Solar Energy Calculator

How this solar energy calculator works

A good solar calculator starts with your electric use, not a sales target. This one works in four steps:

1. Your yearly use. From kWh if you have them, or from your bill: (bill − fixed charge) ÷ rate × 12.
2. System size. How many kW of panels it takes to cover your offset goal, given your state's peak sun hours, the roof direction and system losses. It rounds up to whole panels.
3. Savings. Each kWh the panels make, up to your yearly use, is worth your rate. Net metering rules set how much exported power is worth.
4. 25 years of cash flow. Each year the rate rises by your escalation figure and the panels lose 0.5% of their output. Loan payments and a later inverter replacement come out of the savings. Payback is the point where the running total turns positive.

State sun hours come from NREL data and state rates from EIA 2026 data. Both are averages, so replace them with your own numbers if you have them. The rate on your bill is best. Divide the total bill, minus the fixed customer charge, by the kWh used.

Solar panel calculator formulas and a worked example

kWh per kW per year = sun hours × 365 × direction factor × (1 − losses) × 0.90
System kW = yearly kWh × offset goal ÷ kWh per kW
Panels = system kW × 1,000 ÷ panel watts, rounded up
Cost = kW × 1,000 × cost per watt − incentives
Savings in year n = min(output, use) × rate × (1 + increase)ⁿ⁻¹ × credit share
Output in year n = year-one output × (1 − degradation)ⁿ⁻¹

The 0.90 accounts for inverter efficiency (PVWatts uses 96%) and the output panels lose when they run hot. With PVWatts' default 14% losses, the full derate is 0.86 × 0.90 = 0.774.

Worked example (the default inputs): a $180 monthly bill with a $15 fixed charge, the US average rate of 18.5¢ and 4.98 sun hours, a south roof, 430 W panels, $2.60 per watt, cash, full net metering, 3% yearly rate increases.

1. Use: ($180 − $15) ÷ $0.185 × 12 = 10,703 kWh a year.
2. Output per kW: 4.98 × 365 × 1 × 0.86 × 0.90 = 1,407 kWh.
3. Size: 10,703 ÷ 1,407 = 7.61 kW, or 17.7 panels. Round up to 18 panels = 7.74 kW, about 10,889 kWh in year one. That's 378 sq ft of panels.
4. Cost: 7,740 W × $2.60 = $20,124.
5. Year-one savings: 10,703 kWh × $0.185 = $1,980. The bill drops from $180 to the $15 fixed charge.
6. Over 25 years, savings grow with rates and shrink slightly with panel aging. The running total passes $20,124 in about 9.1 years and ends at about $48,500 net. That's a 241% return on the money spent.
7. CO₂: 10,889 kWh × 0.000672 = 7.3 metric tons in year one, about 172 tons over 25 years.

With a 20-year loan at 7.5% and no money down, the payment is about $162 a month against $165 of average monthly savings in year one. The 25-year net drops to about $29,700 because of roughly $18,800 in interest.

Solar power calculator data: sun hours and rates by state

Peak sun hours are the daily solar energy on a south-facing roof at about 20°, expressed as hours of full 1,000 W/m² sun. Output per kW uses the same 0.774 derate as the calculator. Rates are the average of EIA's March and June 2026 residential prices. Summer and winter rates differ, so your yearly average may vary.

The last column multiplies the two. It shows why sunshine alone does not decide whether solar pays. Massachusetts gets less sun than Texas but has nearly twice the electricity price, so each kW of panels saves more there.

Net metering, payback and what changes the math

Solar panels make most of their power at midday, when most homes use little. Without a battery, a typical home uses roughly 30% to 40% of its solar power as it is made and sends the rest to the grid. What the utility pays for those exports matters a lot.

• Full retail net metering: every exported kWh offsets a kWh you buy later at the same rate. This gives the best payback.
• Partial credit: exports are credited below retail, often near the utility's wholesale or "avoided cost" rate. California's current net billing tariff works this way. In the example, a 35% direct-use share with exports at half of retail cuts year-one savings to about $1,340 and stretches payback to about 12.8 years.
• No export credit: only the power you use directly saves money. Payback in the example stretches to about 22 years, and a battery or a smaller system usually makes more sense.

Where batteries fit. A home battery stores midday solar for the evening, which raises the share of solar you use directly. Under full net metering it adds cost without adding savings, so it mainly buys backup power. Under partial or no export credit, it can win back much of the lost value. To test that here, raise "Solar used directly" to the share a battery would let you use, and add the battery price to the system cost by raising the cost per watt (battery price ÷ system watts).

Other things that move payback: the price per watt you actually get (quotes vary by $1 per watt or more), how fast rates rise, shade, and whether you will need a new roof or a panel upgrade first. Any extra power above your yearly use gets no value here, since most utilities pay little for a yearly surplus.

Paying for solar in 2026: cash, loan, lease and tax credits

The federal tax credit is over for systems you own. The 30% Residential Clean Energy Credit (Section 25D) does not apply to expenditures made after December 31, 2025, under the July 2025 tax law (Public Law 119-21). The IRS counts the expenditure as made when installation is complete, so a 2026 install gets nothing even if you signed or paid in 2025. Some older articles and sales sheets still show the 30%. Leave it out of your math.

Leases and PPAs work differently. The solar company owns the panels and may still claim a separate business credit (Section 48E) under its own deadlines. Projects that begin construction after July 4, 2026 generally have to be in service by the end of 2027. You don't get that credit directly. Ask how it is reflected in the monthly price and the yearly escalator.

Cash gives the best long-term return. A loan spreads the cost, and if the payment is close to your savings, solar can be cash-neutral from the start. Watch for loans with "dealer fees" built into the price. A low APR can hide a sizable markup on the system price. Compare the cash price and the loan price side by side.

State, city and utility rebates still exist in many places. Enter them in the incentives box. The DSIRE database lists programs by state.

What makes the best solar calculator (and how to use one)

• Use 12 months of kWh. One bill can be off by 40% between seasons. Most utilities show a yearly total online.
• Use your real rate. State averages hide big differences between utilities in the same state.
• Check the roof. This solar power calculator assumes you have the space. Count what actually fits with the solar roof calculator. Fire-code setbacks and odd roof shapes often leave less room than you expect.
• Replace an old roof first. Taking panels off later costs about $200 to $300 per panel. See roof replacement cost.
• Compare quotes on cost per watt. Divide each quote's price by its system watts. Then compare with the $2.60 US average and your state's figure.
• Check production for your address. State averages can be off by 10% or more for a coastal, mountain or foggy location. Run your exact roof through NREL's PVWatts and enter the kWh per kW it gives you by adjusting the sun hours.
• Plan for future loads. An EV adds roughly 3,000 to 4,000 kWh a year for 12,000 miles of driving. A heat pump can add more. Raise the offset goal if they are coming.

Sources

Peak sun hours: state averages from NREL PVWatts (National Solar Radiation Database), 20° south-facing roof, as compiled by TheGreenWatt. Electricity rates: average of March and June 2026 residential prices, EIA Electric Power Monthly, Table 5.6.A. Average home use: EIA. CO₂ factor: EPA Greenhouse Gas Equivalencies Calculator (1,405.3 lb CO₂/MWh, AVERT marginal rate). Federal credit: IRS, Residential Clean Energy Credit. Lease and PPA credit deadlines: Solar Power World. Cost per watt: EnergySage (June 2026). Panel removal cost: Fixr.

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