Compare solar proposals by their assumptions, not just payback

Two proposals can show different savings even when their equipment capacities are similar. Compare the assumptions that connect capacity, annual production, bill value and cost before choosing which total to believe.

Keep capacity and production separate

Capacity is a power rating, commonly shown in kilowatts. Annual production is energy over a year, shown in kilowatt-hours. An 8 kW system does not produce 8 kWh every hour of every day. Weather, orientation, shade and losses affect the relationship. Record both figures and ask how the production estimate was obtained.

Check the panel-count arithmetic

An 8 kW target is 8,000 watts. At 400 watts per panel, the capacity-based count is 20 panels. That arithmetic does not prove that the panels fit on a particular roof or satisfy layout constraints. Confirm whether quoted capacity is measured on the same basis, and leave roof and equipment compatibility to a site-specific design assessment.

Look at the annual-use record

A full year of electricity records gives context that one unusually hot, cold or quiet month may miss. Note any expected change in household use instead of silently assuming last year repeats. A simple annual-energy match still says little about when energy is used. Morning, evening and seasonal use can affect how much production is used at home rather than exported.

Find the value assigned to exported energy

A model that values every generated kilowatt-hour at the full retail rate may differ from a tariff that credits exports at another rate. Record whether savings include self-consumption, exports and fixed charges. A single-rate calculator is useful for an explicit scenario, but the quoted bill impact should reflect the actual tariff and timing assumptions.

Reproduce one simple savings calculation

At 11,000 kWh per year and an illustrative $0.16 value per kWh, simple energy value is $1,760 per year. A $25,000 net cost divided by that amount gives about 14.20 years of simple payback. If the annual value becomes $1,400 under another scenario, the same net cost produces about 17.86 years. The equipment has not changed; the value assumption has.

Separate incentives and financing

Use only incentive amounts whose applicability has been checked for the project. A proposed incentive and a confirmed reduction in cost are different inputs. Also distinguish an installed cash cost from a financed payment plan. Simple payback does not model financing, maintenance, future tariff changes or the time value of money.

Ask for the basis of the forecast

Request the location-specific production assumptions and clarify any exclusions in the equipment or installation quote. A useful comparison record includes capacity, panel rating, estimated annual kWh, electricity-value treatment and net cost on matching terms. Keep each proposal’s assumptions rather than copying the lowest payback into an otherwise unrelated model.

Use the tools to organize questions

The system-size calculator and panel-count calculator check capacity relationships. The energy-value calculator and simple payback calculator check a stated scenario. Read the solar quote guide before treating these estimates as a final decision.