Solar Panel Output & Savings Calculator
Estimate solar output and payback: roof area or panel count → system kW, peak sun hours for your state or city, itemized losses, dated tax-credit math.
Your system
Your sun
Your money
| year | kWh made | bill savings | cumulative net |
|---|
What this calculator does
Give it your panel count — or your usable roof area — and it sizes the array in kilowatts, applies the sun your region actually gets, subtracts honest system losses, and prices the result: daily and annual kilowatt-hours, first-year bill savings, net installed cost, simple and escalated payback, and a 25-year cumulative savings chart you can download.
Three choices separate this from most solar widgets on the web. The losses compound. Inverter, heat, soiling, wiring and shade don’t add to a round number — each applies to what’s left, which is why the industry-standard stack lands near 14%, not the 15% you’d get by adding. The incentives are dated. The 30% federal residential credit ended for installs completed after December 31, 2025 — the calculator defaults to no credit and labels the 30% option for systems that still qualify, instead of quietly baking in a credit that no longer exists for most readers. The presets are honest ranges. Peak sun hours here are NREL PVWatts-style annual averages per state and per world city — midpoints you can override, not a number invented per site.
The model, in the open
daily kWh = system kW × peak sun hours × orientation factor × performance factor · annual = daily × 365
- System size — panel count × watts, or usable roof area ÷ panel footprint. The footprint scales with wattage: a 430 W module at ~21% efficiency covers ~2.05 m² (22 sq ft); coverage % carves out setbacks, vents and shade.
- Peak sun hours — annual-average equivalent full-sun hours per day. Presets cover all 50 US states plus DC (~2.9–6.5 PSH) and ~30 world cities (London ≈2.9 to Dubai ≈6.1). Ranges are approximations from NREL-style irradiance data; a rooftop-specific estimate needs your exact address.
- Losses — default a single ~14% (the NREL PVWatts convention), or itemize inverter (~3%), temperature (~5%), soiling (~2%), wiring (~2%), shade/mismatch (~3%) and watch them compound multiplicatively to the same neighborhood.
- Savings — production valued at offset% × retail rate + surplus × export credit. Net-metered systems run offset near 100%; California-style avoided-cost exports run much lower — the slider makes the tariff visible.
- Payback — net cost (gross minus whatever credit still applies) against year-by-year savings: production degrades 0.5%/yr by default, your rate escalates 2.5%/yr. Both the simple division and the escalating model print, because the honest answer is a range.
- Deliberately not modeled — maintenance and insurance, inverter replacement (~year 12), financing interest, roof work, SRECs and state rebates, time-of-use bill shapes. Each can move payback years in either direction; the ROI by state guide covers the state-level spread.
Worked example — 18 panels in California
18 × 430 W = 7.74 kW; California ≈5.9 PSH; ideal tilt; 14% losses → ~39.3 kWh/day, ~14,335 kWh in year one. At $0.17/kWh fully offset: ~$2,437 saved year one. At $2.80/W with no federal credit (post-2025 law): $21,672 net → simple payback ≈ 8.9 years, ≈ 8.3 modeled with escalation — inside the ~25-year panel warranty window either way.
Go deeper
- Peak sun hours explained — what PSH really measures and why it isn’t daylight.
- Federal solar tax credit: 2026 status — what ended December 31, 2025, what remains, dated.
- System sizing math — from your bill to panel count to kWh, formulas included.
- How shade cuts output — the nonlinear string math and what mitigates it.
- Solar ROI by state — why payback doesn’t follow the sun map.
- Lease vs buy — who captures which incentive after OBBBA.
Frequently asked questions
How many solar panels do I need?
Start from your usage, not your roof. A typical US home uses ~10,500 kWh/year; a 430 W panel in a ~5 PSH climate makes roughly 650–700 kWh/year after losses — so most homes land at 15–22 panels (6.5–9.5 kW). The sizing guide walks the math backward from your bill, or switch the calculator to roof-area mode to see what physically fits.
What are peak sun hours, and where do the preset numbers come from?
PSH is the number of hours per day your location averages at full 1,000 W/m² sunlight — an annual average, so a long June day and a short December one are already folded in. US states run roughly 3–6.5 PSH (NREL PVWatts-style ranges); the presets are honest midpoints of those ranges, not site surveys. The peak sun hours guide shows the ranges and how to look up your exact address.
Is the 30% federal solar tax credit still available?
Not for new installations. The §25D Residential Clean Energy Credit — the 30% homeowner credit — ended for expenditures made after December 31, 2025 under the One Big Beautiful Bill Act (P.L. 119-21, signed July 4, 2025). An install completed in 2026 earns $0 federally; one completed by Dec 31, 2025 still claims 30% on the 2025 return (Form 5695), with unlimited carryforward of the unused part. The dated detail is in the tax credit status page.
Why do the losses multiply instead of adding up?
Because each loss eats what the previous ones left. A 5% temperature loss removes 5% of the energy that survived the inverter loss, not 5% of the nameplate — so 3%+5%+2%+2%+3% compounds to 14.1% total, not 15%. Turn on itemize losses above and watch the combined figure; the sizing article shows the multiplication.
What does 'retail offset' mean — and when is it not 100%?
The share of your production that displaces electricity you would have bought at the full retail rate. Under classic 1:1 net metering it's ~100%. Where exports earn a lower avoided-cost rate (California's NEM 3.0 is the big example, or anywhere without net metering), the surplus value drops — set offset to the share you actually consume or net-meter at retail, and give the rest your export rate.
How accurate is the payback estimate?
As accurate as the inputs you give it, and honest about the rest. The model escalates your electricity rate and degrades panel output year by year over 25 years — it does not include maintenance, insurance, inverter replacement (~year 10–15, $1,500–3,000), financing interest, or state incentives (SRECs, state credits, rebates — often worth thousands). Treat the year it prints as a planning number ± a couple of years, not a contract.
Do the numbers I type get sent anywhere?
No. The estimator is one JavaScript file running in this tab — no server does the math, nothing is logged or stored, and the page works offline after it loads. The privacy policy has the details.
Latest articles
Solar Lease vs Buy in 2026: Who Gets What Now
Buying used to win because you kept the 30% credit. That credit ended for 2026 installs — so the lease-vs-buy math changed. Here's the new comparison, honestly run.
Peak Sun Hours, Explained
PSH is the single number that converts your system's kW into daily kWh. What it measures, what it isn't, and the real ranges by region.
How Shade Actually Cuts Solar Output
Shade loss is nonlinear — a chimney shadow on one panel can tax the whole string. The wiring math, the fixes, and when shade kills the deal.
Solar Payback by State: Where the ROI Actually Is
Arizona's sun doesn't guarantee the best payback and Washington's clouds don't guarantee the worst. The math is sun × rate × tariff — worked state by state.
The Federal Solar Tax Credit in 2026: What Ended, What Remains
The 30% homeowner credit ended December 31, 2025 — a dated, honest map of what's left for solar buyers now: carryforwards, lease/PPA pass-throughs, state money.
Solar System Sizing: From Your Bill to Panel Count
Work backward from your bill to a kW target, forward from your roof to a panel count, and check the two agree. All the formulas, honestly stated.