How to Calculate your Energy Load

February 12, 2018

The size of the solar power system you need depends on how much energy you use. The first step in designing a solar power system for your house is to figure out how much power you need to run the various systems and appliances.

Grid-tied Systems

If you are already connected to utility power, this is quite easy. A look at your power bill will tell you how many kWh you are using in any one month. However, the amount of power you use at different times of the year will vary and so will the amount of power that can be harvested from the solar panels. The total power you used over the year is a better number for sizing your solar array, and your usage over several years would give an even more accurate indication.

Off-Grid Systems

If you do not have utility power and it is prohibitively expensive to bring in, then a off-grid system may be a good choice. Sizing this system is a lot more complicated. You will have to make a list of all the appliances and other loads that you expect to use and determine the total power that will be needed.

Types of Loads

Loads can be continuous (running 24 hours a day), or intermittent. Many loads - such as a refrigerator, toaster, or vacuum cleaner - use a large amount of power but only for short periods. The best way to figure out how much power your appliances need is to measure them with a power usage meter (the Kill A Watt meter is one example). Often, something that draws little power (such a standard light bulb at 60 watts) will use more power per day if left on all day than a very large intermittent load such as a coffee maker drawing 900 watts for 20 minutes. For example:

A 60W light bulb, 60W x 24 hrs = 1440 Whr.
A coffee maker uses 900W x 1/3 hr = 300 Whr.

It is also evident from this example that if you use energy efficient compact fluorescent light bulbs, which produce the equivalent of a 60W light bulb but consume only 13W, this bulb would use only 312 Whr in a day even if you left it on all day.

There are some loads that are not obvious and are easily overlooked when calculating load requirements:

The inverter will draw a certain minimum amount of power.
Anything that has a remote control (like the TV or stereo) draws some power at all times so that the remote will be ready to turn it on at anytime.
Computers may also draw power even when they are not turned on.

These types of loads are called “phantom loads” because they are not readily apparent. They draw only a small amount of power. But even a small amount of power, if it is continuous for 24 hours a day, can really add up. These loads can be mostly eliminated by installing a switch, or a switched power bar, that turns off the appliances when not in use.

Once you know the energy loads you're using, the total amount of energy that you use and what the solar array will have to supply, can be calculated.

Calculating Your Total Load

Multiply the power used by each appliance times the hours per day that you use it – this gives you the total number of kWh per day for that appliance. Add up all the appliances and you will have the total number of kWh that you need per day. Adjust the total number for the efficiency of the inverter, typically 90%. Definitions:

Power is given in watts (W) – a thousand watts is a kilowatt (kW).
If you use a kW of power for one hour this is a kilowatthour (kWH). When you look at your utility bill you will see that the power you have used over the month is listed as some number of kWH.

If you use a spreadsheet program you can easily change the hours/day or the power draw of different appliances to find something that will work best for you.

An Example Load Calculation used for the Suncatcher Solar Prototype House

Notes:

The energy rating in KWh/year is based on 416 “Normal Cycle” operations per year and includes the energy required to heat the water. This is more than one load per day, which is not necessary for two people. The usage estimate has been adjusted accordingly.
This energy rating in KWh/year is also based on 416 operations per year. Our usage would be less based on fewer loads through the washer and we often hang the clothes to dry. Again, usage has been adjusted accordingly.

What's Next?

Now that you know how much power you use, we can design a solar power system tailored to your lifestyle. Check out more information on grid-tied systems or off-grid systems and contact us for a free estimate.