How to Calculate Your Solar ROI: A Homeowner's Framework

Most solar quotes include a savings number — “$57,000 over 25 years” or “8.2 year payback” — that is calculated by the installer’s sales software with assumptions you don’t see. Some of those numbers are honest; some are wildly optimistic. The good news is the math is straightforward, the inputs are public, and you can rebuild the whole calculation in about thirty minutes using free tools. This guide walks through that calculation step by step, then flags the assumptions where contractor quotes commonly inflate the result.

Step 1: Calculate Your Net System Cost

Start with the all-in installed price from the quote. This is gross system cost — the number before any tax credit or rebate.

Subtract, in this order:

1. State rebates and utility rebates (these typically reduce the basis for the federal credit).
2. Federal Residential Clean Energy Credit: 30% of the post-rebate cost in 2026.
3. State income tax credits (not rebates — these are claimed separately on your state return and do not reduce the federal basis).

Example:

• Gross system cost: $28,000
• State utility rebate: $1,500 → adjusted basis $26,500
• Federal credit at 30%: $7,950
• State income tax credit (NY, 25% capped at $5,000): $5,000
• Net cost: $28,000 − $1,500 − $7,950 − $5,000 = $13,550

Note the order matters: the federal credit is calculated on $26,500, not on $28,000. The state credit is independent and doesn’t reduce the federal basis. For the mechanics of claiming each credit, see our federal solar tax credit guide.

Step 2: Estimate Annual Production Using PVWatts

The single most important number in the ROI calculation is annual production in kWh. Do not use the contractor’s figure without verifying it against NREL’s free PVWatts Calculator.

To run PVWatts:

1. Enter your address.
2. Enter the system DC size in kW (e.g., 8.0).
3. Module type: Standard (default), Premium for HJT or top-tier TOPCon.
4. Array type: Fixed (roof mount).
5. System losses: leave at default 14% unless you know better.
6. Tilt: equal to your roof pitch (e.g., 22° for a typical 5:12 pitch).
7. Azimuth: 180° = due south, 90° = east, 270° = west.

The calculator returns annual production in kWh, plus monthly breakdown. A correctly designed system in most of the U.S. produces between 1,200 and 1,600 kWh per kW of installed DC capacity per year.

Sanity check: if the contractor’s annual production figure is more than 5% above PVWatts using your inputs, ask why. Common reasons (some legitimate, some not):

• They’re using a higher-end module with custom PAN files (legitimate, accounts for 1–3%).
• They’ve reduced “system losses” below 14% (sometimes legitimate; often optimistic).
• They’ve assumed a flatter tilt or different azimuth than reality (a red flag).
• They’ve used a TMY3 weather file from a sunnier site nearby (usually a small effect).

Step 3: Value the Production

This is the most variable step because it depends on your utility’s net metering policy.

Full retail net metering (still in place in many states, including most of New England, the Midwest, and parts of the South): every kWh you produce is worth your full retail rate, including charges that fund grid infrastructure. The simplest calculation:

• Annual production × retail electricity rate = annual savings

Net billing or partial net metering (California’s NEM 3.0, most newer state policies): production consumed on-site is worth the retail rate; excess exported to the grid is credited at a wholesale or avoided-cost rate, often 70–80% lower. This requires two-component math:

• (Self-consumed kWh × retail rate) + (Exported kWh × export rate) = annual savings

Self-consumption ratio — what fraction of your production is used on-site rather than exported — varies widely. For a typical residential system without a battery, expect 30–50% self-consumption. With a battery, 70–90% is achievable.

Pull your current retail rate from your most recent utility bill (the EIA’s state retail rate tables are a good reference point but reflect averages). Apply it conservatively — don’t extrapolate the highest tier of a tiered rate to your whole production.

Example (full retail net metering):

• Annual production: 11,000 kWh
• Retail rate: $0.18/kWh
• Annual savings: $1,980

Example (NEM 3.0 with 40% self-consumption, no battery):

• 4,400 kWh self-consumed × $0.32 = $1,408
• 6,600 kWh exported × $0.08 = $528
• Annual savings: $1,936
• (And much higher if a battery shifts the self-consumption ratio.)

Step 4: Payback Period — Simple Version

The simple payback period is:

Payback (years) = Net system cost ÷ First-year annual savings

Using the earlier examples:

• $13,550 ÷ $1,980 = 6.8 years simple payback

This is the cleanest number. It tells you when your cumulative savings will equal what you paid. After that point, every dollar of avoided electricity bill is “profit” against the system cost.

Simple payback ignores two things: utility rate increases (which shorten the real payback) and the time value of money (which lengthens it). Both effects are typically modest over a 5–10 year payback window.

Step 5: 25-Year Savings Projection

This is the number contractors love to feature in their quotes. It is also the number most easily inflated by aggressive assumptions. The honest version:

1. Start with first-year production.
2. Apply panel degradation each year (use 0.5%/year unless your panel is rated better — see our panel technology comparison for typical rates).
3. Apply utility rate inflation each year (we’ll get to this in a moment).
4. Sum 25 years of annual savings.
5. Subtract any expected mid-life inverter replacement cost (typically $1,500–$2,500 for a string inverter at year 12–15; $0 for microinverters under warranty).
6. Subtract net system cost.

Realistic utility rate inflation: the EIA’s long-run Annual Energy Outlook projects residential electricity rate growth of roughly 2–3% per year in nominal terms, varying by region. Many contractor models use 4%, 5%, or 6%. A 1-point increase in assumed annual rate inflation can swing the 25-year savings number by 30–50%. This is the single largest source of inflated quote numbers.

For a defensible projection, use the EIA’s historical average for your state (typically 2.0–3.5% annually) or, more conservatively, use 0% and let the actual rate increase be upside.

Step 6: Watch-Outs in Contractor ROI Claims

Common assumptions to interrogate on any quote:

• Utility rate escalation above 4%/year: ask for the source. The EIA Annual Energy Outlook is the most defensible reference.
• Production estimates above PVWatts +5%: ask which losses they reduced from the default 14%.
• Self-consumption ratios above 50% without a battery: ask how the figure was derived.
• Inverter replacement excluded from 25-year math: assume one inverter swap for string systems.
• TOU (time-of-use) optimization claimed without a battery: solar produces midday; high TOU rates are usually evening. Without storage, you can’t capture them.
• “Bill goes to zero” claims: most utilities have non-bypassable monthly customer charges of $10–$30, which solar does not offset.
• Lease/PPA ROI numbers: a leased system’s “savings” are the difference between the lease payment and your old utility bill — your ROI on capital is zero because you didn’t put any capital in. Don’t compare lease and ownership ROIs as if they’re the same thing.

A Defensible Worksheet

Pull these inputs into a spreadsheet:

Run the math. Compare the result to the contractor’s quote. If your numbers and theirs differ by more than 15%, the difference is almost always in the utility rate inflation assumption, the production estimate, or whether the inverter replacement is included.

A Few Honest Defaults for Quick Estimates

When you don’t have time for the full worksheet, these are defensible round numbers in 2026:

• Payback period: 7–11 years for an owned system in a state with full retail net metering. 10–15 years under partial net metering without a battery. Add 1–3 years if no state incentives apply.
• 25-year IRR: 6–10% on a defensibly modeled cash purchase. Anything above 12% deserves scrutiny.
• Net cost after federal credit only: $1.80–$2.40 per watt installed for a typical 6–10 kW residential system in 2026.

ROI is one of three or four factors that should drive a decision to install solar. The others — resilience (especially with a battery), insulation against future rate volatility, property value, and personal preferences about sustainability — don’t reduce to a single dollar figure. But the dollar figure should be honest, and it should be one you built yourself.

Frequently Asked Questions

What’s a realistic payback period for residential solar in 2026? Most owned systems with the 30% federal credit and reasonable net metering land in the 7–11 year range. Systems in states with strong additional incentives can be 5–7 years; systems in states with weak net metering and no state credits run 10–15+ years.

Should I include the system’s effect on home value? Most studies (Berkeley Lab’s long-running solar home premium analysis) find owned solar adds roughly $15,000–$25,000 in value to a typical U.S. home, with regional variation. Leased systems often add little or even subtract value because the lease transfers to the buyer. Be cautious about including resale value in payback math if you plan to stay in the home long-term — you can only realize it once, at sale.

How do I value avoided rate increases? Use the EIA Annual Energy Outlook projection for your region — typically 2–3% per year nominal. Anything higher is a forecast, not a fact.

Does adding a battery improve ROI? Under full retail net metering, usually not — the battery costs more than it saves. Under net billing or NEM 3.0, often yes — the battery captures the spread between import and export rates. Run the math both ways.

What if I move before payback? Owned solar typically recoups most of its remaining value at sale, with some discount. Leased solar can complicate or even reduce a sale price. Plan for this if your time horizon is under 7–10 years.