Electrical Load Calculation

electrical load calculation

How Much Power Does Your Renewable System Need?

Before you start buying components for a home power system, you need to know how much power you use. By taking an inventory of all the electrical loads in your house, and doing a basic electrical load evaluation, you can get a good idea how much power your system needs to produce. If you are designing your power system before building the home, you will need to carefully plan what appliances and electrical systems you will be using.

You can use our automatic Load Evaluation Form to simplify your load calculations. Once you enter the necessary information for each appliance in your home, the form will automatically perform the calculations for you. You will find links to the form and its instructions at the bottom of this page. (You need Adobe's Acrobat Reader 4.x or higher to use the automatic calculations. Otherwise, you will need to perform the calculations by hand.)

List All Electrical Appliances

First, you will need to make a list of everything in your home that uses electricity (or every appliance you plan to have in your home). For the purposes of this evaluation, all these items will be called appliances. Besides the obvious items like televisions, refrigerators and microwaves, appliances you may not immediately think of must also be included.

If you have a telephone that has a transformer on its plug (a "wall wart"), the power draw of the transformer AND the phone must be included. If possible, find out the "idle draw" of appliances like VCRs and microwaves that have digital clocks. Appliances that are only used occasionally, such as power tools, must be included to correctly assess necessary system surge capacity (unless such items will be powered directly off a generator).

Determine Power Draw for Each Item

For each appliance listed, the wattage should be noted, as well as whether it runs on AC or DC current. If wattage information cannot be found on the product labeling or in the manual, amperage and voltage should be noted instead. Most household appliances will run on 115 volt AC power, but some major appliances require 220 VAC instead. The voltage requirements of AC appliances should be easily determined. For DC appliances, the voltage should match whatever your DC system voltage is (or will be), whether 12V, 24V, or more rarely, 32 or 48V.

We have included a table showing average power usage for most common household appliances. You can access it HERE (link opens in new window). The figures in this table are approximate. For your final load calculation, you should obtain specific information from appliance labels and/or manuals wherever possible.

Estimate Appliance Usage Time

For each appliance, you should estimate how many hours per day the appliance is used. A refrigerator may be used seven days a week, but on average the refrigerator motor only runs up to 1/2 of the time, depending on the temperature settings and how warm the house is kept. That would be 12 hours/day. For a clock radio, it would be 24 hrs. A microwave may have a very large wattage rating, but may only be used for 1/2 hour or less per day.

For appliances you only use occassionally (1/2 hour or less per week, total), use 0.1 as your hours per day. This may lead to an slightly high estimation, but when in doubt, always estimate high. It's far better to have a slightly larger system than you need, than to have a system that regularly shuts down due to overloads.

Look for Extra Efficiency

The overall load evaluation is necessary for determining how much power your system needs to produce on an average day, as well as the amount of battery storage you will need. It can also be used to find areas where your electrical consumption can be reduced. Large electric appliances such as hot water tank heaters and electric ovens and ranges are not practical in an alternative energy system. Such large power draw appliances should be replaced with gas or wood-fired alternatives. Household lighting draws can be greatly reduced by using DC lighting. In an existing home, wiring a DC lighting system may not be practical, but lighting loads can still be reduced by using energy efficient compact fluorescent bulbs in as many outlets as possible.

If you are performing calculations for a backup power system, you can use your load calculation to determine which electrical circuits in your home really need to be backed up. Perhaps you could reduce your system size by only backing up your heating system and selected rooms of your house. You may not want to be without power in the room where your computer equipment is kept, but having backup power for a bedroom may not be as important.

Determine AC Power Needs

Once you have finished your total load calculations, you will need to take the information for all the AC appliances on your list, and determine two more figures. First, find the AC appliance on the list with the highest wattage. Then determine how many of the AC items will be connected (plugged in) at the same time. If you have a number of kitchen appliances or power tools, but only use one of each at a time, use the number0 for the highest wattage item in each group. Add the wattage of all the simultaneously connected AC appliances together.

The AC appliance figures are important in determining what size inverter your system will need. Make sure, after performing your general load calculation, that you find out what the start-up power draw is on each AC appliance. Refrigerators, water pumps and other motor-driven appliances will often draw twice as many amps when their motor first starts as they use while their motor is running. Variable-speed power tools and kitchen appliances may have power draws which vary along with their speeds.

You will need to ensure that the surge requirements of your AC electrical loads do not exceed the surge capacity of the inverter you buy. You may wish to use our automatic Load Evaluation form a second time, including only the start-up draw of your AC appliances. The figure resulting from this calculation will serve as a surge capacity minimum for inverter sizing.

Keep in mind that alternative energy systems are generally easy to expand, so your initial system does not necessarily need to be oversized to allow for future load increases. If you electrical use increases, you can easily add more solar panels, batteries or wind turbines, and often a single inverter can be wired to a second inverter to increase AC output.

Load Calculation Form (PDF) | Form Instructions | Power Consumption Table

If you haven't completed a Site Analysis at this point, that's your next step…