How to estimate backup generator needs for an off-grid home?

Calculating Energy Requirements

To estimate backup generator needs, you need to calculate the total energy required from your battery bank during the longest night of the year. In the Northern Hemisphere, this typically occurs on the winter solstice (December 21-22). Use the average daily energy usage (kWh) and the number of hours of solar insolation during the summer months to determine the total energy requirements.

For example, if your off-grid home uses 20 kWh/day and you have 6 hours of solar insolation in the summer, you can calculate the total energy requirements: 20 kWh/day x 365 days/year = 7,300 kWh/year. Divide this by the number of days in the longest night period (December 21-22): 7,300 kWh/year / 14 days = 521.4 kWh.

Accounting for Inefficiencies

However, this calculation doesn’t account for inefficiencies in your system, such as battery losses and generator efficiency. You should add 10-20% to the calculated energy requirements to account for these losses. In this example, 521.4 kWh x 1.15 (10% loss) = 599.9 kWh or 521.4 kWh x 1.20 (20% loss) = 624.7 kWh.

Determining Generator Run Time

Next, you need to determine how often the generator must run to recharge the battery bank. This depends on the size and type of your generator, as well as the capacity of your battery bank. A general rule of thumb is to size the generator to recharge the battery bank in 8-12 hours. Using the smaller calculated energy requirement (599.9 kWh), you would need a generator that can produce 50-60 kW to recharge the battery bank in 8 hours (599.9 kWh / 8 hours = 75 kWh/hour).