Solar Panel Calculator
From annual kilowatt-hours to a system: the PSH × 365 × derate law gives kW, then panels, roof, and honest production.
Solar Panel Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Annual-average model — no seasonal or hourly profile.
- No battery or hybrid sizing.
- No shade analysis; azimuth assumed workable.
In short: 12,000 kWh a year under 4.5 peak sun hours at 80% derate: 9.1324 kW → 23 panels of 400 W = 9.2 kW nameplate. That makes 33.12 kWh/day, 12,088.8 kWh/yr — a 100.7% offset — on about 494.5 ft² of roof. At $3.00/W: $27,600.00 before incentives.
Formula
kW = annual kWh ÷ (PSH × 365 × derate)
panels = ⌈kW × 1000 ÷ panel W⌉
daily = nameplate kW × PSH × derate
offset = daily × 365 ÷ annual kWh
[('PSH', 'peak sun hours — theavgable solar day'), ('0.80', 'end-to-end derate: inverter, soiling, heat, age'), ('21.5 ft²', 'one 400-W panel of roof'), ('130%', 'where oversizing stops paying')]
Worked Example
- Pull 12 months of kWh off the utility bill.
- Set local peak sun hours — 3.5 to 5.5.
- Keep the 80% derate unless you know better.
- Divide by panel wattage; round up to whole panels.
- Check the roof square and the honest offset.
12,000 kWh, 4.5 PSH, 80%: 9.1324 kW → 23 × 400 W = 9.2 kW → 33.12 kWh/day → 12,088.8 kWh/yr = 100.7% offset → 494.5 ft² → $27,600.00 at $3.00/W.
Strengths & Limits Of This Model
Where this engine is strong
- Production re-derived from nameplate, closing the loop
- The offset card shows over- and under-sizing honestly
- Refuses to invent incentive math
Where it stops
- No seasonal storage of surplus
- Roof figure ignores shading losses
Practical Use Cases
Bill-to-system sizing
The kWh number to kW.
Quote checks
Is 30 panels really 8 kW?
Roof budgets
Square feet before the sales visit.
Offset tuning
Sizing to 100%, not to salesmen.
Panel swaps
400 W vs 450 W count.
Methodology & Editorial Standards
kW = annual kWh ÷ (PSH × 365 × derate), with derate a free-entry term (50-100) defaulted to 0.80. Panels = ceil(kW×1000 ÷ panel W); nameplate kW then re-drives production: daily = kW × PSH × derate, annual = daily × 365, offset = annual ÷ usage. Roof = panels × 21.5 ft²; cost = W × $/W with no incentive model. Usage ≤ 0 or past 100,000 kWh, PSH outside 1-8, derate outside 50-100, panels outside 100-800 W refused.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Solar Panel Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How many solar panels does a 2,000 sq ft house need?
It is not the floor area, it is the bill: 12,000 kWh/year under 4.5 PSH at 80% derate wants a 9.13-kW system — 23 panels of 400 W. The same roof with a $400 winter bill doubles that.
What does 80% derate mean?
A panel's nameplate is a lab number; real systems lose to inverters, soiling, heat and aging. 80% end-to-end is the honest planning figure — production math that assumes 100% sells a fantasy.
How much roof do I need for solar?
About 21.5 ft² per modern 400-W panel — the canonical system here asks 494.5 ft², call it two squares. Shading and setbacks can double the ACTUAL requirement.
What is a peak sun hour?
One hour of 1,000 W/m² intensity — the unit solar insolation is sold in. A 4.5-PSH site receives 4.5 of them per average day; the Southwest runs 5.5-6, the Pacific Northwest nearer 3.5.
Should I oversize the system?
Sizing to ~100% of annual usage is the net-metering sweet spot; pushing to 130%+ rarely pays once interconnection caps and buyback rates bite — surplus you bank in June is not always credited in January.
How much does solar cost per watt?
Installed residential pricing clusters in the $2.50-3.50 per watt band — the canonical 9.2-kW system at $3.00 is $27,600 before incentives. This page refuses to model incentive percentages; they change too fast.
Do panels produce on cloudy days?
Yes, at a fraction of rating — roughly 10-25% under heavy cloud. The peak-sun-hour figure already averages weather across the year, so the production math here is honest without a sunshine multiplier.
Does the calculator include battery storage?
No — this sizes production against annual usage. Storage sizing starts from the loads you must carry through an outage, which is the Generator page's question.