List the quantity of each load (i.e., number of light bulbs: 46, refrigerator: 1,
stove:1, freezer: 1, computer: 2, clothes washer: 1, hot water heater: 1,
televisions: 3, radios: 2, stereo: 1, power saws: 2).
List the watts as AC or DC. You will need to evaluate which items require
AC or DC, which items can draw from batteries, and which items will
require an inverter.
List how many hours per day the device will be used. When calculating
hours used, use the time of year the device will be used the most hours per
day. For example, artificial lighting will be used more hours in the winter
than in the summer, when there is more natural light available.
List how many days per week the device will be used. Clothes washers might
be used every day or just a few days per week; power saws may be used just a
few days total per year.
Divide the use days per week by 7 days.
This equals the watt-hours by AC/DC used by a particular device on a
daily basis.
In the case of an existing grid-connected structure, you can use the past year’s
monthly electric bills to verify the average daily load used. Average the monthly
watt usage over a minimum of one and preferably several years. Most electric bills
use kilowatt-hours; to convert them to watts, use the following formula:
Number of kilowatts × 1,000 = watts
Sizing Standalone PV Systems
A standalone PV system is not connected to the grid. The system supplies all the
required energy. Standalone systems store excess solar energy in a battery bank.
This energy is available when insolation is low—for example, on cloudy days or
at night.
You should size PV battery systems according to the three lowest months of
insolation and the highest load demand. The months of lowest insolation are typ-
ically November, December, and January. Keep batteries from becoming depleted
during long periods of low insolation with a gas- or diesel-powered generator.
Consider the following six factors when sizing a standalone PV system: