Solar sizing: watt-hours first, then the sun, then the battery.

Step 1: the budget is watt-hours, not watts.

A device’s wattage tells you how fast it uses energy; how long it runs tells you how much. Multiply the two and sum the day. The battery load budget guide builds that worksheet line by line. In this example the sum is 2,400 watt-hours per day, which is a small household: for scale, the U.S. Energy Information Administration reports that the average residential customer bought about 899 kWh per month in 2022, roughly 30,000 Wh per day (EIA FAQ, page updated January 8, 2024; checked October 27, 2026). A tiny home that keeps heating and cooling off the electrical budget can live an order of magnitude below that. One that does not cannot, and that is the first thing to settle.

Step 2: size for the worst month, not the average.

Peak-sun-hours are a way of stating how much solar energy lands on a panel in a day, expressed as the equivalent number of hours at full-strength sun. The number depends on where you are and when. Use a site-specific lookup for the month you expect the least sun, such as the National Renewable Energy Laboratory’s PVWatts calculator, and design to that month. Here we use 4.0 hours for December. A system sized to the yearly average will be short exactly when the days are shortest.

Step 3: subtract the losses before you divide.

Not all the energy a panel makes reaches an outlet. Wiring, the charge controller, the inverter, panel temperature and dirt each take a share. A combined 25% loss is a common planning figure for small off-grid systems, so the usable fraction is 0.75. The array size is then the daily budget divided by sun hours and by the usable fraction: 2,400 ÷ (4.0 × 0.75) = 800 W. Two or three ordinary residential panels, depending on their rating.

Step 4: the battery is autonomy divided by depth of discharge.

Decide how many sunless days the home must ride through. Two days means the battery must deliver 2 × 2,400 = 4,800 Wh. A battery should not be run to empty: lithium-type batteries are commonly planned at 80% depth of discharge and lead-acid at about 50%. At 80%, the bank must hold 4,800 ÷ 0.80 = 6,000 Wh. On a 12.8 V nominal bank that is about 470 Ah; at 25.6 V, about 235 Ah. Chemistry, voltage and the inverter’s surge rating are then equipment choices, and only now are you ready to make them.

Take three questions into your next call.

1. Is my daily watt-hour figure measured or guessed, and does it include heating, cooling and water heating or exclude them on purpose?
2. Which month did we design for, and what sun-hours figure did we use for this site?
3. What loss factor and depth of discharge are in the quote, and what happens on the third cloudy day?

Equipment quotes are easy to compare once these four numbers are written down beside them. Without them, every quote is right about something different.