How I find my total energy usage or the "loads" of my appliances?

Step 1:   Calculate loads of household appliances (in watts):

a. You can find a listing of common appliance wattage ratings. 

b. You can also use a meter, which gives readings of how much
instantaneous power and energy an appliance is using.

Step 2: Calculate total energy usage (in watt hours per day)

a. Use the load information you just assembled with our load calculator (calculator on left side of this text) to find out how many watt-hours you use per day. 

2.  The label on my appliance does not list Watts, it only lists Amps.  How do I find the wattage?

Converting from Amps to Watts is a simple calculation: Watts =  Amps x Volts

           Our household voltage is 120 Volts AC.  If we have a television that lists 1.5 Amps, we calculate the wattage as follows:

           1.5 Amps x 120 Volts = 180 Watts

3.  My Heater lists kilowatt hours per year.  What wattage do I enter in my load calculator? 

Convert kilowatt hours per year to watt hours per day by the following calculation:

Kwh x 1000 = watt hours per year
Watt hours per year / 365 days per year = watt hours per day

4.  How do I know what size Photovoltaic system I need for my energy usage?

To get a better idea of system size, take the data from your load calculator and use it in one of our system calculators, either for on-grid or off-grid applications.

5. How do I figure out the likely energy production of a given system which lists rated Watts of PV?

Step 1: Look at the solar insolation map below to find you average sun hours per day. 

Step 2: Multiply the rated Watts of your system by the amount of sun hours per day to get the average Watt-hour production number. 

Step 3:  Decrease the calculated amount by about 20% to take into account system inefficiencies. This will provide a more realistic view of the system production. (The 20% reduction is to be conservative.  That way your production should not fall short of your usage.)

Here is an example: 

Calculate the predicted energy production of a 1 kW PV system in Montreal, Qc, Canada.

1. The solar insolation for the Montreal area appears to be about 3 hours.
2. 3 sun hours x 1000w = 3000 watt-hours
3. Decrease that number by 20% (or multiply by 0.8):  3000 x 0.8 = 2400 watt-hours per day = 2.4 kilowatt-hours per day.

Note:  It is important to know that the sun hours for your area change based on time of year.  Average daily sun hours provide an estimate to calculate the approximate size of a system.  If you are completely off the grid (not connected to utility power), and rely on PV to produce all of your electricity, you will want to have more accurate calculations.  To be more accurate, use data close to your area that is broken down by month, compares the angle of PV modules, and includes information on whether or not you have an active tracker for your array. 

Use this chart to help you find the insolation in your area.