How to Read a Solar Quote Without Mixing Up the Numbers

A solar quote may show panel count, system capacity, annual production, bill savings, and payback. Each number answers a different question.

Start with energy use

Collect a full year of electricity use in kilowatt-hours from your utility bills. A single summer bill may misrepresent the rest of the year. The basic sizing tool divides annual use by peak sun hours, days in the year, and a performance factor. It is a planning approximation, not an engineering design. Peak sun hours mean equivalent full-strength sunlight, not the number of daylight hours.

For example, 12,000 kWh per year, 4.5 peak sun hours a day, and an 80% performance factor produce an estimate of about 9.13 kW of DC capacity. A roof with shade or an unfavorable orientation may need a different design. The PVWatts calculator offers a location-based production model for a better comparison with installer forecasts.

Translate capacity into equipment

Panel wattage describes rated output under standard test conditions. Dividing an 8 kW target by 400 W per panel suggests 20 panels. It does not prove that 20 panels fit on the roof or that the inverter and electrical system are appropriate. Ask each installer to show usable roof area, setbacks, shade assumptions, and the equipment model numbers.

Distinguish production from bill savings

Annual production is energy, measured in kWh. Savings are money and depend on your tariff, when you use the energy, and how exported power is credited. Multiplying 11,000 kWh by $0.16 gives $1,760 as a simple value estimate only if every kWh is actually worth that amount. An installer proposal should explain self-consumption and export compensation rather than applying a retail rate to all output without explanation.

Read payback assumptions line by line

Simple payback divides net installed cost by net annual savings. A $25,000 system with $1,800 in annual savings gives about 13.9 years before financing, maintenance, degradation, roof work, and changes in utility rates. Incentives should be counted only if you are eligible under current rules. Ask whether the quoted savings include rate escalation, financing charges, subscription fees, or a battery.

Compare quotes using the same bill history and assumptions. A higher savings figure is not automatically better if it relies on a more optimistic production forecast. Keep the estimates separate from a final purchasing decision and consult a qualified installer for site-specific design.

Ask each installer for the same evidence

Request the proposed DC system size, panel and inverter model numbers, roof layout, shade analysis, annual production estimate, monthly production profile, warranties, installation scope, and total cash price. Ask what utility tariff and export credit were used in the savings estimate. If a quote includes storage, separate the battery cost and benefits from the solar-only system. Comparing a bundled battery proposal to a solar-only proposal by payback alone can obscure the reason the numbers differ.

Review any incentive claim against the rules that apply to your household and installation date. Do not assume a quoted incentive is cash paid to you immediately or that every homeowner can use it. The simple payback calculator deliberately starts with zero incentives so eligibility must be entered deliberately.

Run conservative and optimistic scenarios

Take the installer’s expected kWh, then test a lower annual production and a lower value per kWh. Recalculate payback with recurring costs and compare the result with your expected time in the property. This is not a forecast of resale value or a complete discounted-cash-flow analysis, but it shows which assumptions drive the headline number.

For production, a location-based model such as PVWatts is more informative than a single national peak-sun-hours default. Even a detailed model has uncertainty because weather changes year to year. Use the tools here to understand the quote, then ask a qualified installer to explain discrepancies between your own inputs and its forecast.

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