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How to do an electrical energy audit
A home energy audit is a tally of all of the heat and electricity use in your house. This describes an electrical energy audit, in which you assess the electrical energy demand of the appliances in your home or work in order to find ways to reduce. Often this is done using a spreadsheet and an energy meter such as a KillAWatt.
Many conserve water by turning off faucets while brushing teeth and fixing leaks, but electricity is hard to see. An electrical home energy audit can help you understand your usage, conserve and even fix some leaks.
Before we start, here are some definitions. See Electricity basics for more.
- Current (I)
- Flow of charge measured in amps (A)
- Energy (E)
- Electrical energy measured in watthours (Wh)
- Usually seen in kilowatt-hours (kWh) or (kW-hrs), 1000 watthours.
- Power (P)
- Electrical power, which is energy over time, measured in watts (W)
- Any electrical device plugged into the wall.
- Phantom loads
- Truly frightening beasts that suck energy even when devices are ‘’off’’’.
The basic idea of an electrical energy audit is to analyze the amount of electrical energy used by every device/appliance/load. This spreadsheet will help you complete your electrical energy audit.
Some loads in your home are easy to estimate. For example, light bulbs come with a specific power rating such as 25W. This power rating is the average power draw of the light bulb when it is on. To find the energy use of that bulb you would only need to multiply by the time it was used for. A 25W light bulb on for 4 hours per day would be 100Wh/day (25W*4h/day).
Some loads are more variable. For example, a computer might be rated at 200W, but will only consume that high rate when the computer is working hard (e.g. playing a new video game or rendering 3D). To work around this, you can (a) do the audit assuming the maximum power, (b) look up some average power values online, or (c) use an energy monitor (like a KillAWatt) to find the real average power for your computer and use. Then to find the energy use, simply multiply that average power by the hours that load is used.
Some loads have variable states. For example, a refrigerator will have a few possible states the maximum power is when the compressor is running (and might be 500W), whereas some states are quite low (e.g. 50W). To find the energy use, you can (a) find the average energy values online or maybe even still stuck on your refrigerator from energystar (these values will often annual in the form of kWh/year), (b) use an energy monitor and leave it plugged in for at least 24 hours to find an average daily value. For loads like this in the spreadsheet you will skip all of the power and hours cells and just enter the average Wh/day value in energy cell.
Some loads might not show the power rating in Watts. For most household devices you can instead multiply the input voltage and current ratings to find the input power (because in an ohmic circuit Power = Voltage * Current).
Similar to doing a water audit you will need to take your various power ratings and multiply them by the amount of time each load is utilized. It is often simpler to estimate the amount of time you use a load per day, and then how many days per week. For more intense audits you can use a form next to devices to track their usage.
A final note is that many devices have phantom loads, i.e. loads that are continuously running even when the device is 'off'. Video game consoles often have very high phantom loads. TVs can often have phantom loads close to 10W. Lightbulbs have no phantom load. The energy consumption for these phantom loads can be calculated on their own line in a spreadsheet or together on a line with the device on power. Either way they can pose an interesting formula challenge in the case where you have a device that is on some hours per day and only some days per week. E.g. a TV that is 100W and is on 4 hours per day and 5 days per week has an energy load of 2000 Wh/week (100W * 4 hr/day * 5 day/week), but then its phantom load would be running for all the other time. That time can be calculated in a few ways. The easiest way to calculate the time is just to take the total hours in a week and subtract the hours per week the device is in use. For example there are 168 hours in a week (24 hr/day * 7 days/week), so 168 hr/week - (4 hr/day * 5 days/wk) = 148 hr/wk. So the 10W phantom load would be responsible for 1480 Wh/wk (10W * 148 hr/wk). All together the TV is responsible for 3,480 Wh/week or 3.48 kWh/week from its usage and its phantom load.