Solar & Energy Calculators
Solar payback, EV charging and running costs.
Three calculators for what energy actually costs: solar payback, EV charging and appliance running costs. Each one models the losses as well as the headline figure, which is where the simple version of these sums goes wrong.
All 3 solar & energy calculators
Everything starts from the kilowatt-hour
One number underlies this whole category. A kilowatt-hour is 1,000 watts drawn for one hour, and it is the unit your supplier bills. Running cost is therefore watts × hours ÷ 1,000 × your rate, and that single line answers most household energy questions once you know the wattage and the rate.
Get your rate from your own bill rather than from any average. Electricity prices vary by more than a factor of two across regions, and many tariffs have separate day and night rates, standing charges and tiered blocks that a flat average completely hides.
The losses that never reach the meter
The simple version of each of these calculations quietly assumes a perfect system. Real ones leak, and the leaks are big enough to change decisions.
| Situation | The loss | Effect |
|---|---|---|
| EV charging | Charger efficiency, typically 85 to 90% | Meter reads 6 to 18% more than reaches the battery |
| Solar over 25 years | Panel degradation each year | Later years produce measurably less than year one |
| Solar payback | Electricity prices rise over time | Holding the rate flat understates the saving |
| Standby power | Devices drawing while "off" | Small watts, but 8,760 hours a year |
The EV one is the clearest. You pay for what passes through the meter, not what ends up in the battery, so a charge that puts 60 kWh into the car draws around 66.7 kWh from the wall at 90% efficiency. Any calculator that bills only the energy delivered to the battery understates the cost of every charge.
Solar payback is a long-run estimate, and it moves
A payback period is the most assumption-heavy number on this site. It depends on your installed cost, your generation, how much you use directly rather than export, the export rate, the rate of future price rises, panel degradation and any incentive you qualify for. Change any one materially and the answer moves by years.
Incentives in particular expire. Tax credits are legislated, they have end dates, and a payback figure built on a credit that no longer applies is simply wrong rather than merely optimistic. The solar payback calculator links its current source and states which incentives it is and is not applying, so you can check the position rather than assume it.
Small constant loads add up
People reliably overestimate the cost of brief high-power use and underestimate constant low-power draw. A kettle is enormous for two minutes; a device idling at a few watts runs for all 8,760 hours in the year. The electricity cost calculator handles both, and a month in it is 365 ÷ 12 days rather than 30, so the monthly and annual figures agree with each other instead of contradicting themselves by a few percent.
Solar & Energy Calculators: common questions
How do I work out what an appliance costs to run?
Watts times hours, divided by 1,000, times your electricity rate. A 100 W device left on for 24 hours uses 2.4 kWh a day. Take the rate from your own bill, since regional prices vary by more than a factor of two.
Does charging an EV cost more than the battery capacity suggests?
Yes. Chargers are typically 85 to 90% efficient, so the meter records 6 to 18% more energy than reaches the battery. Putting 60 kWh into the car draws roughly 66.7 kWh from the wall at 90% efficiency, and you pay for all of it.
How accurate is a solar payback estimate?
It is a long-run projection with many assumptions: installed cost, generation, how much you use directly, export rates, future price rises, degradation and incentives. Change one materially and the answer moves by years, so treat it as a range.
Is standby power worth worrying about?
It adds up, because the hours do. A few watts is trivial for an hour and meaningful across all 8,760 hours in a year. People reliably overestimate brief high-power use and underestimate constant low-power draw.