Power, Energy and the Loads List

The Kill-A-Watt meter is an excellent tool you can use to measure the wattage of a device.

So how do we do that? There are simple power and energy meters available that can do this task for you. Such meters are typically plugged into a wall outlet and the device to be measured is then plugged into the meter. A display screen shows information about the device's use of electricity. Most meters of this type measure power drawn by the device (in Watts) and energy consumed over time (in Watt-hours and/or kWh). To measure the electricity used by the fridge, plug it into the meter and leave it plugged in for a week or a month. (The longer the time period, the more accurate the average will be). At the end of the time period, read the display and divide that figure by the time unit. Most meters will use hours. Knowing there are 24 hours in a day, you can then determine the average daily, weekly or monthly Wh or kWh energy consumption by simple arithmetic.

The value of an accurate loads list for your off-grid system cannot be overstated. If you're off the grid, you must produce every Watt-hour that you need. A bad guess up front may mean insufficient energy when it counts. Even seemingly insignificant differences among devices can yield wildly different results when calculated over long periods of time. For example, if we had substituted an equivalent incandescent light bulb to our CFL above, and left the incandescent on for the same 66 hours and 40 minutes, it would use over 4 kilowatt-hours of electricity—four times the energy used by the comparable CFL! In preparing for an off-grid electric system, it absolutely pays to spend the time to measure or carefully estimate your intended loads. Not only will you ensure that you end up with a renewable energy system that meets your actual needs, you'll usually find ways to conserve energy or increase efficiency which reduces the size of the system you'll need. A smaller system is a more cost effective system and a loads list is the most important tool to get you there.