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. Sq ft cue only: a typical ~1,500 sq ft home often lands near ~16 × 400 W panels (~750 kWh/mo) and ~2,000 sq ft near ~19 panels (~900 kWh/mo)—always prefer the bill, not floor area. Commercial default on the form: 40 kWh/day → 10 kW ≈ 25 panels. Next: solar production calculator. Hub: Solar calculators hub.
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.
Quick sizing examples (input → result)
Planning estimate only—assumptions shown. Re-run the calculator with your bill kWh and site peak sun hours.
- Home bill: 900 kWh/month → 30 kWh/day; PSH 5.0; η 0.80; 400 W modules → 7.5 kW ≈ 19 panels. Meaning: array sized to offset typical monthly use before shading/interconnection checks.
- Low-sun site: 25 kWh/day; PSH 3.5; η 0.80; 400 W → 8.9 kW ≈ 23 panels. Meaning: lower peak sun hours need a larger DC array for the same daily load—confirm PSH before inverter quotes.
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.
- 1,500 or 2,000 sq ft home? Prefer bill kWh. Rough cue: ~1,500 sq ft → ~16 × 400 W; ~2,000 sq ft → ~19 × 400 W (at 5.0 PSH / η 0.80)—re-run with your monthly kWh.
- 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 calculators 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)
Peak Sun Hours by US City (Annual Average)
Peak sun hours (PSH) = equivalent hours of full-sun irradiance (1,000 W/m²) per day. This is the single most important input for solar sizing. Values below are annual averages from NREL PVWatts — use monthly values for seasonal system design.
| City | State | Avg PSH (kWh/m²/day) | Summer PSH (Jun–Aug) | Winter PSH (Dec–Feb) | Solar Climate Note |
|---|---|---|---|---|---|
| Phoenix | AZ | 6.5 | 7.2 | 5.0 | Best in US; minimal seasonal variation |
| Los Angeles | CA | 5.8 | 6.8 | 4.2 | Excellent; coastal fog reduces morning output |
| Miami | FL | 5.2 | 5.8 | 4.0 | Good; afternoon thunderstorms reduce output |
| Houston | TX | 5.0 | 5.8 | 3.5 | Good; high humidity and cloud cover |
| Atlanta | GA | 4.8 | 5.5 | 3.2 | Moderate; seasonal variation |
| New York | NY | 4.2 | 5.2 | 2.5 | Moderate; large seasonal swing; snow cover issue |
| Chicago | IL | 4.0 | 5.0 | 2.2 | Moderate; cold winters reduce panel efficiency slightly |
| Seattle | WA | 3.5 | 5.0 | 1.5 | Lowest major US city; very cloudy winters; summer decent |
Source: NREL PVWatts v8, fixed tilt at latitude, 14% system losses. Actual values vary by roof orientation, tilt, shading, and local weather. For precise design, use PVWatts or SAM with your exact address and roof parameters.
Worked Solar Sizing Examples (3 Real Scenarios)
Example 1 — 5 kW Residential (Phoenix, AZ, 30 kWh/day)
Usage: 30 kWh/day (900 kWh/month) · Location: Phoenix (6.5 PSH) · Panel: 400 W each · System losses: 14%
Step 1 — Required array output: 30 kWh/day ÷ 6.5 PSH = 4.62 kW DC
Step 2 — Account for losses: 4.62 kW ÷ (1 − 0.14) = 4.62 ÷ 0.86 = 5.37 kW DC → round up to 6 kW
Step 3 — Panel count: 6,000 W ÷ 400 W/panel = 15 panels
Step 4 — Roof area: 15 × 17.6 ft² (400W panel ≈ 21.2 ft² including spacing) ≈ 320 ft² of south-facing roof
Expected production: 6 kW × 6.5 PSH × 0.86 × 30 days ≈ 1,006 kWh/month (exceeds 900 kWh usage by 12% — good for future EV charging or battery). Estimated cost: $15,000–$18,000 before incentives, ~$10,500–$12,600 after 30% federal ITC.
Example 2 — 20 kW Small Commercial (Los Angeles, 80 kWh/day)
Usage: 80 kWh/day (2,400 kWh/month) · Location: Los Angeles (5.8 PSH) · Panel: 450 W commercial · Losses: 12%
Step 1: 80 ÷ 5.8 = 13.8 kW
Step 2: 13.8 ÷ 0.88 = 15.7 kW → round up to 20 kW (standard commercial inverter size)
Step 3: 20,000 ÷ 450 = 45 panels (3 strings of 15)
Step 4: 45 × 24 ft² ≈ 1,080 ft² flat roof with ballasted racking
Expected production: 20 × 5.8 × 0.88 × 30 ≈ 3,066 kWh/month. Note: this is 28% over usage — commercial systems often oversize to account for future growth and demand charge management. Consider 15 kW (34 panels) if budget-constrained: 15 × 5.8 × 0.88 × 30 = 2,300 kWh/month (96% of usage).
Example 3 — 100 kW Industrial Rooftop (Houston, 400 kWh/day)
Usage: 400 kWh/day (12,000 kWh/month) · Location: Houston (5.0 PSH) · Panel: 500 W high-efficiency · Losses: 10% (optimized commercial system)
Step 1: 400 ÷ 5.0 = 80 kW
Step 2: 80 ÷ 0.90 = 88.9 kW → round up to 100 kW (standard commercial/industrial tier)
Step 3: 100,000 ÷ 500 = 200 panels (10 strings of 20, or 20 strings of 10 depending on inverter config)
Step 4: 200 × 27 ft² ≈ 5,400 ft² roof area (≈ 1,800 ft² per 33 kW)
Expected production: 100 × 5.0 × 0.90 × 30 ≈ 13,500 kWh/month (113% of usage). Inverter: 2× 50 kW or 4× 27.6 kW string inverters. ROI note: At $0.12/kWh industrial rate, savings ≈ $1,620/month = $19,440/year. System cost ~$200,000–$250,000 before incentives → simple payback ~10–13 years (faster with demand charge reduction and MACRS depreciation).
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 a 1,500 or 2,000 sq ft home?
Prefer the electric bill over square footage. Rough cue only (5.0 PSH, η 0.80, 400 W modules): ~1,500 sq ft homes often use near ~750 kWh/month → ~16 panels; ~2,000 sq ft near ~900 kWh/month → ~19 panels. Two homes the same size can differ 2× in kWh—enter your monthly kWh above (or use the monthly ladder table) instead of sizing from floor area alone.
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.
Are peak sun hours the same as hours of daylight?
No. Daylight can be 12+ clock hours while peak sun hours (PSH) compress irradiance into equivalent full-sun hours (often 3.5–6.5). Using clock daylight oversizes or undersizes badly—enter PSH from the peak sun hours calculator.
Can I size solar panels from house square footage alone?
Not reliably. Two 2,000 sq ft homes can differ 2× in kWh. Prefer monthly bill ÷ 30 for daily kWh, then run this calculator. Floor area is only a weak cue in the monthly ladder table.
What comes after panel count—inverter or battery?
Confirm array kW here, then yield (PV production) and strings/MPPT. Add battery bank sizing when you need autonomy days—panel count alone does not size storage.
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.