Energy Cost Calculator (kWh)

How It Works

The running cost

Nothing more to it than that. The interesting part is what happens to the number afterwards.

What a solar kilowatt-hour is worth

A kilowatt-hour from your own panels is worth one of three amounts depending on where it goes:

• Used in the home as it is made: the full retail rate, because it is a kilowatt-hour you did not buy. This is the number you enter above.
• Exported: the export rate, which is a fraction of retail. The solar payback calculator defaults to $0.17 retail against $0.05 export, and it escalates the two separately because retail carries network and policy charges that export compensation does not.
• Exported from a UK plug-in kit: nothing. There is no export payment on the socket route, which is why the plug-in solar savings calculator prices spilled generation at zero.

So moving a load from evening to midday is worth the gap between the two rates on every kilowatt-hour it shifts: $0.12 on the payback tool's defaults, the full 25.7p on a plug-in kit. On a time-of-use tariff the same logic runs the other way round for what you still buy: a load you cannot shift into daylight belongs in the cheap overnight window, and the rate to enter above is the one that applies when it actually runs, not the day rate.

Worked example: from a bill line to an array

A 1,000 W load running 5 hours a day for 30 days at $0.30 per kWh:

• 5 kWh a day, 150 kWh a month, $45.00 a month, $547.50 a year
• That $547.50 is the ceiling on what solar can save on this load, and it is only reached if the load runs while the panels are producing
• Covering 5 kWh a day at 4.5 peak sun hours with the system size tool's 18% loss factor takes 5 ÷ (4.5 × 0.82) = 1.36 kW of array, which rounds up to four 400 W panels, 1.6 kW, making about 2,155 kWh a year
• Off grid the same 5 kWh a day divided by a 0.8 depth of discharge is 6.25 kWh of lithium bank per day of autonomy before round-trip losses, which is the arithmetic the battery bank size calculator starts from

Solar water pumping runs the same arithmetic in the other direction. A borehole pump moving 4 m³/h against 30 m of head needs about 0.33 kW of hydraulic power, roughly 0.64 kW at the terminals once pump and motor efficiency are allowed for; the pump power calculator gives that figure from the duty point. Six hours a day is 3.84 kWh, and at 4.5 peak sun hours that is about a 1.1 kW array to run it with no grid at all.