A method you can check
A rooftop solar system can reduce electricity bought from the grid, but not every generated kilowatt-hour necessarily offsets a retail-priced kilowatt-hour. The value depends on when the home uses electricity, the terms of the export agreement, storage, and local tariffs.
Direct self-consumption
Solar energy used in the home while panels are generating can offset some purchases from the grid. A dishwasher running in the middle of the day may use some current solar production; a device used at night cannot directly consume that daytime production unless energy is stored or exported and later compensated according to the tariff rules.
Exported generation
Surplus electricity may flow to the grid. Some jurisdictions credit exported energy at the full retail rate, others use a different export rate, and some systems operate under limits or specific settlement rules. You need the actual contract or local program rules to estimate the value correctly.
Simple payback is only a first screen
Dividing the installed price by estimated annual savings gives a simple payback period. That calculation usually leaves out panel degradation, inverter replacement, maintenance, insurance, financing, discounting, tariff changes, taxes, and future export policy. A detailed decision requires an annual cash-flow model rather than one ratio.
Use a range of assumptions
Ask for a site-specific annual generation estimate and understand what shading, roof orientation, weather, and system losses were included. Compare a conservative, central, and optimistic scenario. Test how the result changes when self-consumption or export compensation changes. If a battery is proposed, include its full installed cost, efficiency losses, usable capacity, and expected replacement timeline rather than treating stored energy as free.
The Solar Savings & Payback Estimator exposes the assumptions used in its calculation.
Related resources
Continue with the Electricity Cost Calculator, the Home Energy Planner, or the Appliance Comparison Calculator depending on the question you are trying to answer.
References and further reading
The U.S. National Renewable Energy Laboratory's PVWatts Calculator estimates photovoltaic production worldwide and explains that results depend on system and site assumptions. The U.S. Energy Information Administration describes why electricity prices vary by location.
WattWise's simple payback estimate is not a PVWatts simulation or financial forecast. Use site-specific production, current import/export rules, installed quotes, maintenance assumptions, and local professional advice before making a purchase decision.