The useful cost of rooftop solar is the price of a complete, working system compared with the electricity bill it changes over time. Start with your last 12 months of bills, at least two itemized proposals, the condition of your roof and your utility’s current solar tariff.
This guide is for U.S. households comparing rooftop photovoltaic systems. Local rates, export rules and project requirements matter more than a national headline price. Keep a solar-only quote separate from an optional battery: storage adds a different cost and backup capability.
Put the quotes on the same basis
Ask each installer to identify panel capacity in kilowatts DC, inverter capacity in kilowatts AC, estimated annual generation in kilowatt-hours, equipment models and the complete cash price. A kilowatt describes power; a kilowatt-hour describes energy accumulated over time. An 8 kW array producing 10,000 kWh in a year has two different, useful specifications.
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| Item | What to request | Why it affects the decision |
|---|---|---|
| System and production | DC panel size; AC inverter size; annual and monthly kWh; shading and loss assumptions | A larger nameplate array can still perform poorly on a shaded roof. |
| Installed scope | Permits, interconnection, electrical upgrades, roof work and removal/reinstallation costs | An omitted job can outweigh a lower panel price. |
| Service responsibility | Equipment, workmanship and roof-penetration terms; labor and shipping coverage | A panel warranty and an installer service obligation cover different risks. |
| Payment | Cash price, financed amount, APR, fees, term and total payments | The advertised monthly payment can hide a higher purchase price. |
For an initial normalization, divide solar-only cash price by DC watts. A fictional $24,000, 8,000 W system is $3.00/W. Use this only between proposals with comparable scope. The DOE installation guide explains why roof assessment, installer selection and electricity-use history belong early in the process.
Check the production estimate and tariff
Ask for the location, roof tilt and orientation, shading assessment, system losses and weather assumptions behind the estimate. Compare the proposed monthly pattern with your bills. PVWatts provides an independent starting estimate; an installer’s site assessment should explain differences. Use the weather-based estimate as a planning value and allow for year-to-year variation.
Then obtain the actual utility tariff. Establish how self-consumed electricity reduces purchases, how exports are credited, when credits expire and which charges remain. A retail energy price and an export credit can be very different. Time-of-use billing requires matching production and consumption to tariff periods. Calculate bill savings using the tariff and the timing of generation and consumption together. The DOE homeowner guide describes the role of consumption, generation, financing and utility compensation.
Work through a transparent savings example
Both scenarios hold prices and production constant and assume all avoided purchases use the stated rate. They exclude financing, degradation, replacements, tax effects and the time value of money. Fixed utility charges remain payable; neither figure predicts a zero bill. The downloadable worksheet keeps these limits beside the arithmetic.
Extend the calculation across the ownership period
For each year, estimate production, self-consumption, export credit, remaining utility charges, maintenance and any replacement expense. Use the equipment performance warranty and a realistic service plan to set assumptions. Keep low, central and high cases for rate changes and output rather than selecting only the most favorable combination.
For a cash purchase, start with the purchase and installation expense, subtract incentives actually received at their expected dates, then add operating costs and subtract bill reductions over the chosen years. For a loan, model the down payment and scheduled payments instead of also counting the full financed principal as an upfront expense. Discount future cash flows if comparing their present value. A lease or power-purchase agreement needs its own payment escalation, transfer and end-of-term terms.
U.S. federal credit status, checked September 7, 2026: the IRS residential clean energy credit page states that the credit is unavailable for property placed in service after December 31, 2025. Do not deduct an assumed 30% credit from a new 2026 project. Check the relevant agency’s current rules for any state or local program and document eligibility before including it.
Turn the comparison into a decision
Request written answers for any unresolved scope, tariff or financing assumption. A credible quote should let you reproduce the central calculation and see what happens if savings are lower. If the roof needs attention soon, price the roof and future panel handling together before committing.
Ordinary grid-connected solar may stop supplying the home during an outage. Ask explicitly about approved islanding equipment, battery capacity and which circuits receive backup. Use the backup-power planning guide to identify essential loads, and the conservation guide to assess whether reducing consumption changes the system size you need.
Solar quote and savings worksheet
Solar quote and savings worksheet — plain-text download. Save a copy and fill it in with your own information. The file includes instructions, assumptions and references so it can be used independently.