header image

Estimating Home Energy Usage

How Much Energy Do You Use?
When trying to determine the size of your system, you’ll need to understand your electrical consumption. You can’t just add up the watts for everything because seldom are they all on the same time. What you do need to know is how much energy do you need every day for typical usage.

wattage plate markingsYou can usually find the wattage of most appliances stamped on the bottom or back of the appliance, or on its nameplate. The wattage listed is the maximum power drawn by the appliance. Many appliances have a range of settings (for example, the volume on a radio), the actual amount of power consumed depends on the setting used at any one time.

Using the information provided on this page, you will be able to estimate your energy usage and figure out the number of batteries and the number of PV panels you'll need.

Appliance Typical Wattage
Clock radio 10
VCR/DVD 17–21
Laptop 50
Aquarium 50–1,210
Window Fan 55–250
Ceiling Fan 65–175
Radio (stereo 70–400
Computer CPU 120
Monitor 150
Water pump (deep well) 250–1,100
Clothes washer 350–500
Refrigerator 16 cu ft 725
Furnace Fan 750
Heater (portable) 750–1,500
Microwave oven 750–1,100
Dehumidifier 785
Toaster 800–1,400
Coffee maker 900–1,200
Clothes iron 1,000–1,800
Vacuum cleaner 1,000–1,440
Dishwasher 1,200–2,400
Hair dryer 1,200–1,875
Toaster oven 1,225
Clothes dryer 1,800–5,000
Water heater (40 gal) 4,500–5,500
Flat screen TV 133
Projection TV 170
Formula for Estimating Energy Consumption
You can use this formula to estimate an appliance's energy use:

(Wattage × Hours Used Per Day) ÷ 1,000 = Daily Kilowatt-hours (kwh)
where 1 kilowatt (kw) = 1,000 Watts

Note: To estimate the number of hours that a refrigerator actually operates at its maximum wattage, divide the total time the refrigerator is plugged in by three. Refrigerators, although turned "on" all the time, actually cycle on and off as needed to maintain interior temperatures. One third of the time is a usable ballpark estimate of daily load.

Household Usage and System Sizing Example:
Flat Screen TV: 120 Watts × 4 hours/day = 480 watt hours daily
Personal Computer and Monitor: 120 Watts x 2 hours/day = 240 watt hours daily
5 - 60 watt light bulbs: (5 x 60 = 300) 300 Watts x 4 hours/day = 1,200 watt hours daily
Deep well pump: 1,000 Watts x 30 Min/day = 500 watt hours daily
Total = 2,420 watt hours a day

In this example we will be using a Trojan T-105 6 Volt battery having a total capacity of 1,100 watts.  We do NOT want to run the battery down more that 25% to get the longest life time from the battery, so that gives us 275 watt hours from one battery.

battery bank connections2,420 watts used daily ÷ 275 = 8.8. So you would need a battery bank of 8.8 batteries to maintain your usage without a re-charge.  Because most inverter systems are 24 Volts, your battery bank should be made up of multiples of four 6 volt batteries which equals 24 volts and 275 watt hours. Three strings of four batteries are needed to meet the requirement in this example.

pv panelsIf you want to recoup your usage from solar PV modules using a typical 185 watt solar module you would need:
185 x 5(average # of sun hours) = 925 watts hours available
2,420 ÷ 925 = 2.6 modules
Round up to three panels.

house appliances