Solar Panel Calculator

How to use this calculator

1. Enter system size (kw) — The calculator loads with a worked example already filled in, so you can see what a realistic set of figures looks like before replacing them with your own.
2. Fill in the remaining fields — Every field has a sensible default. Change the ones that apply to you and leave the rest.
3. Read the result — The answer updates as you type — there is nothing to submit. The rows beneath the headline show how it breaks down.
4. Expand the schedule — Where a year-by-year table is offered, open it. The breakdown over time usually shows something the single headline figure does not.
5. Share or print — Use the share button to copy a link with your figures already filled in, or print the result — the print stylesheet strips the navigation and adverts.

What this calculator works out

To use the Solar Panel Calculator, enter your system size (kW), peak sun hours per day and system efficiency. The result — annual saving — updates as you type, with no button to press and nothing to submit. The calculation runs entirely in your browser, so the figures you enter are never sent anywhere.

How this is calculated

The solar panel calculator applies its formula to the values you enter and returns the result immediately.

A worked example

Take the values this page loads with:

• System size (kW): 4
• Peak sun hours per day: 3.5
• System efficiency: 80%
• Electricity price per kWh: $0.28
• Installation cost: $7,000.00

On those figures the annual saving is 1,144.64. Broken down:

• Generation per day: 11.20 kWh
• Generation per year: 4,088 kWh
• Monthly saving: 95.39
• Payback period: 6.1 years
• Saving over 25 years: 21,616.00

Change any field above and every figure here recalculates — these numbers are produced by the same formula the calculator runs, so they cannot drift apart from it.

What the figures mean

The headline figure answers the immediate question. These are the ones worth paying attention to:

• Generation per year — the figure worth watching when you change your inputs.
• Payback period — the figure worth watching when you change your inputs.
• Saving over 25 years — the figure worth watching when you change your inputs.

Where a schedule is shown, expand it: the year-by-year breakdown usually reveals something the single headline number hides.

What this calculator assumes

The efficiency factor covers inverter losses, temperature derating, shading and dirt. Payback assumes every unit generated replaces one you would have bought — in practice you export some at a much lower rate unless you have a battery.

Why it runs in your browser

Every calculation on this page is arithmetic, and arithmetic does not need a server. Running it locally means the result is instant, it keeps working if your connection drops, and — the part that matters — your salary, your debts and your medical measurements are never transmitted to us. There is no figure on our side to store, log or lose.

Where this calculator stops

A calculator applies the rules it is given to the figures you enter. It cannot see the rest of your circumstances, and real financial decisions are rarely decided by a single number.

Treat the result as a starting point for a conversation — with a lender, an accountant or an adviser — rather than as an answer. Where an institution will quote you a real figure, that figure is the one that counts: it will include fees, your credit profile and terms this page knows nothing about.

Keeping the figures current

The formula behind this page does not change, but the assumptions you should bring to it do — interest rates, inflation and market returns all move. Revisit the inputs rather than trusting a figure you calculated a year ago.