Power modes “truth gate”

By identifying cut off points between the different power modes, for a given appliance, it was possible to use a “truth gate” to determine which power mode was responsible for each time interval‟s energy consumption. Each potential power mode was given a “truth gate” column, which produced either a “TRUE” or “FALSE” output depending on whether the value contained in the corresponding moving average cell, fell within a specified value range. This was achieved by using Excel‟s “IF” function, which was linked to the “truth gate” criteria cells and could be adjusted for each appliance. Table 5-13 shows the “truth gate” criteria cells for a television.

Table 5-13 Truth gate criteria cells for an LCD television in household 13

Although the “truth gate” provided an automated method to apportion electricity consumption to different power modes, it was limited due to two main effects: (i) the moving average added higher power consumption values to cells that were actually zero or in a low power mode (and were classified as the “active” power mode); (ii) time intervals where low power electricity consumption was occurring, but below the 1 Wh threshold, 0

5 10 15 20 25

29 Jun 30 Jun 01 Jul 02 Jul 03 Jul 04 Jul 05 Jul 06 Jul

Mon 29/6/2009 to Sun 5/7/2009

Max Min

Active NA 15.00

Active Standby

Passive Standby 15.00 1.00 Off Standby 1.00 0.00

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displayed zero electricity consumption (and were classified as “mains off”). Examples of these effects are shown in Figure 5-10.

Figure 5-10 Examples of the limitations of the truth gate in apportioning power mode electricity consumption correctly in an appliance worksheet prior to manual correction

A number of methods were tested to mitigate these problems. For example, by specifying that the “truth gate” formula only gave a “TRUE” output if an energy consumption

measurement was also in column D, it was possible to prevent “TRUE” outputs from being displayed in intervals where no electricity consumption was recorded. In order to correct cells where low power electricity consumption was known to be occurring, it was found that the simplest method was to correct the “truth gate” columns manually. This involved copying and re-pasting the “truth gate” columns as “values”, and altering periods of different power mode usage appropriately. The example presented previously, in Figure 5-7, shows data for a LCD television where the zeros in the “truth gate” columns have been manually manipulated to correctly show passive standby electricity consumption.

Despite this method being relatively time consuming it offered a number of benefits.

Firstly, this technique allowed the researcher to become very familiar with the data and better understand the characteristics of the AMS. Thus, it was possible to identify distinct patterns in the data (such as the number of zeros in between a series of 1 Wh

measurements), which helped to correctly allocate electricity consumption to the different No electricity

consumption actually occurred in this interval

Standby power electricity

consumption was actually occurring in this interval

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standby power modes. Secondly, the process of data screening resulted in a more in-depth analysis of the data. Any small variation in the energy consumption recorded was examined in detail and it is believed that many potential errors in the data were reduced.

A further example of the benefit of manual screening is evident in Figure 5-11. As

highlighted in section 5.5.6, when a higher power mode is used for a portion of an interval, the value of electricity consumption can be similar to that of a lower power mode. Figure 5-11 shows that at the end of the period of active use, a 1 Wh measurement (Column E:

Row 125) results in a value that could be interpreted as a measurement caused by the television‟s passive standby power mode (Column E: Row 133). However, it can be seen that there are no intervals with a zero energy consumption value, prior to the 1 Wh

measurement in Row 125. Therefore, it can be assumed that this electricity consumption was from the active mode, because there was not sufficient time for the standby power mode to produce the 1 Wh energy consumption measurement. Thus, in order to allocate some energy consumption values to the appropriate power mode, it was necessary to evaluate the preceding and following intervals. This is an activity that would have been difficult to conduct automatically.

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Figure 5-11 Example analysis data for a LCD television recorded in household 13

Although at times laborious, the benefit of manual screening is also evident in other research. For example, Isaacs et al. (2006a) highlight that manual screening prevents unusual, but genuine, data from being excluded from research. The authors‟ state:

HEEP has not used automatic data screening procedures. We visually checked every data channel when it arrived, during and after initial processing, and before and during analysis. More than 10,000 channel years of data have been inspected. Some really weird usage patterns were followed up and in most cases found to be genuine.

(Isaacs et al., 2006a p7)

Active power consumption that could be interpreted as standby power electricity

consumption

Passive standby power electricity consumption

161 5.6.2.1.3 Potential errors in power mode analysis

Power mode electricity consumption

Despite every effort being made to reduce errors in the data analysis process, it must be recognised that the 1Wh resolution of the AMS introduces a degree of error. An example of this can be seen in Figure 5-11 previously, which shows data for a television. Figure 5-11 shows that when the television is in the passive standby power mode, around 13 zero energy consumption measurements were recorded prior to the 1 Wh measurement (e.g.

Column D: Row 133 to 147). But, only seven zero energy consumption measurements were recorded prior to the 1 Wh measurement following active use (Column D: Row 133).

This suggests that around half of the energy for this interval, actually resulted from the previous use in the active power mode. This type of effect was also apparent in reverse, when an appliance was put into a fully active mode from a standby power mode. Thus, electricity consumed during standby power modes could be allocated to the active power mode, when the appliance has been turned on prior to a 1 Wh measurement being recorded (e.g. the appliance has become active with up to twelve preceding zero energy consumption measurements).

The potential error from this effect is dependent on the number of times an appliance was turned from one power mode to another and the time of use (i.e. the point before the next incremental 1 Wh measurement). As a result, it is difficult to quantify the effect of the error with any certainty. Although work was undertaken to attempt to tackle this problem, it became apparent that this would be a difficult and very time consuming process. Due to the equal chance that an appliance would be turned from one power mode to another, midway between 1 Wh measurements, it was decided that this error was, to a degree, likely to cancel itself out (if one accepts the law of averages) and this form of error was accepted as a constraint of the AMS‟s 1 Wh resolution.

Duration of power mode use

A pragmatic approach was also taken to uncertainty regarding the measurement of the duration of appliances in low power modes. Due to the 1 Wh resolution, it is difficult to know exactly when some appliances were used in a low power mode, because a number

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of zero energy consumption measurements are likely to have occurred prior to a 1 Wh measurement. For example, Figure 5-12 shows data for a DVD player that was put into a standby power mode.

Figure 5-12 Electricity consumption data for a DVD player, recorded at household 2, apportioned to the passive standby power mode

The first energy consumption measurement occurred at 13:30 (Column D: Row 3211) but, it is uncertain exactly when the activation occurred, because the appliance could have been activated up to around 9 intervals previously (this is also more uncertain due to the variation in the duration of the intervals at this time). Although preceding (or following) intervals could have been estimated for such cases, it was decided to start and end the allocation of power modes at intervals containing energy consumption measurements.

Appliance could have been activated in the standby power mode anywhere in this time period

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This was because a WGM could have been at a midpoint between registering the 1 Wh and any estimate, even based on good reasoning, could have been incorrect and could have introduced subjective error into the results. It was considered more prudent to accept error from the resolution of the AMS, rather than from the researcher‟s subjectivity.

Although this approach will undoubtedly cause a degree of error, this will be an

underestimation of appliances duration in particular standby power modes rather than the overestimation. Therefore, results concerning the duration of particular standby power modes are a minimum usage, but based on solid consumption measurements rather than subjectivity.

Unclassifiable standby power and unknown electricity consumption

The apportioning of electricity consumption measurements to power modes was often straightforward for many of the appliances monitored in this study. Appliances active and standby power modes were usually easily identifiable (e.g. the LCD monitor shown in Figure 5-9) and other appliances frequently remained in the same power mode throughout the monitoring period (e.g. STBs, VCRs, DVD players, routers, printers and audio

equipment). In such cases, observations made while installing and uninstalling the monitoring equipment allowed the correct power mode to be applied. The selection of standby power modes could also be confirmed at the interview by discussing the patterns of appliance use with householders. Nevertheless, it must be accepted there is the potential for some human error in some cases (e.g. VCRs set to record broadcasts) when differentiating between standby power modes was more difficult.

However, it is believed that such error was kept to a minimum by using an “unclassifiable standby” (U/C) column. This was used in cases where it was not possible to ascertain that electricity consumption occurred through the use of a specific standby power mode, but it was known that the appliance was in a standby power mode. This was due to some appliances standby power modes having very similar power consumption values (e.g.

within 1 or 2 W) or due to periods of inconsistent polling. By entering a “TRUE” output in the unclassifiable standby column, it was possible to allocate energy consumption to overall standby power consumption totals, whilst avoiding the introduction of error into the results for specific power modes.

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In a few cases, it was not possible to ascertain whether an appliance was in an active or a standby power mode. This resulted from either missing data (e.g. very long time intervals) or from appliances having similar power values for active and standby power modes (this mainly affected digital radios and telephony equipment). For such cases, the data were removed from power mode calculations by being recorded in a separate “Unknown”

energy consumption column.

The identification of computers active and passive standby power modes

A number of issues related specifically to desktop and laptop computers. Similar to the MTP‟s (2006b) investigation of home computers, it was often difficult to discern when computers entered active standby or passive standby power modes due to automatic low power management settings. This was due to computers often operating in a wide range of power loads while active (e.g. depending on software requirements and the number of applications in use).

Although reports from householders suggested that the majority of the computers monitored in this study did not have power management settings enabled, it is possible that computers with factory default settings may have entered into standby power modes.

An additional issue for laptop computers is that standby power measurements can also be influenced by batteries state of charge (EES, 2006). Therefore, it is possible that active standby and passive standby power electricity consumption from computers may have been inadvertently attributed to the active power mode. The results presented for desktop and laptop standby power mode electricity consumption must therefore be viewed as a conservative estimate.

The calculation of network appliances standby power consumption

In a number of households network appliances (e.g. STBs, routers, modems and

telephones) often remain continuously in an active power mode, even when a television or computer was not being used. Similar to the approaches used in ESS (2006) and the REMODECE project, in such cases, electricity consumption was categorised as a form of active standby, because the appliances were not providing their primary functions. A

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report from REMODECE states that for routers and STBs “standby is calculated as the consumption in the hours when the associated PC or TV is not in use” (Grinden and Feilberg, 2008 p7). This thesis has also adopted this approach.

Values for STBs standby power electricity consumption were calculated with the following method. The total duration value for active television use was divided by the total duration of the corresponding active STB use. This produced the fraction of the two week

monitoring period that each STB was being used to actively view broadcast material. This fraction was used to calculate the amount of electricity consumption that could be

apportioned to the active and active standby power modes (shown in the equations 5, 6 and 7).

Equation 5

Equation 6

Equation 7

This approach was also applied to households that used AV boosters and AV senders continuously in an active mode. In households 1, 2 and 6, VCRs were occasionally used to record television programmes. Therefore, the active use of VCRs was incorporated into the calculations to account for the STBs active electricity consumption when connecting the VCRs to broadcast material.

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A similar approach was also used for routers and modems by dividing the total duration of active computer use by the duration of router and modem use for each household.

However, for one of the households (household 12) the Internet access was delivered through a device that also connected the household‟s telephones to the service provider‟s telephone network. This device was categorised as always being in an active power mode, due to it continuously providing one of its primary functions (i.e. connecting the

telephones to the external network), even though telephones were not in continuous use.

A limitation of the calculations is that active standby values may be underestimated. For STB use in households 1, 3 and 6 a portion the active VCR use may have been to view video cassettes rather than for recording. Similarly, electricity consumption values gained for routers and modems will not account for the simultaneous use of computers in a household. Thus, although the results gained provide a much more representative assessment of the appliances electricity consumption, they may underestimate active standby power consumption from network appliances and must be viewed as a conservative evaluation.