Going solar is a five-figure decision that pays off over decades, and the single most important question is simple: how long until the system pays for itself? This solar panel calculator estimates the annual electricity your rooftop array will produce, the dollar savings against your utility bill, the net cost after tax credits and rebates, and the payback period — then projects the total return over the typical 25-year panel warranty.
Slide the inputs and every figure updates instantly. Tune the system size to match a quote on your desk, raise or lower the electricity rate to stress-test a future rate hike, and watch the payback period move.
How to Use
- Enter the System Size in kilowatts. A typical US rooftop install is 5–8 kW; a 400-watt panel counted 20 times gives an 8 kW array. This is the first and most important number on the page.
- Set your Peak Sun Hours. This is not the number of daylight hours — it is the equivalent hours per day the sun shines at full 1,000 W/m² intensity. Most of the continental US lands between 4 and 6; NREL's PVWatts tool will give you a precise figure for your ZIP code.
- Type your Electricity Rate from a recent utility bill, in dollars per kWh. The US average hovers around $0.15, but California, Hawaii, and New England can run $0.25–$0.40.
- Enter the installer's Gross System Cost — the sticker price before incentives. As of 2025, installed US residential solar runs roughly $2.50–$3.50 per watt, so a 6 kW system lands near $15,000–$21,000.
- Add your Tax Credits & Rebates. The federal Investment Tax Credit (ITC) currently knocks 30% off the net cost for most households; state and utility rebates may add more.
- Read the result card: annual production in kWh, annual bill savings, the net cost after incentives, the estimated payback period, and the 25-year savings and ROI.
The engine uses the standard NREL PVWatts derate model: annual production equals system kilowatts times peak sun hours times 365, times an 0.85 derate factor that accounts for inverter loss, panel soiling, temperature, wiring, and DC-to-AC conversion.
The Solar Production Formula
Annual energy output follows a simple, physics-based formula that PVWatts and nearly every US installer uses:
Annual kWh = System kW × Peak Sun Hours × 365 × 0.85
Where:
- System kW is the rated DC capacity of the array.
- Peak Sun Hours is the location-specific average daily sunlight at standard test conditions.
- 365 converts daily production into annual production.
- 0.85 is the combined system derate factor — NREL's default, covering inverter efficiency (~96%), panel mismatch, soiling, temperature loss, wiring resistance, and DC-to-AC conversion.
Worked example on a 5 kW system in a 5-sun-hours climate: 5 × 5 × 365 × 0.85 = 7,756 kWh per year. At a $0.15/kWh electricity rate that is about $1,163 in annual bill savings. After a $12,000 gross price minus $3,600 of incentives (net cost $8,400), the payback period is $8,400 ÷ $1,163 ≈ 7.2 years. Beyond that, the panels keep producing for another 18+ years of the 25-year warranty, converting savings into pure profit.
If annual savings come out to zero (for example, a 0 kW system), the payback figure is reported as "n/a" rather than divided by zero.
What Moves the Numbers Most
Solar economics are sensitive to a few variables more than others:
- Electricity rate — every cent per kWh counts directly. At $0.30/kWh, the same system saves twice as much as at $0.15, slicing the payback period nearly in half.
- Sun hours — Arizona (6.5 hours) produces far more per kW than Seattle (3.5 hours). This is why identical panel counts pay back at very different speeds across states.
- Incentives — the 30% federal ITC is non-refundable, so you owe enough federal tax to actually claim it. Homeowners with low tax liability should confirm eligibility before signing.
- Installed cost per watt — shopping multiple quotes typically saves $2,000–$5,000 on a residential install, which directly shortens payback.
- Net metering policy — if your utility credits excess exports at the full retail rate, the calculator's savings projection is realistic. If exports are paid at the much lower wholesale rate, actual savings drop.
Aim for a payback period under 8–10 years in a high-rate, high-sun state; 10–13 years is normal in moderate markets; beyond 15 years usually signals a system that is overpriced or undersized for the roof.
Examples
Example 1 — Typical suburban install. A 5 kW array, 5 peak sun hours, $0.15/kWh rate, $12,000 gross cost, $3,600 ITC. Annual production is 7,756 kWh, annual savings $1,163, net cost $8,400, payback 7.2 years. Over 25 years the system saves about $29,086 against the net cost — a 25-year ROI of roughly 246%.
Example 2 — High-rate, high-sun home. A 6.6 kW system in southern California at 5.5 sun hours and a $0.18/kWh blended rate, quoted at $18,000 with $4,500 of incentives. Annual production is about 11,262 kWh, annual savings $2,027, net cost $13,500, payback 6.7 years. Over 25 years the homeowner nets roughly $37,179 above the investment — a 25-year ROI near 275%.
Example 3 — Large commercial or barn array. A 100 kW system on a rural barn at 5 sun hours, $0.15/kWh, $2,000,000 installed cost with no incentives. Annual production is 155,125 kWh, annual savings $23,269, but the payback stretches to 86 years — a clear signal that at this installed price per watt the project does not pencil out, and the quote should be renegotiated.
FAQ
How much will solar panels save me per year?
Multiply your system size in kW by your local peak sun hours, then by 365 and 0.85 to get annual kWh, and multiply that by your electricity rate. On a typical 5 kW US system at $0.15/kWh, expect about $1,100–$1,200 in annual bill savings. Use the calculator above for your exact numbers.
What is a good solar payback period?
Across the United States, a typical residential payback period is 6–12 years, with the best markets (California, Texas, Arizona, Florida) reaching 5–8 years. Anything under 10 years is strong; 10–15 is acceptable; above 15 usually means you are overpaying for the system or your local electricity rate is too low to justify it.
How is peak sun hours different from daylight hours?
Daylight hours count every minute the sun is up, from dawn to dusk — roughly 12 hours a day on average. Peak sun hours compress all of that into equivalent hours at full 1,000 W/m² noon-intensity sunlight. A location with 14 hours of dawn-to-dusk daylight may still only offer 5.5 peak sun hours. NREL's PVWatts map lists the exact value for your ZIP code.
What does the 0.85 derate factor mean?
Real-world panels never deliver their nameplate output. The 0.85 factor (used by NREL's PVWatts) bundles inverter efficiency, panel mismatch, wiring losses, dust and snow, temperature-related power loss, and DC-to-AC conversion into a single haircut. You can think of it as "expect about 85% of the math on a perfect day."
Does the federal 30% solar tax credit apply to me?
The federal Investment Tax Credit (ITC) currently allows residential homeowners to deduct 30% of the total system cost (equipment plus installation) from their federal income tax. It is non-refundable, so you must owe enough federal tax in the year of installation and future years (with carry-forward) to claim it. Always confirm with a tax professional, as rules change by year.
Should I buy solar panels in cash or finance them?
Cash purchases deliver the shortest payback period and the highest lifetime ROI, because you pay no interest. A solar loan shifts the up-front cost to $0 down but adds interest, extending payback by 2–5 years and reducing 25-year profit by several thousand dollars. Leases and power purchase agreements (PPAs) require no up-front cash but cut your savings dramatically and transfer ownership value away from you.
What happens after the 25-year warranty?
Modern silicon panels typically degrade at about 0.5% per year, so a 25-year-old panel still produces roughly 85–87% of its original output. Inverters usually need one replacement around year 10–15 ($1,500–$3,000), but the panels themselves keep generating. Many systems continue saving money for 30–40 years.
Disclaimer
This solar panel calculator is provided for informational purposes only and does not constitute financial, tax, engineering, or installation advice. Estimates are based on the NREL PVWatts derate formula and the inputs you enter; actual production varies by roof angle, shading, azimuth, panel degradation over time, local weather, net metering policy, and installer quality. Tax credit and rebate figures are approximations — always confirm eligibility and final pricing with a licensed solar installer and a qualified tax professional before committing to a purchase.