Your electricity bill is the sum of every appliance’s usage, but the bill never shows the split. Calculating it appliance by appliance takes ten minutes and usually reveals one or two surprises. The unit that matters is the kilowatt-hour, kWh.
The formula
Energy (kWh) = power (watts) x hours used / 1,000. A 1,500-watt heater running 2 hours uses 1,500 x 2 / 1,000 = 3 kWh. Cost = energy x your price per kWh. At $0.17 per kWh, those 3 kWh cost $0.51. Find the wattage on the appliance label, in the manual, or with a plug-in meter for older appliances whose labels have faded.
For appliances that cycle on and off, like fridges and heaters with thermostats, the label wattage is the running power, not the average. A fridge rated 150 watts does not use 150 watts for 24 hours. Either use the yearly kWh figure from its energy label, or measure with a plug meter for a week and scale up.
Worked example: one evening
Take a living room evening: a 100-watt TV for 3 hours (0.3 kWh), a 60-watt lamp for 4 hours (0.24 kWh), a 1,500-watt heater for 2 hours (3 kWh), and a 400-watt games console for 1 hour (0.4 kWh). Total: 3.94 kWh. At $0.17 per kWh that evening costs about $0.67. The heater alone is 76% of it, which is the usual pattern: anything that makes heat or cold dominates.
Scale the same list to a month. If that evening repeats daily, the heater alone is 3 x 30 = 90 kWh, about $15.30 a month. A 2,000-watt water heater running 1 hour a day is 60 kWh a month. Small electronics, phone chargers and LED lamps barely register beside them.
Finding the hidden hogs
Old fridges, electric water heaters, tumble dryers, and pool pumps are the classic hidden hogs. A dryer at 2,500 watts for 1 hour per load uses 2.5 kWh per load; three loads a week is 30 kWh a month. Standby power is real but usually small: a TV on standby at 1 watt costs about 0.7 kWh a month, so hunt the big loads before unplugging everything at night.
Work through the house room by room and list each appliance once, with watts and daily hours. Rank the list by monthly kWh. The top two or three rows are where any saving effort belongs; the bottom rows are not worth changing your habits for.
From kWh to your bill
Add the monthly kWh for all appliances, then multiply by your tariff rate from your bill. Watch for tiered tariffs, where usage above a threshold costs more per kWh, and for separate day and night rates if you have them. If your calculated total is far from the billed total, check for an appliance you missed, a tariff change, or an estimated meter reading.
Estimating a whole-house bill
Add the categories to estimate a monthly bill. Example household: heating 300 kWh, water heating 150 kWh, cooking 100 kWh, laundry and drying 60 kWh, electronics and entertainment 80 kWh, lighting 40 kWh. Total: 730 kWh. At $0.17 per kWh that is $124.10 for the month, before any standing charge or taxes.
Compare that estimate with your actual bill. If the bill is much higher, suspect an appliance you missed, an old fridge, electric heating you forgot, or an estimated meter reading. If it is much lower, your usage assumptions are probably generous. Either way, the category breakdown tells you where to look first.
Try it faster with the tool
You can follow every step above by hand, but the free Online Energy Consumption Calculator does the arithmetic for you. Enter your numbers and it gives the answer right away, so you can check your hand calculation or skip straight to the result.
Quick tips
- kWh = watts x hours / 1,000.
- Heat and cold appliances dominate; chargers barely register.
- Rank appliances by monthly kWh and attack the top rows.