What Size Solar System Do I Need for 30 kWh per Day?

Thirty kilowatt-hours is the off-grid reading of an average U.S. electricity bill. The same energy, sized as a grid-tied year, is a much smaller array — the methods are not interchangeable.

Full breakdown: what size solar system.

A day this large is where fuel backup enters the design. See solar vs generator for off-grid. The yearly version of the average bill is on what size solar system for an average home.

Array watts at three sun figures

Thirty thousand watt-hours uses the same three sun figures as the smaller daily pages: 4, 5, and Phoenix December 4.75. Keep the annual 6.54 kWh/m²/day out of the winter division.

How to size it

First, the EIA bridge. Average electricity sold to a U.S. residential customer in 2022 was 10,791 kWh a year, about 899 kWh per month. 899 ÷ 30 = 29.97 kWh per day, which rounds to the 30 kWh this page divides. The rest is Battle Born's array step, not a second opinion about the bill.

1. 30,000 ÷ 4 = 7,500 W. Plus 20% = 9,000 W. Plus 30% = 9,750 W.
2. 30,000 ÷ 5 = 6,000 W, then 7,200 W and 7,800 W.
3. 30,000 ÷ 4.75 = 6,315.8 W, then 7,578.9 W and 8,210.5 W. Use December's 4.75, not the annual 6.54 kWh/m²/day, when the question is a winter day in that sunny climate.
4. Illustrative 400 W panels at the 30% / 4-hour result: 9,750 ÷ 400 = 24.375, which rounds up to 25 panels. The 5-hour 30% case rounds up to 20 panels. The December 30% case rounds up to 21 panels.

Hold this next to the average-home page before you buy either number. 10,791 ÷ 1,755 is about 6.1 kW when the job is a Phoenix year of grid-tied production. This page's 4-hour off-grid result is 7,500 W before margin — several kilowatts larger — because an off-grid day has to be met from that day's sun, and Battle Born then adds 20 to 30 percent. Same household energy, different denominator.

Battery amp-hours from the same watt-hours

A 30,000 Wh day at the example 12 V is 2,500 Ah, and two days is 5,000 Ah. That 12 V comes only from 2,400 Wh ÷ 12 V = 200 Ah. The battery bank sizer is the storage step.

Finish with your own usage in the solar size calculator and PVWatts. Raising system voltage changes amp-hours for the same watt-hours; it does not change the array watts in the table above.

FAQ

Why connect 30 kWh/day to 899 kWh/month?

EIA says the average home uses about 899 kWh per month and 10,791 kWh a year. 899 ÷ 30 days = 29.97 kWh per day, which is the 30 kWh this page sizes.

Why isn't this the same 6 kW as the average-home page?

The average-home page divides annual kilowatt-hours by Phoenix's 1,755 kWh per kW per year. This page divides one day's watt-hours by 4 to 5 peak sun hours and adds 20 to 30 percent. A grid-tied year and an off-grid day are different jobs.

What is the 4-hour array, in panels?

30,000 ÷ 4 = 7,500 W, or 9,000–9,750 W with margin. 9,750 ÷ 400 rounds up to 25 illustrative panels.

What is the 12 V battery for 30 kWh?

30,000 ÷ 12 = 2,500 Ah for one day and 5,000 Ah for two days. Twelve volts is the example voltage in that article. It is not a rule that a bank this large must be built at 12 V.

Related questions

• What size solar system for 10 kWh per day
• What size solar system for an average home
• What size solar system for 1,000 kWh per month

All of these questions are listed on the topic map.

Sources

• Battle Born Batteries, How to Size Solar Panels for a Deep Cycle Battery System — accessed Oct 2, 2026.
• Battle Born Batteries, How to Size a Deep Cycle Battery Bank — accessed Oct 2, 2026.
• NREL/NLR PVWatts Version 8 API documentation — accessed Oct 2, 2026.
• PVWatts calculator — accessed Oct 2, 2026.
• U.S. Energy Information Administration, How much electricity does an American home use? — accessed Oct 2, 2026.