How the solar savings calculator works
The solar savings calculator estimates how much money a solar panel system can save you over its lifetime by comparing what you generate against what you would otherwise pay the grid. You enter your system size, local sunshine, electricity rate, system cost and a few optional details, and the tool projects savings year by year across a 25-year analysis period.
The core idea is simple: every kilowatt-hour (kWh) your panels produce and you use is a kWh you do not have to buy. Multiply that avoided purchase by your electricity rate, year after year, and you get your cumulative savings.
What drives your solar savings
System size and sunshine
Your system's output depends on its size in kilowatts (kW) and how much usable sunlight, or peak sun hours, your location receives per day. A location with more peak sun hours produces more energy from the same size system. The calculator applies a performance ratio of about 0.8 to account for real-world losses from wiring, temperature, inverter efficiency and dirt on the panels, so first-year generation works out to:
Year 1 generation (kWh) = system size (kW) x peak sun hours x 365 x 0.8
Electricity rate and self-consumption
Savings scale directly with your electricity rate: the higher the rate you avoid paying, the more each kWh is worth. Self-consumption, also called offset, is the percentage of solar generation you actually use in your home at the full retail rate rather than exporting it, often at a lower rate. If your utility pays little for exports, lowering your self-consumption assumption gives a more realistic savings figure. The default is 100 percent, meaning every kWh generated replaces a kWh you would have bought.
Rising electricity prices
Electricity prices tend to rise over time. The calculator lets you set an annual price increase, with 3 percent as a common default in many markets, which compounds each year and increases the value of the energy your system produces. This is one reason solar savings often accelerate in later years of ownership.
Panel degradation
Solar panels lose a small amount of output capacity each year, typically around 0.5 percent, as materials age. The calculator applies this degradation to reduce generation slightly year over year, so your savings estimate reflects a realistic, gradually declining output rather than assuming panels perform at year-one levels forever.
The formulas behind the numbers
Putting it together, for any year "n" in the 25-year period:
Generation in year n = year 1 generation x (1 - degradation)^(n-1)
Rate in year n = electricity rate x (1 + annual increase)^(n-1)
Savings in year n = generation x self-consumption x rate
The calculator adds these up year by year to build a cumulative total, then compares that running total against your system cost to find your payback period, interpolated to a fraction of a year. After 25 years it reports net savings, meaning total savings minus system cost, and return on investment, where ROI = net savings / cost.
Maintenance and inverter replacement
Solar systems are largely maintenance free, but you should still budget for occasional cleaning and monitoring, and for replacing the inverter roughly once during the system's life. Inverters typically last 10 to 15 years, so a 25-year savings projection should reasonably assume one replacement. This is not automatically built into every simplified estimate, so treat the headline savings figure as slightly optimistic unless you separately budget for that cost.
Worked example
The table below illustrates how cumulative savings might grow for an example system, assuming a rate of 0.15 per kWh, 3 percent annual price inflation, 100 percent self-consumption and 0.5 percent annual degradation. Figures are illustrative only, not real outputs.
| Year | Annual savings (example pattern) | Cumulative savings |
|---|---|---|
| 1 | Baseline year | Equal to year 1 |
| 5 | Higher, due to rate inflation | Several times year 1 |
| 10 | Higher still, despite mild degradation | Growing steadily |
| 15 | Continues rising | Often nearing full system cost |
| 20 | Near peak annual value | Typically well above system cost |
| 25 | Slightly moderated by degradation, still elevated by rate gains | Full 25-year total |
Your own results depend heavily on local sunshine, rates and system cost, which is why running the calculator with your own numbers matters more than any generic example.
Cautions about the assumptions
This calculator produces an estimate, not a guarantee. Actual savings depend on your real electricity usage pattern, whether your utility uses net metering or net billing, weather variability year to year, and any changes to tariffs or incentive programs. Incentives vary widely: the US residential tax credit has historically been around 30 percent as an illustration, the UK offers VAT relief and Smart Export Guarantee payments, Australia has STC rebates, and India and Pakistan both have rooftop and net metering support schemes, but details and amounts change by region and over time, so check current local rules before making a financial decision. Results here are for planning purposes only and are not financial advice.