TO CALCULATE HOW MUCH OFF-GRID POWER YOU WOULD USE: Start by finding how many watts each appliance needs. Be sure you use watt figures from special energy-efficient appliances recommended for solar electric homes, like compact fluorescent bulbs and solar-electric designed refrigerators. Do not add appliances that should be propane fueled.
Then multiply listed watts of each appliance by the number of hours per day, on average, that appliance runs. This gives watt-hours per day for that light or appliance. Do this for each appliance. The total for all appliances is your total watt-hours needed each day.
Figures below show some appliances commonly used in independent solar homes. Substitute your own daily hours for each and add other appliances not listed. Refrigerators come on and off on demand by thermostat, so running time per day is not known. Our Kill-a Watt meter will accurately test watt-hours used per day for any AC appliance up to 1875 watts.
| Appliance | Watts | Run Time (Hr/Day) | Watt-Hours/Day |
|---|---|---|---|
| 10CF Freezer, standard | 1000 | ||
| 12/24V NovaKool 4CF (w/added insulation) | 300 | ||
| 32″ LCD TV | 140 | 3 | 420 |
| Ceiling Fan, 12/24V DC | 5-20 | 6 | 30-120 |
| Ceiling Fan, AC | 60 | 6 | 360 |
| CF bulbs, equal to 15 watts | 15 | 4 | 60 |
| Clothes Washer | 700 | 3/4 | 525 |
| Coffee Maker | 300 | 1 | 300 |
| Computer | 100 | 4 | 400 |
| Food processor | 200 | 1/20 | 10 |
| Laser Printer, in operation | 90 | 1/4 | 23 |
| LED bulbs, equal to 5 watts | 5 | 4 | 20 |
| Microwave oven | 1260 | 1/4 | 315 |
| Refrigerator/Freezer (Energy Star) | 1000 | ||
| Satellite receiver | 20 | 3 | 60 |
| Small 4CF Apartment Fridge | 945 | ||
| Stereo | 40 | 4 | 160 |
| Toaster | 1200 | 1/10 | 120 |
| Vacuum Cleaner | 550 | 1/4 | 138 |
| Well pump 120V AC (100 gal/day) | 100 | 1/3 | 350 |
| Well pump DC (100 gal/day) | 100 | 1 | 100 |
| Window Air Conditioner | 660 | 6 | 4000 |
HOW MANY WATTS OF SOLAR PANELS MODULES ARE NEEDED?
On a fully sunny day, each solar module produces the equivalent of six hours of its maximum charging ability. Divide your total watt-hours needed each day by six. Theoretically, this calculation tells you how many rated watts of solar modules are needed to produce your day’s power from a day’s sunshine.
However we now add 50% more solar watts to allow for solar module derating (actual working watts is less than theoretical maximum rating) and for power loss in wiring, batteries and inverter. This gives the watts of solar you need to install if every day is fully sunny.
Since our location is not sunny every day, in the northwest corner of the U.S. we must add another 60% to 100% to the total in an attempt to make up for our cloudy, short winter days. The percentage to add for areas in the 48 states is shown on the U.S. map below. In the desert southwest showing 0%, nothing needs to be added.e now add 50% more solar watts to allow for solar module derating (actual working watts is less than theoretical maximum rating) and for power loss in wiring, batteries and inverter. This gives the watts of solar you need to install if every day is fully sunny.
This final number is the total rated solar watts you should install to meet your level of energy consumption on average in your climate. Because weather changes year to year, and because seasons vary more in some areas than in others, this estimate is a rough figure, but close enough to work. This process is explained in more detail, with work sheets, in books listed in back of our catalog. If you are sizing a system for a home with utility power, please reference our Grid-Tie sizing article.
If you’d like to improve your off the grid homes or start one, call us 208-263-4290 or contact us online to get started on building a custom off grid living.