Solar Panel Savings Calculator

What model does this tool use?

Generation is system kW × a built-in kWh-per-kW figure for the country. Household use is estimated as monthly bill ÷ a built-in electricity rate × 12. The tool treats up to 85% of generation as used at home, capped at that household use, and the rest as exported. Benefit is used energy × the built-in rate plus exported energy × the built-in export rate. Payback is net cost ÷ annual benefit, and the 25-year figure is 25 × benefit − net cost.

What figures are built in for each country?

Electricity rate per kWh, install cost per watt, incentive share, kWh per kW per year and export rate respectively: US 0.16, 2.95, 30%, 1,500, none; UK 0.29, 2.00, none, 950, 0.15; India 6, 55, 30%, 1,400, none; Canada 0.17, 3.50, none, 1,200, none; Australia 0.32, 1.25, none, 1,600, 0.06. All are in local currency, carry no date and are not current market figures. Only the system size, bill and roof area are inputs.

Why does the tool estimate my usage from the bill?

It has no field for kWh, so it divides your monthly bill by the built-in rate. If your actual tariff is higher or lower than that rate, the estimated usage will be wrong in the opposite direction, and so will the share of solar output that counts as used at home. Enter a bill for electricity only, in the local currency.

How is the system size chosen?

A size above 0 in the System Size box is used as entered, and the roof area is then ignored. A size of 0 means “auto”: the tool takes roof area × a built-in kW per square metre (0.15 in the US, UK and Canada, 0.12 in India, 0.17 in Australia) and caps it at 20 kW for the US and Australia, 15 for the UK and Canada, and 10 for India. The result line then shows the size used.

How is the incentive handled?

For the US and India the tool deducts a flat 30% of the gross installation cost; for the other three countries it deducts nothing and shows a note. The 30% is a figure built into this tool, not a statement of current law, and real incentive schemes have eligibility rules, caps and end dates that change. Check with the scheme administrator or an adviser whether an incentive applies to you.

What happens if I produce more than I use?

The surplus is treated as exported and is paid at the built-in export rate, which is 0 in the US, India and Canada. A small bill with a large system therefore stalls the savings: a 5 kW US system on a $40 monthly bill shows $480 of annual benefit, against $1,020 at $150, and payback of 21.5 years against 10.1. The estimate does not model real net-metering or export rules.

What is not modelled?

Panel degradation, rising or falling electricity prices, maintenance, inverter replacement, financing costs, batteries, tax effects, shading, roof direction and export limits. Years are all treated as identical, so the 25-year figure is a flat multiple of one year’s benefit.

What does the tool do with zeros and cleared boxes?

A bill of 0 means nothing is used at home, so all output is exported: in the US the benefit is $0 and payback shows “n/a (no annual benefit)”. A roof area and size of 0 give a 0 kW system and zero cost. A cleared box counts as 0, and a system size of 0 means the system is sized from the roof area (4.5 kW for the default 30 square metres). Negative numbers are treated as 0, and a negative 25-year result is labelled a shortfall.