An energy audit is the first stepping stone to building a solar system that supplies your needs at the best economical price point.
Too often people see a "deal" and impulse buy, then learn what they've bought won't work for their needs and there's no way to upgrade/scale-up - they have to dump it and that's an expensive lesson to learn.
See: https://diysolarforum.com/resources/system-energy-audit-and-sizing-spread-sheet.12/ --
Spreadsheet Quicklink
From the spreadsheet, you'll see the formula for a given device is the average amount of power it draws and how long you use it per day. A simple table might look like this:
| Device | Apparent
Watts (power) | number of hours
needed/day | watt-hours/day
(energy) |
|---|
| C-pap machine | 70 | 6 | =6 x 70 = 420 |
| fan | 120 | 6 | =120 x 6 = 720 |
| frig | 160 | 7 | =160 x 7 = 1120 |
| cell | 20 | 1 | =20 x 1 = 20 |
The sum of the "Apparent Watts" column shows the maximum inverter
continuous size. Inverters have two sizes,
continuous and
peak. The peak rating is how much power the inverter can burst to for very short periods of time. Devices that have induction motors (e.g., air conditions, pumps) may take as many as 10x the rated number of amps. If the LRA is on the motor, usually the inrush will be around that value. You can use a
clamp meter that measures in-rush to get this value, ask the manufacturer, or possibly find it on Google. Sometimes you can reduce the in-rush needed with a
soft-starter.
The value for watts is often printed on the device, for example, a 60 watt light bulb consumes 60 watts. Sometimes though, the value printed is the maximum value and sometimes it's the reactive rather than the apparent power (see below). You can measure the actual power used with a
watt meter. Make sure you measure over a reasonable period of time, for example, a refrigerator running the defrost cycle consumes a lot of power, but typically only runs once a day for 10 to 20 minutes.
The right-most column shows the total power per day. You can use that to calculate the number of solar panels and size a battery to fit your needs.
Other helpful links:
Watts Vs. Apparent WattsOn the side of your device, you might see something like 80 watts. That might tell you the "real power" the device consumes. If it's listed as 80 VA then it's apparent power.
Anything that consumes power with capacitance or inductance is going to have a reactive power penality. What the device actually consumes is always the apparent power.
You can get the Apparent Watts for your device by actually measuring it with a watt meter, or by using the power factor if known (apparent power = real power/power-factor).
Resistive devices (e.g., a toaster) are around 1. Most devices are around .95, modems around .7, cable boxes around .65. | |
