What Size Solar System Do I Need for 1,500 kWh per Month?
Fifteen hundred kilowatt-hours a month is above both the U.S. average month and Louisiana's average month. The array has to be sized to that bill, not to the national average.
Full breakdown: what size solar system.
Panel-count context for a whole house: how many solar panels.
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Kilowatts, two ways
| Method | Array | 400 W panels, rounded up | Source |
|---|---|---|---|
| Phoenix year: annual kWh ÷ 1,755 | 10.26 kW | 26 | PVWatts v8 |
| 4 peak sun hours, then ÷ (1 − 0.14) | 14.53 kW | 37 | Battle Born hours; PVWatts v5 losses |
| 5 peak sun hours, then ÷ (1 − 0.14) | 11.63 kW | 30 | Battle Born hours; PVWatts v5 losses |
Counts in the last column divide by an illustrative 400 W and round up. The yield behind the 1,500 kWh Phoenix cell is 1,755 kWh/kW/year for that fixed array (tilt 20°, azimuth 180°, 14% losses) at lat 33.45, lon −112.07.
How to size it
At this bill the useful comparison is Louisiana, not the U.S. average. Do the Phoenix division first so the kilowatts are tied to 1,755, then run the generic day so a cloudier baseline is on the same page.
- Annual energy: 1,500 × 12 = 18,000 kWh.
- Phoenix array: 18,000 ÷ 1,755 = 10.26 kW. That 1,755 is AC kilowatt-hours per kilowatt of DC for the Phoenix inputs in the PVWatts v8 run. Unrounded, an illustrative 400 W panel count rounds up to 26.
- Daily energy: 1,500 ÷ 30 = 50.00 kWh. The 30-day step compares the bill with a monthly average; EIA does not print the number 30.
- 4 peak sun hours: (50.00 ÷ 4) ÷ (1 − 0.14) = 14.53 kW, which rounds up to 37 illustrative 400 W panels.
- 5 peak sun hours: (50.00 ÷ 5) ÷ (1 − 0.14) = 11.63 kW, which rounds up to 30 panels.
Louisiana is the high state in the EIA FAQ at 14,774 kWh a year. 14,774 ÷ 12 = 1,231 kWh per month. 1,500 − 1,231 = 269, so this bill is 269 kWh above Louisiana's average month, or 1,500 ÷ 1,231 = 1.22 times that month. Against the U.S. month of about 899 kWh, 1,500 ÷ 899 = 1.67. Sizing to the national average would miss this bill on purpose.
10.26 kW is the sunny-year answer and 14.53 kW is the 4-hour answer with 14% losses. They are allowed to disagree: one uses Phoenix's full year, including December at 4.75 and an annual plane-of-array of 6.54, and the other uses Battle Born's 4 peak sun hours. Enter the house in PVWatts or the solar size calculator before ordering panels.
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FAQ
How many kilowatts offset 1,500 kWh a month in Phoenix?
18,000 kWh a year ÷ 1,755 kWh per kW = 10.26 kW. On an illustrative 400 W panel that rounds up to 26.
What do 4 and 5 peak sun hours give?
1,500 ÷ 30 = 50.00 kWh/day. (50.00 ÷ 4) ÷ (1 − 0.14) = 14.53 kW. At 5 peak sun hours the same steps give 11.63 kW.
How many illustrative panels is the generic high end?
The unrounded 4-hour result rounds up to 37 panels of 400 W. The 5-hour result rounds up to 30. The 400 W figure only turns watts into a whole-module count.
Does this replace a PVWatts run at my house?
No. 14.53 kW assumes 4 peak sun hours, and 10.26 kW assumes Phoenix. A 1,500 kWh bill elsewhere is a new PVWatts case.
Related questions
- What size solar system for 1,000 kWh per month
- What size solar system for 2,000 kWh per month
- What size solar system for an average home
All of these questions are listed on the topic map.
Sources
- NREL/NLR PVWatts Version 8 API documentation — accessed Oct 2, 2026.
- PVWatts calculator — accessed Oct 2, 2026.
- NREL PVWatts Version 5 Manual, A. Dobos, September 4, 2014 — accessed Oct 2, 2026.
- Battle Born Batteries, How to Size Solar Panels for a Deep Cycle Battery System — accessed Oct 2, 2026.
- U.S. Energy Information Administration, How much electricity does an American home use? — accessed Oct 2, 2026.