Solar Water Heater Savings Calculator

Work out solar water heater savings instantly with clear inputs, formula shown and shareable results.

About Solar Water Heater Savings Calculator

Heating water takes 4.186 kJ per litre per degree, so 160 litres raised 35 degrees needs 6.5 kWh a day. A solar thermal system typically supplies 50-70% of that across a year — most of it in summer — which sets both the saving and the long payback.

Formula

Water heating energy

kWh = litres x 4.186 x temperature rise / 3600; saving = kWh x solar fraction x 365 x price

Worked example

With the default values below, Solar Water Heater Savings Calculator returns Annual saving of $128.34. Change any input above and every figure here is recalculated in your browser.

Inputs used

Daily hot water use: 160 L
Temperature rise required: 35 °C
Solar contribution: 60 %
Energy price: 0.09

Results

Annual saving: $128.34
Annual energy saved: 1,426 kWh
Daily heating energy: 6.51 kWh
Payback on a 4000 system: 31.17 years

Step by step

1Values used
2Water heating energy
3Annual saving= 128.34
4Annual energy saved= 1,426 kWh
5Daily heating energy= 6.51 kWh
6Payback on a 4000 system= 31.17 years

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why is the solar fraction not higher?

Winter irradiance is a fraction of summer and demand is highest then. A system sized for winter would massively overheat in July.

Is solar thermal or PV better now?

PV plus a diverter to the immersion heater is often cheaper per kilowatt-hour and more flexible, which is why solar thermal installations have declined.