Frequently Asked Questions
How do you calculate wattage?
Multiply volts by amps for a DC circuit or a resistive AC load. For AC motors and electronics, multiply volts × amps × power factor. If you only have BTU/h, divide by 3.412142. The calculator accepts all three input methods and produces the same effective wattage for the rest of the calculation.
How many watts does a refrigerator use?
A modern fridge draws 100 to 200 W while the compressor runs, but it cycles at about 35% duty. Average draw works out to 35 to 70 W continuously. Older fridges can peak at 300 W or higher. Enter the rated wattage and set duty cycle to 35% for an accurate yearly figure.
How much does it cost to run a 1,500 W space heater?
At £0.24 per kWh, running a 1,500 W heater for 4 hours a day costs about £1.44 daily, £43.20 monthly, and £518 yearly if used year-round. Winter-only use over 5 months drops the yearly figure to around £216. Space heaters are among the most expensive appliances per hour of use.
How many watts does an air conditioner use?
A window AC draws 900 to 1,500 W. A split system for a whole room sits at 1,500 to 2,500 W. Central air pushes 3,000 to 5,000 W. The BTU/h input method converts cooling capacity to watts, but real AC electrical draw is lower than the raw conversion suggests because the unit moves heat rather than generating it.
What is a duty cycle on an appliance?
Duty cycle is the percentage of time an appliance draws full power while switched on. A fridge cycles at about 35%. An AC during cooling season sits around 65%. Continuous loads like a TV or desktop PC run at 100%. Entering the right duty cycle is the difference between an accurate estimate and one that’s three times too high.
How do you convert BTU to watts?
Divide BTU per hour by 3.412142. A 12,000 BTU air conditioner equals roughly 3,517 W of cooling capacity. For a heat pump or AC, the electrical draw is typically one-third of that because the unit moves heat rather than creating it. The calculator uses the BTU figure directly for cooling loads.
How much does it cost to run a tumble dryer?
A 3,000 W dryer used twice a week for an hour at 90% duty uses about 281 kWh per year. At £0.24/kWh, that’s £67 annually. Heat pump dryers cut the figure to about £22 because they run at 800 to 1,000 W instead of 3,000. The cost gap pays back the higher purchase price within three to four years.
What uses the most electricity in a home?
Heating and cooling top the list, followed by water heating, then large appliances like fridges and dryers. In most homes, HVAC accounts for 40% to 50% of the bill. Water heating adds another 15% to 20%. Fridges and freezers sit around 7%. Lighting and electronics make up the remainder. The calculator helps you check any single appliance against these averages.
Do appliances use power when switched off?
Yes. Standby power draws 0.5 to 5 W per device in most homes. Across 20 always-on devices, that’s 10 to 100 W continuously, adding 88 to 876 kWh per year. The cost ranges from £21 to £210 depending on your rate. Unplugging unused electronics or using switched power strips eliminates most of this draw.
Why does my energy bill not match the calculator?
Standby loads, variable duty cycles, seasonal usage shifts, and tariff structure all sit outside the calculation. The tool models one appliance at one power level over a fixed schedule. Real households mix appliances, weather, and habits in ways that no single-appliance estimate can capture. Use the calculator to compare appliances against each other, not to predict the whole bill.