Introduction
Solar sizing looks like a black box — you type in a monthly bill and a number of panels comes out. In reality it’s four short arithmetic steps, and understanding them lets you sanity-check any quote, adjust for your real roof, and spot when an installer’s proposal doesn’t add up.
The Solar Panel Calculator runs these exact steps: daily usage from your monthly kWh, system size from sun hours with a derating factor, panel count from panel wattage, and roof area from panel dimensions. This guide derives each step, works the full example in both metric and imperial, and cross-checks the results so you can trust the math.
The Four Formulas
1. Daily usage: Monthly kWh ÷ 30 = kWh per day. (Use 30.4 for precision; 30 is fine for planning.)
2. System size (kW): Daily kWh ÷ Peak Sun Hours ÷ 0.8. The 0.8 is a derating factor — a single multiplier that accounts for inverter efficiency (~96–98%), panel heat losses, dust and soiling, wiring losses, and age-related degradation. It means we assume the system delivers 80% of its lab-rated output in real conditions.
3. Panel count: (System kW × 1,000) ÷ Panel wattage, rounded up to a whole panel.
4. Roof area: Panel count × panel area. A typical residential panel covers ≈1.8 m² (≈19.4 ft²).
Worked Example: 900 kWh/Month, 5 Peak Sun Hours
Household usage: 900 kWh/month (≈ the US average). Location: 5 peak sun hours/day. Panels: 400 W each.
Step 1 — Daily usage. 900 ÷ 30 = 30 kWh/day.
Step 2 — System size. 30 ÷ 5 ÷ 0.8 = 6 ÷ 0.8 = 7.5 kW.
Step 3 — Panel count. 7,500 ÷ 400 = 18.75 → round up to 19 panels. (Installed capacity becomes 19 × 400 W = 7.6 kW.)
Step 4 — Roof area. 19 × 1.8 m² = 34.2 m² ≈ 34 m². Imperial: 19 × 19.4 ft² = 368.6 ft² ≈ 366–370 ft². Cross-check: 34.2 m² × 10.764 ft²/m² = 368 ft² — both unit systems agree.
Annual output check. A kilowatt of solar generates roughly 1,000–1,400 kWh/year depending on sun hours. At 5 sun hours, expect the upper-middle of that range: 7.5 kW × ~1,200 = ≈9,000 kWh/year — a reasonable offset against the home’s 10,800 kWh/year usage. The estimating guide shows how to pick a target offset when your roof or budget won’t cover 100%.
Why the Derating Factor Matters
Skipping the 0.8 derating is the most common DIY sizing error. Without it, the same example gives 30 ÷ 5 = 6.0 kW — a system that’s undersized by 20% and will underproduce every year of its life. The losses it captures are real:
- Temperature: panels lose roughly 0.3–0.4% of output per °C above 25 °C cell temperature — a roof in summer easily costs 8–12%.
- Inverter conversion: DC-to-AC conversion keeps 96–98%.
- Soiling and wiring: dust, pollen, and wire runs take another few percent.
- Mismatch and aging: panels in a string perform to the weakest, and output drifts down ~0.5%/year.
Together these justify the 20% haircut. Professional designs use more granular loss stacks, but 0.8 is a sound planning figure.
Sensitivity: How Sun Hours Change the Answer
Peak sun hours sit in the denominator, so small changes swing the result. Same 900 kWh/month, 400 W panels:
| Peak sun hours | System size | Panels | Roof area |
|---|---|---|---|
| 4.0 | 9.4 kW | 24 | 43 m² (465 ft²) |
| 4.5 | 8.3 kW | 21 | 38 m² (406 ft²) |
| 5.0 | 7.5 kW | 19 | 34 m² (366 ft²) |
| 5.5 | 6.8 kW | 17 | 31 m² (329 ft²) |
| 6.0 | 6.3 kW | 16 | 29 m² (310 ft²) |
Check one row: 4.0 sun hours → 30 ÷ 4 ÷ 0.8 = 9.375 kW → 9,375 ÷ 400 = 23.4 → 24 panels → 24 × 1.8 = 43.2 m² ≈ 43 m²; 24 × 19.4 = 465.6 ft² ≈ 465 ft². The spread from 4 to 6 sun hours is 24 vs 16 panels — a 50% difference for the same electricity bill. Location matters more than almost anything else in the calculation.
Panel Wattage: Same Math, Fewer Panels
Higher-wattage panels don’t change the system size — 7.5 kW is 7.5 kW — but they change the count and roof area. At 450 W: 7,500 ÷ 450 = 16.7 → 17 panels → 17 × 1.8 = 30.6 m² ≈ 31 m² (330 ft²). Two fewer panels and ~3 m² less roof for the same energy. That’s the trade-off the material guide weighs against price.
Rounding Rules
- Always round panel count up. 18.75 panels means 19 — rounding down leaves you short every sunny day.
- Round roof area up to the next whole m² or 10 ft², and remember it excludes walkways, fire setbacks, and racking clearances your installer adds.
- Round system size to one decimal (7.5 kW) for discussion; installers will land on a nearby whole-panel multiple anyway.
For what these numbers typically cost installed — and how to turn kWh into payback years — see the cost guide. If you want the full sizing walkthrough with your own bill in hand, start with the complete guide.
Related Guides
- Complete Guide — the full solar sizing overview with reference tables.
- Estimating Guide — reading your bill and measuring your roof.
- Cost Guide — per-watt pricing, savings, and payback math.
- Common Mistakes — the errors that distort these formulas.
- Pro Tips — planning advice for better sizing decisions.
- FAQ — quick answers to common solar sizing questions.
Get Your Number
Four steps — daily usage, system size with derating, panel count rounded up, roof area — and every number cross-checks between metric and imperial. Run your own figures through the Solar Panel Calculator, verify the arithmetic against the worked example above, and bring the result to a licensed installer for a site-specific design.
Disclaimer: Calculator results are planning estimates based on the inputs you enter. Actual system size, panel count, output, and cost depend on roof conditions, shading, orientation, equipment choices, and local rates. Verify sizing with a licensed solar installer or electrician — and check requirements with your local code authority — before signing contracts or starting work.
