How Many Solar Panels for 1,000 kWh a Month?

1. Turn the monthly bill into a daily target

Solar math works per day, because sun hours are quoted per day. Use the 12-month average from your utility's usage history, not one bill.

• Daily energy: 1,000 kWh ÷ 30.42 days = 32.87 kWh/day

The 30.42 is simply 365 ÷ 12, the length of an average month. A home at 1,000 kWh a month sits somewhat above the national average reported by the EIA, which is typical for a house with central air conditioning, an electric dryer or a pool pump.

2. What "peak sun hours" means

One peak sun hour is one hour of sunlight at 1,000 W per square meter, the intensity at which panels are rated. A location with 4.5 peak sun hours receives the same daily energy as 4.5 hours of that full-strength sun, even though the sun is up for twelve.

The values in this guide are rounded planning numbers drawn from the national solar resource data that NREL publishes (NSRDB). They are averages for a tilted, south-facing array over a whole year. For your own address, run PVWatts: it uses the same data and gives month-by-month production.

Why size on the yearly average and not on the darkest month? Because a grid-tied system under net metering banks the summer surplus and draws it back in winter. Sizing for December alone would roughly double the array in the northern states and leave it exporting most of its summer output. Off-grid cabins are the exception: there, the worst month sets the size, and the battery and generator plan matters as much as the panel count.

3. System losses

A real array delivers less than its nameplate. Heat, wiring, inverter conversion, dust, snow and panel mismatch all take a share. PVWatts uses 14% as its default, so the system runs at 0.86 of its rated output.

That number assumes a clean, unshaded roof. If a chimney or a tree shades part of the array in the afternoon, plan for 20% or more (see section 7).

4. The calculation

System size (kW) = daily kWh ÷ (sun hours × (1 − losses))

32.87 ÷ (4.5 × 0.86) = 8.49 kW

Panels = kW × 1000 ÷ panel watts, always rounded up:

8,494 W ÷ 400 W = 21.24 → 22 panels

Rounding up gives 8.8 kW installed. Estimated yearly output is 8.8 kW × 4.5 h × 365 × 0.86 = 12,430 kWh, against 12,000 kWh of yearly use. The small surplus covers the slow output loss every panel goes through over the years.

5. The same bill, five US regions

This is where most online answers go wrong: they give one panel count for the whole country. Sun hours vary by almost a factor of two between Arizona and western Washington, and the array size follows.

Two caveats. First, regional averages hide big local differences: coastal fog, mountain elevation and lake-effect cloud all move the number. Second, the low-sun regions also have the strongest seasonal swing. In the Pacific Northwest, December output can be a small fraction of July output, so the array leans heavily on net metering credits from summer.

Not using exactly 1,000 kWh? The chart below gives the system size for any monthly average between 400 and 1,600 kWh. The relationship is linear: at 4.5 sun hours, each extra 100 kWh a month adds about 0.85 kW.

6. Panel wattage changes the count, not the kW

Area is nearly identical in each row because it is driven by module efficiency, here about 20%, or 200 W per square meter. Larger panels mean fewer pieces, fewer roof attachments and less labor, but they are heavier and harder to fit around vents and hips.

7. Shade, orientation and losses

• East or west roof: expect noticeably less yearly output than a south-facing roof at the same tilt. PVWatts lets you enter the real azimuth; do that before adding panels to compensate.
• Partial shade: with 20% losses instead of 14%, the case needs 9.13 kW, or 23 panels. Microinverters or optimizers limit the damage from one shaded module.
• Flat roofs: racking at a modest tilt helps rain clean the glass and usually raises output compared with flat-laid panels.

8. Inverter and interconnection

A common design ratio puts the panel kW at about 1.1 to 1.3 times the inverter's AC rating, because panels rarely hit full nameplate. At 1.2, the 8.8 kW array pairs with roughly 7.3 kW of inverter capacity; with microinverters, check that each unit accepts a 400 W panel.

Grid-tied systems are built under NEC Article 690 and connected under the utility's interconnection rules. Expect a building permit, an electrical inspection and a utility approval before the system can be switched on. Rapid shutdown requirements for rooftop arrays apply in jurisdictions on recent code editions.

9. Reading the panel label and the proposal

The back-sheet label and datasheet tell you what you are buying:

• Pmax (W): the rated power at standard test conditions, the 400 in this case.
• Voc and Isc: open-circuit voltage and short-circuit current, used to size strings and protection.
• Temperature coefficient of Pmax: how much power drops per degree Celsius of heat, usually a fraction of a percent. Lower is better in hot climates.
• Proposal production estimate: it should be in kWh per year, not only in kW. Compare it with 12,430 kWh for the 8.8 kW system at 4.5 sun hours. If a proposal promises much more, ask for its assumptions.

10. Installation checklist

• [ ] Twelve months of usage history, not one bill
• [ ] PVWatts run for the actual address, tilt and azimuth
• [ ] Shade assessment of every roof plane used
• [ ] Roof condition: re-roofing before installation is far easier than after
• [ ] Main panel capacity checked for the solar backfeed breaker or a supply-side connection
• [ ] Permit, inspection and utility interconnection approval in writing
• [ ] Monitoring set up so you can compare real output with the estimate

11. Common mistakes

• Sizing from a summer bill. A July bill with heavy air conditioning oversizes the array; a spring bill undersizes it.
• Using one national panel count. The 22 panels of a Midwest roof become 16 in Arizona and 28 near Seattle.
• Ignoring future loads. An EV or a heat pump can add hundreds of kWh a month. Plan inverter and roof space now.
• Aiming for exactly zero. Fixed utility charges stay on the bill no matter what. Covering 90 to 100% of use is the practical target.

FAQ

How much does one 400 W panel produce? At 4.5 sun hours and 14% losses, about 47 kWh a month.

What if I only want to cover 80% of the bill? The system drops to 6.80 kW, or 17 panels.

How many panels in Arizona? At 6.0 sun hours the case needs 6.37 kW, which is 16 panels of 400 W.

We are buying an EV that will drive 30 miles a day. What changes? At 3.5 mi/kWh plus 10% charging losses, that is about 287 kWh more a month. The system grows to 10.93 kW: 28 panels.

Do I need batteries? Not to offset the bill under net metering. Batteries add backup power and help where export credits are low, but they do not change the panel count above.

Case summary

Different bill, different region or a shaded roof? Enter your own numbers in the solar panel calculator; it runs exactly this formula.