Working out what something costs to run is nearly always the same calculation: find the rate, find the quantity, multiply. The difficulty is that utilities quote rates in units nobody uses conversationally, and hide a fixed charge underneath that has nothing to do with how much you use.
Electricity: one equation, two charges
Electricity is billed in kilowatt-hours. A kWh is a kilowatt sustained for an hour, so a 2 kW heater running for three hours uses 6 kWh, whatever it achieves in the room.
The formula is just kWh = watts ÷ 1000 × hours, then multiply by your unit rate. A 60 W bulb left on all day uses 1.44 kWh; a 3 kW kettle boiling for three minutes uses 0.15 kWh. The electricity cost calculator does this per appliance, per day or per year.
The standing charge is separate. Most tariffs add a fixed daily amount regardless of usage, which is why a bill is never zero even for an empty house — and why comparing tariffs on unit rate alone is misleading. A low unit rate with a high standing charge can cost more if you use little.
What the label on the appliance actually means
The wattage printed on a device is normally its maximum draw, not its average. This matters most for anything that cycles. A fridge rated at 150 W does not draw 150 W continuously — its compressor runs perhaps a third of the time, so the real figure is far lower than multiplying by 24 suggests.
Heating elements are the honest ones: kettles, toasters, immersion heaters and electric heaters draw close to their rating whenever they are on. As a rule, anything that makes heat is expensive to run and anything with a chip in it is not. Standby power is real but small — worth tidying up, not worth agonising over.
If you are comparing figures quoted in different units — joules, BTU, kilowatt-hours — the energy converter and power converter handle the translation.
Water, where the pricing model matters more than the usage
Water is billed one of two ways, and which one you are on changes what is worth doing.
Metered billing charges for what you use, usually per cubic metre — 1,000 litres, which is more than it sounds. There is often a separate charge for wastewater on top, calculated from the same reading, so a litre saved can save twice.
Unmetered billing charges a fixed amount based on the property rather than usage. On an unmetered bill, using less water saves nothing at all — the case for saving it is environmental rather than financial.
For scale: a typical shower uses somewhere around 10 litres a minute, a bath 80 litres or so, and a dripping tap can waste several thousand litres a year without ever looking dramatic. The water bill calculator works from household habits rather than asking you to guess a total.
Solar: payback is about what you use, not what you generate
Solar payback is system cost divided by annual saving, and the saving is where the reasoning usually goes wrong.
Electricity you generate and use yourself saves you the full retail rate. Electricity you export is bought back at a much lower one — often a fraction of what you pay to buy it. So the same panels save very different amounts depending on when you use power. A household that is out all day and cooks at night exports most of what it makes; one that runs the washing machine at midday keeps far more of the value.
That is also the real argument for a battery: it shifts generation into the evening, converting cheap exports into avoided expensive imports. The solar savings calculator works through payback from system cost, generation and the share you actually consume.
The costs that are easy to miss
Subscriptions are the clearest example of a bill that grows without any single decision to make it grow. Each one is individually small and monthly, which is exactly why the total surprises people — the right unit to think in is the annual one. The subscription tracker normalises monthly and yearly plans onto the same basis.
Driving has the same shape. Fuel cost for a journey is distance divided by efficiency times price, and the fuel cost calculator handles both mpg and litres per 100 km so you can compare a route, or a car, without converting first.
And in a shop, the only honest comparison between two sizes is price per unit — packaging is designed to make that hard, and the larger box is not reliably cheaper per gram. The unit price calculator settles it in a couple of taps.
Where the money actually is
If you only do one thing: heating, hot water and anything else that turns electricity into heat dominate a household bill. Lighting and electronics are usually a rounding error beside them. Time spent on the thermostat, the hot water schedule and insulation is worth more than time spent unplugging chargers — and it is worth measuring rather than guessing, because intuition about which appliances are expensive is reliably wrong.