Home energy calculators that show their working
Most energy sites tell you a technology is great and then ask for your phone number. These do the arithmetic in front of you instead — every intermediate figure visible, every assumption written down, nothing transmitted anywhere.
Solar Payback Calculator
How long until a rooftop array pays for itself
Work out the net cost of a solar array after incentives, the year-one saving, the simple payback period and the 25-year picture — with utility inflation and panel degradation both accounted for.
Open the calculator →Solar Array Sizing Calculator
How many panels it takes to cover your bill
Turn your annual electricity consumption into an array size, a panel count and a roof area — then see how much of your bill each option actually offsets, and where adding panels stops helping.
Open the calculator →Battery Backup Sizing Calculator
How much storage your outage actually needs
Add up the loads you genuinely need to keep running, choose how long you need them, and get the usable and nameplate battery capacity plus the continuous inverter rating to look for.
Open the calculator →Heat Pump Sizing Calculator
A screening estimate of your heating and cooling load
Estimate the heating and cooling capacity a home needs from its floor area, climate zone and envelope quality — the sanity check to run before an installer hands you a quote.
Open the calculator →Insulation Savings Calculator
What an R-value upgrade is actually worth per year
Put a number on an insulation upgrade. Enter the assembly area, the R-value before and after, your heating degree days and your fuel price, and see the annual saving and the payback period.
Open the calculator →Water Heating Cost Calculator
What hot water costs on each type of heater
Compare a heat pump water heater, an electric resistance tank, a gas tank and a tankless unit side by side on the same household demand — annual running cost, ten-year cost, and how long a switch takes to pay back.
Open the calculator →Appliance Running Cost Calculator
What each thing in your house costs per year
Build a list of what is plugged in, and see the annual running cost of each item ranked worst to best. The usual result is that two or three loads account for most of the bill, and everything else is noise.
Open the calculator →EV Charging Cost Calculator
Cost per mile at home, and against petrol
Work out what charging an electric car at home actually costs per mile and per year, compare it against the petrol car it replaced, and see what an off-peak tariff or your own solar does to the number.
Open the calculator →Why these exist
Every one of these questions — will a heat pump work here, is a battery worth it, how long until solar pays for itself — has an answer that depends on numbers specific to one building. Generic advice cannot give you that answer, and the calculators that could are mostly lead-capture forms wearing a calculator's clothes.
So these show the intermediate steps rather than a single verdict, list every assumption including the ones that make the result look worse, and take no input off your device. When a result is bad news, they say so.
The guides on this site exist to explain what the calculators are doing and where their inputs come from. If you find a figure that is wrong, tell us — corrections are the messages we act on fastest.
Built by Vano Gelashvili
Vano Gelashvili is a software developer who builds the calculators on this site. He is not an HVAC engineer, and says so on every page: what he brings is the arithmetic, worked openly against published data.