What Size Solar System Do I Need for 1,000 kWh per Month?

A 1,000 kWh bill is a little above the U.S. monthly average. The Phoenix year and the 4-hour generic estimate do not land on the same kilowatts.

Full breakdown: what size solar system.

Panel-count context for a whole house: how many solar panels.

Kilowatts, two ways

The 1,000 kWh rows use the same Phoenix run: 1,755 kWh per kW, lat 33.45, lon −112.07, fixed open rack, tilt 20°, azimuth 180°. 400 W is only the divisor for the panel column.

How to size it

A 1,000 kWh month is the worked example people expect from a bill, so the loss categories behind the 14% term are spelled out here and not repeated as a list on the other monthly pages. Start with the year, then the day.

1. Annual energy: 1,000 × 12 = 12,000 kWh.
2. Phoenix array: 12,000 ÷ 1,755 = 6.84 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 18.
3. Daily energy: 1,000 ÷ 30 = 33.33 kWh. The 30-day step compares the bill with a monthly average; EIA does not print the number 30.
4. 4 peak sun hours: (33.33 ÷ 4) ÷ (1 − 0.14) = 9.69 kW, which rounds up to 25 illustrative 400 W panels.
5. 5 peak sun hours: (33.33 ÷ 5) ÷ (1 − 0.14) = 7.75 kW, which rounds up to 20 panels.

EIA's average month is about 899 kWh. 1,000 ÷ 899 = 1.11, so this bill is about 1.11 times that average. The matching annual average is 10,791 kWh; 12,000 ÷ 10,791 = 1.11 as well. On the Phoenix yield those years are 12,000 ÷ 1,755 = 6.84 kW here, and 10,791 ÷ 1,755 = 6.15 kW (about 6.1 kW) on the average-home page.

The 14% in the generic column is PVWatts' default total system losses. The version 5 manual lists the default categories as soiling 2%, shading 3%, mismatch 2%, wiring 2%, connections 0.5%, light-induced degradation 1.5%, nameplate rating 1%, and availability 3%. The manual's point is that the 14% total is not a simple sum of those lines. This page uses the 14% total only, as (1 − 0.14).

The gap from 6.84 kW to 9.69 kW is the weather assumption, not a second bill. Phoenix's year yields 1,755 kWh per kW; the generic column forces the day through 4 peak sun hours and 14% losses. December in that Phoenix run is 4.75 against an annual 6.54. Your roof goes in PVWatts; your appliance list goes in the solar size calculator.

FAQ

How many kilowatts offset 1,000 kWh a month in Phoenix?

12,000 kWh a year ÷ 1,755 kWh per kW = 6.84 kW. On an illustrative 400 W panel that rounds up to 18.

What do 4 and 5 peak sun hours give?

1,000 ÷ 30 = 33.33 kWh/day. (33.33 ÷ 4) ÷ (1 − 0.14) = 9.69 kW. At 5 peak sun hours the same steps give 7.75 kW.

What is inside the 14% loss?

PVWatts version 5's default total system losses are 14%. The manual lists soiling 2%, shading 3%, mismatch 2%, wiring 2%, connections 0.5%, light-induced degradation 1.5%, nameplate rating 1%, and availability 3%, and it says that 14% is not a simple sum of those categories.

Does this replace a PVWatts run at my house?

No. The 1,755 kWh per kW figure is one Phoenix array at 14% losses. 4 to 5 peak sun hours is a baseline, not this roof's December. Run 12,000 kWh at the real address.

Related questions

• What size solar system for 500 kWh per month
• What size solar system for 1,500 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.