Project solar savings from system size, sun hours, derate, and your utility rate — year-one savings, 25-year totals, net cost after the federal credit, and payback.
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FORMULA USEDSolar Panel Savings Calculator
How solar savings are computed
A photovoltaic system is rated in kilowatts (kW), but what you bank is kilowatt-hours. Annual production follows the standard energy-model formula: kW × peak sun hours per day × 365 × derate factor. The derate — typically 0.78–0.85 — shaves the rating for inverter conversion losses, wiring resistance, panel soiling, and heat.
From production to dollars
Each kWh your panels produce is a kWh you do not buy, so year-one savings are simply production × your rate. Over 25 years this model adds 3% annual utility-rate inflation and subtracts 0.5% per year of panel degradation, the industry-standard figure. The 30% federal residential clean energy credit is applied to the gross installed cost to get net cost, and payback is the year cumulative savings overtake it.
Reading payback the right way
Payback in the 8–12 year range is typical for US residential systems against a 25–30 year warranted panel life. If your result lands far outside that band, the drivers are almost always electricity rate (high-rate states pay back faster), sun hours, and installed cost per watt — in that order.
Worked example
A 6 kW system at $3.00/W, 5 peak sun hours, 85% derate, and a $0.13/kWh rate.
Production: 6 × 5 × 365 × 0.85 = 9,308 kWh in year one.
Cumulative savings pass $12,600 around year 9.4; the 25-year total is roughly $41,000.
How to use it
Enter system size in kW DC and your local peak sun hours (a solar quote lists both).
Adjust the derate only if you know shading or equipment specifics.
Put in the quoted $/W, the federal credit percentage, and your utility rate.
Compare payback and lifetime net benefit before signing any contract.
Frequently asked questions
How much do solar panels save on electricity bills?
It depends entirely on production and your rate. A typical 6 kW US system generates roughly 9,000 kWh a year, worth about $1,200 at a $0.13 rate and $2,000 where rates run near $0.22.
What is the payback period for residential solar panels?
Most US systems pay back in 8 to 12 years after the federal credit. High electricity rates and strong sunlight push it toward the low end; cheap power and cloudy climates push it past 15.
What derate factor should I use?
Start at 85% for a clean, unshaded roof. Heavier shading, snow, or older inverters justify dropping toward 78%. Quotes sometimes assume aggressive efficiency, so compare their production estimate against the formula here.
Does the 30% federal tax credit apply to the whole cost?
In this model, yes — 30% of gross installed cost comes off your federal taxes if you owe enough to use it. State or utility rebates would reduce the net cost further but are not included in the default inputs.
Do solar panels degrade over time?
Yes, most manufacturers warrant about 0.5% output loss per year, leaving roughly 87.5% of original production at year 25. The 25-year savings figure accounts for this along with 3% annual electricity-price inflation.