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Home energy maths you can check.

Free home energy calculators. Every formula and source is shown, so you can check the maths.

31 calculators290 sources

Illustration of a home with solar panels, a wall battery, an electric car and a heat pump

  • Battery rebate

    $3,367

    on 13.5 kWh until 31 Dec 2026

  • EV charging

    $2,681

    a year less than petrol

  • Heater

    $70

    a winter on reverse-cycle

  • Air conditioner

    $511

    a year for a 7.1 kW split

Australia

All Australian tools
BatteriesBattery rebate calculator$3,367 on 13.5 kWh until 31 Dec 2026BatteriesBattery buy now or wait calculator$555 lost on 13.5 kWh by waiting past 31 Dec 2026BatteriesBattery size calculator12.4 kWh for 3 people and 10 kW of solarBatteriesAdd a battery to solar calculator10.2 years payback on 10 kWh until 31 Dec 2026BatteriesBlackout backup calculator6.3 days on a full Powerwall 3BatteriesBattery runtime calculator10.8 hours for an 80 W TV on 100 Ah lithiumBatteriesCaravan fridge battery calculator68 Ah for 3 days of a 46 L fridgeBatteriesBattery charge time calculator3.5 hours for 100 Ah lithium on a 25 A chargerBatteriesInverter size calculator2,700 W for a 2,000 W heater and a fridgeAppliancesAppliance running cost calculator$128 a year for a fridgeHeating & coolingHeater running cost comparison$70 a winter on reverse-cycleHeating & coolingAir conditioner running cost calculator$511 a year for a 7.1 kW splitTransportEV charging cost vs petrol calculator$2,681 a year less than petrolElectricityElectricity cost per kWh calculator32c/kWh Australian average usage priceElectricitySolar Sharer and Midday Power Saver calculator$127 a year less in Ausgrid, dishwasher moved to middaySolarSolar feed-in tariff calculator$624 a year for 10.5 kW in SydneyAppliancesAir fryer vs oven cost$29 a year saved at 3 meals a weekAppliancesHeat pump hot water cost and rebate$793 a year saved vs a peak-rate electric tankAppliancesCost to run a dishwasher$0.32 a load on normalAppliancesGas vs induction cooktop$325 a year saved by leaving gasAppliancesStandby power cost$135 a year on standbyHeating & coolingWood heater vs reverse-cycle$1,900 a winter of bought wood in VictoriaHeating & coolingWhat one degree costs you$30 to $60 one degree on an example $600 heating seasonHeating & coolingEvaporative cooling running cost$143 a summer for a 10.4 kW ducted system, power plus waterTransportFuel cost calculator$33.84 Sydney to Canberra, one wayTransportFuel consumption calculator8 L/100km example: 500 km on 40 LTransportEV charging time calculator5 h 23 min MG4 Urban 54, 20 to 80%, 7 kW wall chargerElectricityWatts to amps calculator10.43 A for 2,400 W at 230 VElectricityElectricity bill calculator$582 a quarter, 2 people in Sydney, Ausgrid default offerElectricityPeak and off-peak times calculator34.8% break-even peak share, AusgridSolarSolar EV charging calculator1.45 kW of panels makes the energy for 12,600 km a year (Model Y AWD 600 km, Sydney)

Next rebate drop · 1 January 2027

A 13.5 kWh battery loses $555 of rebate after 31 December.

The STC factor falls on 1 January and 1 July until the program ends in 2030. The same battery installed a few months later gets a smaller discount.

Until 31 Dec 2026
$3,367
From 1 Jan 2027
$2,812

How it works

  1. Enter your numbers

    Use the figures from your quote or bill. Leave a rate blank and we use the average rate for your area.

  2. See the working

    Every result shows the formula behind it, step by step, with your numbers filled in.

  3. Check it before you buy

    Compare the result with your quote, your bill or the energy label. Every calculator lists its sources.

Every number has a source

We prefer official sources: regulators, government schemes and published tariffs. Every data file carries the date we last reviewed it, and we re-check each one at least every 90 days.

  • 50 topics · 290 sources
  • Reviewed 23 September 2026
See every source

Questions

Is WattMaths free?

Yes. There is no sign-up. The calculators run in your browser and we don't store what you enter.

How often are the rates updated?

Each calculator shows the date its rates were last checked. We review every rate at least every 90 days.

Do you sell batteries or take commissions?

No. WattMaths is free. We plan to pay for it with ads. Ads are labelled, and they never change a result.

Check the maths

Each calculator works through its result step by step, lists its sources, and shows the date it was last checked.

How we calculateAbout