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. I've sized hundreds of systems at the Portlandia Electric Supply counter — from a 4kW cabin array to a 40kW shop roof — and the math has never once been the hard part. The hard part is getting people to read their actual utility bill instead of guessing. This calculator-style guide turns your monthly kWh into a system size in kW and a panel count, with charts 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.

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

Run it once by hand so the charts make sense: 1,200 kWh/month ÷ 30 = 40 kWh/day; ÷ 4.5 PSH = 8.9; ÷ 0.78 = 11.4 kW. That's the whole calculation — everything below is just that division done at scale and formatted for reading.

The 0.78 is the standard system derate: real-world losses from inverter efficiency, wiring, soiling, temperature, and module 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. One number from your bill (monthly kWh), one number from the sun table below, and you're two divisions from an answer that's within 10% of any professional proposal we've ever audited.

PSH is the single biggest variable in the formula, and it swings the answer by nearly 2:1 across the country. Annual averages for a fixed, south-facing, tilt-equal-to-latitude array (NREL long-term data):

Read it this way: a kW of solar in Phoenix produces 86% more energy per year than the same kW in Seattle. Panels cost roughly the same in both places — which is why payback periods differ so wildly between states even before incentives. East/west roofs cost you about 15% off these numbers; heavy shading costs more, and that's where microinverters earn their premium over a single string inverter.

System kW shown at three sun levels; panel counts for 400W-class panels at 4.5 PSH:

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 completely. For specific usage scenarios, our guides on panels for 1,500 kWh per month and panels for 10,000 kWh per month go deeper on those two brackets.

Panel wattage moved fast — 380W is now the budget tier and 450W+ is standard residential. Here's the same system in different panel classes:

Panel counts round up, which is why a "10kW system" quotes as 10.0kW (25 × 400W) from one installer and 10.35kW (23 × 450W) from another — both are honest. Higher-wattage panels matter most when roof space is the constraint, and on premium panels like the REC Alpha Pure line you're also buying a lower degradation rate that pays out in year 15, not year 1.

Buyers think in kW; bills come in kWh. Convert once and for all — annual kWh = system kW × PSH × 365 × 0.78:

Check the chart against itself: 38kW at the US average produces ~4,056 kWh/month — which is exactly the usage it was sized for. The math closes. If your numbers don't close like that, one of your three inputs is wrong, and it's usually the PSH guess — people remember the sunny weekend in May, not the gray fortnight in November.

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. If you're pairing that much solar with storage, our guide on battery counts for big systems handles the other half of the design.

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.

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. I've sized both houses on the same street more than once.

Modern 400–450W residential panels measure about 17–18 sq ft each, and code setbacks plus walkways push the installed figure to roughly 44–50 sq ft per kW:

A typical 3,000 sq ft ranch has 1,500–1,800 sq ft of roof total, of which maybe 60% faces a usable direction — so a 14kW system fits comfortably and a 25kW system usually doesn't. Ground mounts and carports exist for exactly this constraint, and they're worth pricing before you shrink the system: a ground mount adds racking cost but gives you perfect tilt, perfect orientation, and no roof-penetration warranty arguments with your roofer. Shade is the other silent roof-shrinker — the south roof that's clear at noon in June can sit behind a bare-branched oak's shadow line from October through March, and tree growth over the system's 25-year life is the projection nobody includes in year-one math. For a sense of scale at the big end, our piece on how big a 25kW array actually is walks the footprint, and how big a 1000W panel is answers the question we get from people picturing one giant module (it's panels, plural — it always is).

• 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's need buys you margin for year 10 without paying for capacity you'll never use.
• Future loads: adding an EV (+250–400 kWh/month for 1,000 miles of driving at 3–4 mi/kWh) or a heat pump later? Size the inverter and roof plan now — the string inverter sizing guide covers the DC-to-AC ratio decisions.

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 ~18,200 kWh/year → ~$2,550/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. Running a smaller system against big loads? What a 3kW system actually runs calibrates expectations, and the off-grid battery sizing guide covers systems where the grid isn't part of the picture at all.

Everything in this guide keys off one number, and it's amazing how many people can't find theirs. On most US residential bills, look for "kWh used" or "energy usage" in the summary box — not the dollar total, not the "delivery charges," the kilowatt-hours. Better: use twelve months, not one. A July bill in Texas or a January bill in Minnesota can run double the shoulder months, and sizing to your worst month oversizes the system for ten months of the year while sizing to your best leaves you short. Most utility web portals show a 12-month usage graph on the account page; a few will even export the interval data. Add the twelve numbers, divide by 12, and that monthly average is the input for the chart above. If you heat with electricity, weight toward the winter months — resistance heat and heat-pump auxiliary strips are the loads that make "average" advice dangerous. And if the bill shows demand charges (kW, not kWh) — common on commercial accounts — you're in a different pricing world where solar shaves consumption but batteries shave demand; call us, because the sizing logic changes.

The chart above assumes a grid connection — the utility is your battery, absorbing surplus at noon and backfilling at midnight. Off-grid flips the problem: you're not sizing to annual energy, you're sizing to the worst stretch of weather you refuse to run a generator through. The off-grid version of the formula uses your daily load and the worst-month PSH (December, not the annual average — 2.0–2.5 hours across much of the northern US), then multiplies array size by 1.3–1.5 for cloudy-week margin. A cabin using 10 kWh/day that's perfectly happy with a 3.5kW grid-tied array needs 8–12kW off-grid in the same location, plus 20–30 kWh of battery for 2–3 days of autonomy. That's not a typo — off-grid costs 2–3× the array for the same lifestyle, which is why we walk every off-grid caller through load reduction first: propane for heat, cooking, and water heating can cut the electrical load 40% and the system cost right along with it. The storage math for that design lives in the battery bank sizing guide.

Annual averages hide the swing. A fixed-tilt array in the northern half of the country produces roughly 60% more energy in June than in December — longer days, higher sun angle, and (in cold climates) snow-free panels versus snow-covered ones. For grid-tied sizing this barely matters; the annual numbers net out. For anything with batteries it matters enormously, because December production and January consumption are the binding constraints: your battery bank drains deepest exactly when the array produces least. Practical consequence: a "correctly sized" grid-tied system can still leave a battery empty for a week of December storms, and that's not a defect — it's physics. The fix is either generator backup for the deep-winter gap, a 20–30% larger array than the annual math suggests, or honest load-shedding rules for the worst weeks. We recommend the generator for nearly everyone north of the 40th parallel; the customers who skip it call us in January. Southern buyers get the milder version of the same lesson — summer monsoon stretches and hurricane weeks test the bank the same way, just with better sun on either side of the storm.

Once the array is sized, the inverter question arrives immediately, and the answer is not "match the kW." String inverters are routinely undersized relative to the array on purpose — a DC/AC ratio of 1.2–1.35 is standard practice, meaning a 10kW array on a 7.6kW inverter. The array only touches nameplate output for a few cool, clear hours a year, so the "lost" production from clipping runs 1–3% annually while the smaller inverter saves real money and runs at a more efficient operating point the other 350 days. Push past 1.4 and the clipping math starts eating the savings. Microinverter systems make the ratio decision per-panel instead, which is why they shrug off shade and orientation mixing at the cost of a higher price per watt. Off-grid and hybrid systems invert the logic entirely — there, the inverter must cover your surge loads (well pumps and compressors pull 3–5× running watts at startup), so the array-to-inverter ratio matters less than the surge rating. Our hybrid inverter guide covers that architecture.

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. The battery bank sizing guide linked above walks the full 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 12,800 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.

Send us your kWh from your last bill — we'll size the system and quote it within one business day. And once you know the panel count, the MC4 and PV wire guide specs the wiring side of the same array.