Solar sizing is the process of calculating the correct number of solar panels, battery capacity, and inverter size for a given electrical load. Proper sizing ensures that the system meets energy needs year-round without excessive cost or waste. The two main sizing methods are the daily energy method and the peak sun hour method.
The first step is determining daily energy consumption in kilowatt-hours. Each appliance's wattage is multiplied by the hours of daily use. A typical off-grid homestead uses 5 to 20 kilowatt-hours per day, depending on the number of appliances and lifestyle. Lighting accounts for 10 to 20 percent, refrigeration for 20 to 30 percent, water pumping for 10 to 20 percent, and entertainment devices for 10 to 15 percent.
The required solar array size is calculated by dividing the daily energy requirement by the average peak sun hours for the location. Peak sun hours vary from 3 to 6 hours per day depending on latitude and season. A homestead using 10 kilowatt-hours per day in a location with 4 peak sun hours requires a 2.5 kilowatt array, assuming 100 percent system efficiency. Accounting for inverter losses, wire losses, and panel derating (total system efficiency of 70 to 80 percent), the actual array size is 3.1 to 3.6 kilowatts.
Battery capacity is expressed in amp-hours at a specific voltage. The required capacity is calculated by multiplying the daily energy consumption by the desired days of autonomy (typically 2 to 5 days for off-grid systems) and dividing by the battery voltage and depth of discharge. Lithium Batteries can be discharged to 80 percent depth, while lead-acid batteries are limited to 50 percent to preserve cycle life. A 48-volt battery bank with 10 kilowatt-hours of usable storage requires approximately 200 amp-hours at 48 volts.
In temperate climates, winter solar production may be 30 to 50 percent lower than summer production. Off-grid systems are sized for the worst month of the year, typically December in the northern hemisphere. A backup Generators is used to supplement solar during extended cloudy periods rather than oversizing the solar array for winter conditions.