Calculating your Load 1

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

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 like the Kill A Watt shown on the right. It will tell you the power usage over time and this model also acts as a timer that lets you program ON and OFF times for your appliances. It’s a handy energy saver and can also be used as a vacation timer.

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 lightbulbs, which produce the equivalent of a 60W lightbulb but consume only 13W, thisbulb 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 a small amount of power but, as we have seen, 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.

The total amount of energy that you use, and with 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%.


  • 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

Load Rated Power (W) Hrs/day kWh/day
Water pump 1200 0.3 0.36
Septic pump 800 0.05 0.04
Kitchen Appliances:
Fridge 404 kWh/yr As rated 1.10
Coffee Maker 900 0.25 0.23
Toaster 900 0.2 0.18
Toaster Oven 1300 0.1 0.13
Electric Frying pan 1200 0.02 0.024
Slow Cooker 110 .1 0.011
Lighting and fans:
10 lights @ 15 Watts 150 2 0.3
Ceiling fans (2 @ 12W ea.) 24 4 0.1
Office and Entertainment:
Television (27”) 100 2 0.1
VCR 30 0.5 0.015
DVD Player 30 1 0.03
Radio 2 15 0.03
Stereo 20 2 0.04
Aquarium lights 45 3 0.14
Aquarium filters 20 24 0.48
Laptop Computer 50 6 0.3
Printer 10 1 0.01
Modem and wireless router 40 4 0.16
Washer (front loading) – see Note 1 227 kWh/yr 0.5 * rated 0.3 1
Dryer (110 V) – see Note 2 398 kWh/yr 0.1 * rated 0.1 2
Iron 1100 .05 0.05
Small Power Tools 600 .1 .06
Car Block Heater 1200 .01 .01
Battery chargers 6 2 .01
(tools, cell phones, camera, etc.)
Total AC Load: 4.31
@ 90 % inverter efficiency: 4.74


  1. 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.
  2. 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.

We welcome any questions or comments.  Please leave them in the Comments section below.


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