Solar System Size Calculator: How Many Panels Do You Need? (Chart)

Every solar quote starts from the same three numbers: how much power you use, how much sun your roof gets, and what panel you bolt to it. This calculator-style guide turns your monthly kWh into a system size in kW and a panel count — with a chart you can read directly, the derating factors pros use, and worked examples including the 4,000 kWh/month question that lands in our inbox constantly.

The sizing formula

System size (kW) = Monthly kWh ÷ 30 ÷ Peak sun hours ÷ 0.78

The 0.78 is the standard system derate: real-world losses from inverter efficiency, wiring, soiling, temperature, and mismatch eat roughly 22% of nameplate output. Peak sun hours (PSH) is the daily average of full-sun equivalent — Phoenix sees ~6.5, Kansas City ~4.5, Seattle ~3.5.

Solar system size chart (monthly kWh → system size → panel count)

System kW shown at 4.5 peak sun hours (central US average); panel counts for 400W-class panels:

Monthly usage	Daily kWh	System size @ 4.0 PSH	@ 4.5 PSH	@ 5.5 PSH	400W panels @ 4.5 PSH
500 kWh	16.7	5.3 kW	4.8 kW	3.9 kW	12
750 kWh	25.0	8.0 kW	7.1 kW	5.8 kW	18
1,000 kWh	33.3	10.7 kW	9.5 kW	7.8 kW	24
1,500 kWh	50.0	16.0 kW	14.2 kW	11.7 kW	36
2,000 kWh	66.7	21.4 kW	19.0 kW	15.5 kW	48
3,000 kWh	100.0	32.1 kW	28.5 kW	23.3 kW	72
4,000 kWh	133.3	42.7 kW	38.0 kW	31.1 kW	95

Using 450W panels instead? Multiply panel count by 0.89. Using 380W? Multiply by 1.05. And if your utility doesn't offer 1:1 net metering, size for daytime consumption rather than total usage — exporting at avoided-cost rates changes the economics.

How many solar panels for 4,000 kWh per month?

This is the single most-searched version of the question, so here it is directly: 4,000 kWh per month is 133 kWh per day. At the US-average 4.5 peak sun hours with standard derating, that needs a 38 kW system — about 95 panels of 400W each, or roughly 84 panels at 450W. In a high-sun market (5.5 PSH) it drops to ~31 kW and 78 panels. That's a large residential or small commercial array — 1,600–1,700 square feet of roof — and it's squarely in the range where commercial-grade panels and three-phase string inverters start making sense. Users asking this usually have electric heat, a pool, an EV fleet, or a shop building; all solvable, but size honestly.

Worked example: 1,500 kWh/month home in the Midwest

Monthly usage 1,500 kWh (a typical all-electric 3,000 sq ft home): 50 kWh/day ÷ 4.5 PSH ÷ 0.78 = 14.2 kW system → 36 × 400W panels, or 32 × 450W. Roof space at ~17.5 sq ft per panel: ~630 sq ft of unshaded south/west roof. Pair it with a 10–15 kWh battery for evening coverage — see our battery backup runtime calculator for that half of the math.

Worked example: how many panels for a 3,000 sq ft home?

Square footage is a proxy, not a measurement — but the correlation holds: a 3,000 sq ft US home averages 1,200–1,600 kWh/month depending on climate and HVAC fuel. That lands on a 12–16 kW system, 30–40 panels of 400W. Pull your actual bill; two identical floor plans can differ by 2× if one has a pool pump and electric resistance heat.

Factors that move the chart

• Roof orientation and tilt: east/west arrays produce ~15% less than south at the same tilt; flat or shallow roofs need tilt racking or accept the loss.
• Shading: even partial shade on a string system is brutal; microinverters or optimizers recover most of it.
• Panel degradation: quality mono panels degrade ~0.4–0.5%/year; sizing 5% over today buys you margin for year 10.
• Future loads: adding an EV (+250–400 kWh/month) or heat pump later? Size the inverter and roof plan now.

What does that system cost in 2026 — and what's the payback?

Sizing is half the decision; the other half is dollars. Current US installed benchmarks: residential rooftop runs $2.50–$3.25 per watt before incentives for equipment-plus-install, and equipment-only (panels, inverter, racking — what a supply house sells you) is roughly $0.80–$1.30 per watt depending on panel class and inverter architecture. Applied to the chart:

• The 1,500 kWh/month example (14.2 kW): ~$35,000–$46,000 turnkey, or ~$12,000–$18,000 in equipment for a DIY/pro-install split.
• The 4,000 kWh/month example (38 kW): ~$95,000–$124,000 turnkey — at that scale, commercial pricing and three-phase design typically shave 15–25% off per-watt residential rates.
• The 30% federal Investment Tax Credit applies to both equipment and labor on residential systems — on the 14.2 kW turnkey example that's $10,000–$14,000 back.

Payback math with a $0.14/kWh utility rate: the 14.2 kW system produces ~18,700 kWh/year → ~$2,600/year offset → roughly 9–13 years simple payback before incentives, 6–9 years after the ITC, faster in high-rate states ($0.20+/kWh cuts it to 4–6 years). Panels are warranted 25 years; the back half of the system's life is nearly free power. That's why the sizing chart matters more than the price sheet: an oversized system dumps production at avoided-cost rates, and an undersized one leaves bill savings on the table. Size to usage, then let payback fall where it falls.

Frequently asked questions

How many solar panels do I need for 1,000 kWh per month?

At 4.5 peak sun hours: a 9.5 kW system — 24 panels of 400W. High-sun states: ~20 panels. Low-sun: ~27.

How big is a 25 kW solar system?

About 62–63 panels of 400W, roughly 1,100 sq ft of array, producing ~90–115 kWh/day depending on location — that's 2,700–3,400 kWh/month, enough for a large all-electric home or light commercial building.

How many batteries for a 4kW solar system?

Match storage to your overnight load, not array size: most 4kW-system homes use 400–700W overnight average, so one 10–14 kWh LiFePO4 battery covers evening-to-morning. Our battery bank sizing guide walks the math.

Can I install a bigger system than my usage?

Check your utility first — many cap residential systems at 100–120% of historical usage for net-metering eligibility.

How much roof do I need per kW?

Roughly 44–50 sq ft per kW with modern 400–450W residential panels (about 17.5 sq ft per panel plus setbacks and walkways).

What's the difference between kW and kWh when sizing?

kW is the system's nameplate power — the chart's output. kWh is the energy it produces over time — what offsets your bill. A 10 kW system in a 4.5-sun-hour region produces roughly 14,000 kWh per year after derating.

Should I oversize the inverter for future panels?

Within limits, yes — string inverters commonly run 1.2–1.35 DC-to-AC ratios with minimal clipping loss. Planning a 20% array expansion into the inverter choice now is cheap; replacing an undersized inverter later is not.

Panels in stock, priced to move

Portlandia Electric Supply stocks residential and commercial modules ready to ship: the REC Alpha Pure 400W black mono, the Qcells Q.PEAK DUO 400W, and the value-leader Canadian Solar HiKu6 395W. Browse all Solar Panels, Monocrystalline Panels, or skip the design work with a pre-engineered Solar Panel Power System Kit. Send us your kWh from your last bill — we'll size the system and quote it the same day.