How Much Power Do You Use?
To Calculate How Much 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 | Hours/Day | Watt Hours/Day |
| Microwave Oven- Average | 1260 | 1/4 | 315 |
| Microwave Oven- with timer knob | 900 | 1/4 | 225 |
| Food Processor or Blender | 200 | 1/20 | 10 |
| Toaster | 1200 | 1/10 | 120 |
| Clothes Washer- standard | 700 | 3/4 | 525 |
| Clothes Washer | 200 | 3/4 | 150 |
| Vacuum Cleaner | 1000 | 1/4 | 250 |
| Electric Blanket | 180 | 4 | 720 |
| DC Power Bed-Warmer | 60 | 4 | 240 |
| Refrigerator/Freezer- standard | 1500 | ||
| Small Apartment Fridge 4 cu.ft. | 945 | ||
| 12/24V RV NovaKool 4 cu.ft. | 300 | ||
| 10 cu. ft. Freezer- standard | 1000 | ||
| Window Air Conditioner- small | 660 | 6 | 4000 |
| Ceiling Fan AC | 60 | 6 | 360 |
| Ceiling Fan DC | 5-20 | 6 | 30-120 |
| Water Well Pump- 120V AC, 100 GPD | 1000 | 1/3 | 350 |
| Water Well Pump- DC, 100 GPD | 100 | 1 | 100 |
| Standard 60W Light Bulb | 60 | 4 | 240 |
| CFL Light Bulb- Eq to 60W | 15 | 4 | 60 |
| Computer | 100 | 4 | 400 |
| Laser Jet Printer- operating | 90 | 1/4 | 23 |
| 19" Color TV | 85 | 3 | 255 |
| 32" LCD TV | 140 | 3 | 420 |
| Satellite Receiver | 20 | 3 | 60 |
| Quality Stereo | 40 | 4 | 160 |
How Many Watts of Solar 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.
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.
