (105) is determined by:

9. The method (200) according to any of the preceding claims, wherein - selecting the appliance (105) with the lowest rank; and setting its grade to a predetermined number; and - iteratively selecting the appliance (105) with the next higher rank; determining a distance between the selected appliance (105) and the next lower ranked appliance (105); and setting the rank of the selected appliance (105) to the determined distance plus the rank of the next lower ranked appliance (105). ​ 10. The method (200) of claim 9, wherein, The distance is determined along a first principal component direction of the selected appliance (105) with respect to the previous appliance (105).

11. The method (200) of claim 10, wherein, A principal component analysis is performed on the appliance (105) data.

12. The method (200) according to any of the preceding claims, wherein The method (200) further comprises digitally signing the electronic document (145) containing the determined grade.

13. The method (200) according to any of the preceding claims, wherein Parameters of the domestic appliance (105) are continuously collected over a usage time of the domestic appliance (105).

14. An apparatus (110) for grading domestic appliances (105), the apparatus comprising the following elements: - an interface (125) for retrieving life cycle parameters of a plurality of domestic appliances (105); and - a processing device (120) adapted to: - order the appliances (105) into a directed graph (400) using the life cycle parameters as ordering criteria; - cause that along a path through the graph (400) there is a Pareto improvement between consecutive appliances (105); - determine a life cycle rank of each appliance (105) depending on its position in the path of the graph (400); and - determine a grade of the appliance (105) based on the life cycle rank of the appliance (105).

15. A system (100) comprising the apparatus of claim 14 and a plurality of domestic appliances (105).

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