How Solar Panel Calculators Work
A transparent look at the methodology, data sources, and accuracy of online solar estimators
The Core Calculation
All solar panel calculators — including this one — are ultimately solving the same equation. The physics is straightforward; the accuracy comes from the quality of the input data.
// Step 1: Convert bill to monthly usage
monthlyKwh = monthlyBill ÷ electricityRate
// Step 2: Annual consumption
annualKwh = monthlyKwh × 12
// Step 3: System size
systemKw = annualKwh ÷ (sunHours × 365 × orientation × shade × 0.80)
// Step 4: Panel count (round up)
panelCount = ⌈(systemKw × 1000) ÷ panelWattage⌉
// Step 5: Actual annual production
annualKwhProduced = panelCount × panelWattage ÷ 1000 × sunHours × 365 × 0.80
// Step 6: Financial outputs
annualSavings = annualKwhProduced × electricityRate
paybackYears = (systemKw × 1000 × costPerWatt) ÷ annualSavings
Where the Data Comes From
☀️ Peak Sun Hours — NREL PVWatts
The National Renewable Energy Laboratory (NREL) maintains a solar radiation database derived from satellite imagery and ground stations across the US. Their PVWatts tool calculates annual average peak sun hours for any US location. Our calculator uses NREL's state-level annual averages, which have a typical accuracy of ±5% compared to ground-measured values.
Source: NREL National Solar Radiation Database (NSRDB) — pvwatts.nrel.gov
⚡ Electricity Rates — EIA
The U.S. Energy Information Administration (EIA) publishes average retail electricity rates by state for the residential sector monthly. Our calculator uses the 2024 annual average residential rates. Individual utilities within a state vary — your actual rate may be 10–30% above or below the state average. You can override the rate in our calculator if you know your actual $/kWh from your utility bill.
Source: EIA Form EIA-861, 2024 Residential Electricity Rates by State
💰 Install Cost — Lawrence Berkeley National Lab
Lawrence Berkeley National Laboratory's "Tracking the Sun" dataset tracks actual installed solar system costs across hundreds of thousands of US installations. Our calculator uses LBNL state-level average cost-per-watt ($/W) figures for residential systems, which range from about $2.55/W in low-cost states to $3.10/W in high-cost states. Installer quotes you receive may differ by ±20–30% based on your specific roof, equipment tier, and local competition.
Source: LBNL Tracking the Sun 2024, emp.lbl.gov/tracking-the-sun
The 0.80 Derate Factor — Unpacked
Solar panels are rated at Standard Test Conditions (STC): 25°C (77°F) cell temperature and 1,000 W/m² irradiance. Real-world conditions rarely match STC. Here's how the losses compound:
| Loss Factor | Typical Range | Our Assumption |
|---|---|---|
| Inverter efficiency | 95–98% | 96% |
| DC wiring losses | 1–3% | 2% |
| AC wiring losses | 0.5–2% | 1% |
| Temperature derating | 3–10% | 5% |
| Soiling / dust | 1–5% | 2% |
| Shading (none selected) | 0–15% | 0% |
| Panel mismatch | 1–3% | 2% |
| Combined system efficiency | 75–88% | ~80% |
This aligns with the default PVWatts "DC to AC derate factor" of 0.86 × ~0.96 inverter = 0.825, rounded to 0.80 conservatively. Using a more generous 0.85 would reduce calculated panel counts by ~6%.
What Makes a Reliable Solar Calculator
State-specific sun hours, not national averages
National average sun hours mask wide state variation. A calculator using 4.5 hrs/day for everyone will over-estimate production in Ohio and under-estimate in Arizona.
State-specific electricity rates
Massachusetts pays $0.29/kWh; Idaho pays $0.11/kWh. Using a national average rate would make Massachusetts solar look less attractive and Idaho solar look better than reality.
Orientation and shading adjustments
A south-facing roof in Texas is very different from an east-facing roof in the same house. Good calculators adjust production estimates for both.
Transparent assumptions
The calculator should show you the rate used, sun hours used, and key assumptions — so you can verify or override them. Opaque calculators are harder to trust.
Clear scope of estimate
The calculator should clearly state what's included in the payback estimate (does it include the 30% ITC? degradation over time? escalating utility rates?) — ours doesn't include the ITC, which means your actual payback is shorter than shown.
Try the Calculator
See all assumptions shown transparently — rate used, sun hours, and more
Limitations of All Solar Calculators
Be aware of what no online calculator can account for:
Your specific roof geometry
A complex multi-pitch roof with multiple orientations requires a professional shade analysis. Calculators use a single orientation input.
Local shading from trees, chimneys, and neighbors
The shade options (none/light/moderate/heavy) are approximations. Nearby obstructions that shade panels at specific times of day can significantly reduce production.
Your utility's specific tariff structure
Time-of-use rates, demand charges, minimum bills, and interconnection fees all affect real-world savings in ways a simple calculator can't capture.
Panel degradation over time
Solar panels lose ~0.5% output per year on average. A 25-year savings estimate should account for this — most simple calculators use a fixed annual production figure.
Future electricity rate changes
If utility rates increase 3–5%/year (as historical trends suggest), solar savings grow over time — most simple calculators use a fixed current rate.
Frequently Asked Questions
How accurate are online solar panel calculators?▾
What is the PVWatts methodology used in solar calculators?▾
What are 'peak sun hours' in solar calculations?▾
What is the 0.80 derate factor in solar calculations?▾
Why do different solar calculators give different estimates?▾
Disclaimer: This article and calculator are for educational and planning purposes only. Solar system performance varies with site conditions, equipment, and installation quality. This is not financial, engineering, or tax advice. Data sources: NREL PVWatts, EIA 2024, LBNL Tracking the Sun 2024.