Solar Panel Calculator: how many panels do I need?
Instant how many solar panels calculator: enter daily kWh (or convert from a monthly bill) and peak sun hours → required array kW and approximate panel count. Works for homes and commercial roofs—planning estimate, not a shopping kit.
Quick answer
How many solar panels do I need? First size the array: Array kW ≈ Daily kWh ÷ (Peak sun hours × System efficiency). Home bill example: 900 kWh/month ≈ 30 kWh/day, 5.0 PSH, 0.80 derate → 7.5 kW ≈ 19 × 400 W panels. Commercial default on the form: 40 kWh/day → 10 kW ≈ 25 panels. Next: solar production calculator. Hub: Solar calculator.
Quick Solar Panel Sizing Calculator
Defaults: 40 kWh/day · 5.0 peak sun hours · 0.80 system efficiency.
40 kWh/day ÷ (5.0 × 0.80) = 10.0 kW ≈ 25 × 400 W panels.
Advanced Solar Panel Sizing Calculator
Site presets
Solar Panel Sizing Results
Engineering disclaimer
Planning estimate only. Confirm irradiance, shading, and utility interconnection with site data and OEM software before procurement.
Results
10.0 kW array · ~25 panels @ 400 W — default example.
Array kW by peak sun hours
Same daily kWh and derate; highlights how PSH changes array size.
| Peak sun hours | Array kW | Approx. modules |
|---|
People also ask
- How many solar panels do I need? Array kW ÷ (module kW) after sizing—e.g. 7.5 kW ÷ 0.4 kW ≈ 19 × 400 W from a 900 kWh/month bill.
- How many for 25 kWh/day? At 5.0 PSH × 0.80 → 6.25 kW ≈ 16 × 400 W panels.
- How many watts of solar panels do I need? Array watts = array kW × 1000; module count ≈ array watts ÷ module watts.
- What is the 20% rule? Optional headroom buffer—prefer explicit η derate over a blind +20%.
- What is the 120 rule? AC interconnection / busbar constraint—not this DC array formula.
Planning guidance
This page sizes DC array nameplate kW for homes and commercial roofs—not a bid-ready design. Prefer your bill kWh over floor-area rules of thumb.
- Bill first: Monthly kWh ÷ 30 → daily kWh, then run the calculator (presets cover 500 / 900 / 1,500 kWh/mo).
- Derate honestly: Soiling and high temperature often pull efficiency below 0.80—see losses table below.
- Load first (off-grid / plant): off-grid load or factory load.
- Yield next: Convert array kW to annual kWh in the PV energy production calculator.
- BOS: Strings in series-parallel, then MPPT sizing.
- Path: Full workflow on the Solar calculator hub.
Last updated: 2026-08-13. Planning estimate only—confirm irradiance, shading, and interconnection before procurement.
Typical scenarios
- Home (bill-based): 500–1,500 kWh/month → convert to daily kWh; PSH 4.0–5.5 typical US mid-latitudes.
- Warehouse roof: 60–120 kWh/day process + lighting offset—PSH 4.5–5.5 typical mid-latitudes.
- Office campus: 15–40 kWh/day daytime-heavy load—pair with production estimate for bill offset.
- Low-sun site: PSH 3.0–3.8 drives larger arrays—confirm before inverter/MPPT quotes.
- Off-grid cabin: Start from daily kWh in the load calculator, then size array for worst-month PSH.
How to size solar panels: step-by-step
- Daily kWh load — Roll up energy in off-grid load or from meters. Example: 40 kWh/day.
- Peak sun hours — Location PSH from peak sun hours or irradiance data. Example: 5.0 h.
- System efficiency — Combined derate η (often 0.75–0.85). Example: 0.80.
- Array kW — Daily kWh ÷ (PSH × η) = 40 ÷ (5 × 0.80) = 10.0 kW.
- Module count — (Array kW × 1000) ÷ module W. At 400 W modules → 25 panels for the 10 kW example.
- Area check — Rough roof/ground area from module footprint × count (+ aisles).
- String layout — Configure Voc/Isc in series-parallel, then MPPT sizing.
Long-form method: how to size a commercial solar array.
Daily kWh → how many panels (400 W ladder)
Assumptions: 5.0 peak sun hours, η = 0.80, 400 W modules. Re-run the calculator if your PSH, derate, or module watts differ.
| Daily kWh | Array kW | ~Panels (400 W) |
|---|---|---|
| 10 | 2.5 | 7 |
| 20 | 5.0 | 13 |
| 30 | 7.5 | 19 |
| 40 | 10.0 | 25 |
| 60 | 15.0 | 38 |
| 100 | 25.0 | 63 |
Monthly bill kWh → how many panels
Convert bill energy with daily kWh ≈ monthly kWh ÷ 30, then the same formula. Assumptions: 5.0 PSH, η = 0.80, 400 W modules. Floor area (sq ft) is a weak proxy—prefer the bill.
| Monthly kWh | Daily kWh | Array kW | ~Panels (400 W) | Rough home cue |
|---|---|---|---|---|
| 500 | 16.7 | 4.2 | 11 | ~1,200 sq ft efficient |
| 750 | 25.0 | 6.3 | 16 | ~1,500 sq ft typical |
| 900 | 30.0 | 7.5 | 19 | ~2,000 sq ft average use |
| 1,000 | 33.3 | 8.3 | 21 | Larger / higher AC use |
| 1,500 | 50.0 | 12.5 | 32 | ~3,000 sq ft high use |
| 4,000 | 133.3 | 33.3 | 84 | Very high / multi-dwelling |
Solar panel sizing by scenario
Illustrative screens at η = 0.80 and 400 W modules—re-run with your PSH and derate above.
| Scenario | Daily kWh | PSH | Array kW | ~Panels (400 W) |
|---|---|---|---|---|
| Home ~900 kWh/month | 30 | 5.0 | 7.5 | 19 |
| Home ~1,500 kWh/month | 50 | 5.0 | 12.5 | 32 |
| Small office (grid-tied offset) | 30 | 5.0 | 7.5 | 19 |
| Off-grid cabin | 5 | 4.0 | 1.6 | 4 |
| RV / van | 3 | 5.0 | 0.75 | 2 |
| Warehouse / small commercial | 100 | 5.0 | 25.0 | 63 |
| Telecom hut | 5 | 5.0 | 1.25 | 4 |
Common mistakes
- Using daylight clock hours instead of peak sun hours.
- Ignoring system losses (inverter, wire, soiling, temperature)—forcing η = 1.0.
- Sizing only for summer load when winter is the binding month.
- Forgetting surge watts when selecting inverters (energy ≠ peak power).
- Mixing dissimilar modules in one series string without mismatch analysis.
System losses explained (summary)
Combined η ≈ 0.75–0.85 is common for planning. Detail and deeper factors: solar system losses & derating.
| Loss source | Typical % |
|---|---|
| Inverter / conversion | 5–10% |
| DC/AC wiring | 2–3% |
| Soiling (dust/snow) | 2–7% |
| Temperature (above STC) | 5–15% |
| Mismatch / light shading | 2–10% |
| Combined planning band | ~15–25% (η ≈ 0.75–0.85) |
Formula (quick reference)
Array kW = Daily kWh ÷ (Peak sun hours × System efficiency)
How many panels ≈ (Array kW × 1000) ÷ Module watts — or equivalently Daily kWh ÷ (Module kW × PSH × η), then ceil.
Method walkthrough: how to size a commercial solar array. For derate η, read solar system losses & derating.
Formula and sources
Array kW = Daily kWh ÷ (PSH × η_system). Confirm with site irradiance and OEM design software before procurement.
- NREL PVWatts — production / irradiance context
- NFPA / NEC (NFPA 70) — Article 690 PV wiring and listing context
- Solar system losses & derating (CalcPanel)
Frequently Asked Questions
How many solar panels do I need?
Size the array first: Array kW = Daily kWh ÷ (PSH × η), then panels ≈ (Array kW × 1000) ÷ module W. From a monthly bill, use daily ≈ monthly ÷ 30. Example: 900 kWh/month → 30 kWh/day, 5.0 PSH, η 0.80 → 7.5 kW ≈ 19 × 400 W panels. Use the monthly ladder table above for other bill screens.
How many solar panels do I need for my house?
Prefer the electric bill over square footage. Typical US homes often land near 750–1,000 kWh/month; at 5.0 PSH and η 0.80 that is roughly 16–21 × 400 W panels. A 2,000 sq ft label alone is too coarse—two homes the same size can differ 2× in kWh.
How many solar panels do I need for 25 kWh per day?
At 5.0 PSH and η 0.80: 25 ÷ (5 × 0.80) = 6.25 kW ≈ 16 × 400 W panels. At 3.5 PSH the same load needs ≈ 8.9 kW ≈ 23 panels.
How to figure out how many solar panels I need?
Convert the bill to daily kWh (monthly ÷ 30), enter peak sun hours and η, then read array kW and panel count above. Example: 900 kWh/month → 30 kWh/day → about 19 × 400 W at 5.0 PSH / 0.80. Commercial method write-up: how many panels commercial formula.
How to calculate how many solar panels you need?
panels ≈ Daily kWh ÷ (PSH × η) × 1000 ÷ module W. Same as Array kW × 1000 ÷ watts. Use Quick Calculator defaults or Home/Warehouse presets.
How do I calculate solar panel sizing?
Divide daily kWh by peak sun hours times system efficiency. Example: 40 ÷ (5 × 0.8) = 10 kW. Then module count ≈ (10 × 1000) ÷ module watts.
How do I calculate solar panel array size?
Same formula: Array kW = Daily kWh ÷ (PSH × η). Use the Advanced calculator above for derate, monthly bill convert, and optional module wattage.
Is this a solar system size calculator?
Yes for PV array kW (photovoltaic). Enter daily or monthly kWh and peak sun hours to size the DC array—not astronomy “solar system” pages, and not consumer panel+inverter shopping kits.
How many watts of solar panels do I need?
First get array kW, then array watts = array kW × 1000. Example: 10 kW → 10,000 W; at 400 W modules ≈ 25 panels. This page absorbs the “panel wattage calculator” intent.
What is the 20% rule for solar panels?
Some consumer guides add ~20% capacity headroom for soiling, aging, or load growth. Prefer an explicit system efficiency η (often 0.75–0.85) in this calculator; add a contingency only when your forecast justifies it—not as a blind multiplier on every job.
What is the 33% rule in solar panels?
Some consumer guides suggest oversized arrays vs inverter (~33%). That is a rule of thumb—not a substitute for site PSH, derate, and OEM voltage/current windows. Size array kW here, then verify inverter/MPPT limits separately.
What is the 120 rule for solar panels?
Interconnection discussions sometimes reference a 120% busbar/breaker rule under certain NEC editions. That constrains AC interconnection hardware, not this DC array kW formula. Follow the AHJ and applicable NEC edition on your project.
Can I mix 100W and 200W solar panels?
Mixing dissimilar modules in one series string causes mismatch (current limited by the weakest). Prefer identical modules; if unavoidable, parallel separate strings and check Voc/Isc in the series-parallel calculator.
What peak sun hours should I use?
Use location peak sun hours from irradiance maps or site data—not clock hours of daylight. Start with peak sun hours.
How is this different from the energy estimator?
This sizes PV array kW from solar resource and derate. The energy estimator focuses on facility kWh cost, not PV array sizing.
What is the next step after array kW?
Estimate annual yield in the PV energy production calculator, configure strings, then size MPPT controllers.
How it works
Daily AC energy demand is divided by peak sun hours and a combined system efficiency (inverter, wiring, soiling, temperature). The result is DC array nameplate kW for planning—not a bid-ready design.
Optional module wattage converts array kW into an approximate module count for BOM screening.
How many watts of solar panels do I need?
Array watts (W) = Array kW × 1000. Example: 40 kWh/day at 5.0 PSH and η=0.80 → 10.0 kW → 10,000 W of nameplate. At 400 W modules → about 25 panels. Use this calculator when search intent is “solar panel wattage” or “how many watts do I need”—same formula as array sizing.
Worked examples
- Home 900 kWh/month (≈30 kWh/day) at 5.0 PSH, η=0.80
30 ÷ (5 × 0.80) = 7.5 kW. At 400 W modules ≈ 19 panels.
- 40 kWh/day at 5.0 PSH, η=0.80
40 ÷ (5 × 0.80) = 10.0 kW. At 400 W modules ≈ 25 panels.
- 80 kWh/day warehouse, 4.5 PSH
80 ÷ (4.5 × 0.80) ≈ 22.2 kW array ≈ 56 × 400 W panels.