Electricity Bill Calculator

What this calculator does

An electricity bill arrives as one number, which is useless for deciding what to do about it. This calculator breaks it down to the level where decisions actually get made: what one appliance costs to run.

You pick an appliance or type in its wattage, say how many hours a day it runs, and enter what you pay per unit of electricity. It returns the cost per day, per month and per year. Run it a few times for the things you use most and you have something far more actionable than a monthly total: a ranked list of what is actually consuming your money.

Watts, hours, and the unit you are actually billed in

Electricity is sold by the kilowatt hour, and the whole calculation is about converting an appliance into that unit.

A watt is a rate: how fast a device draws power at any instant. A kilowatt hour is a quantity: one thousand watts drawn for one hour. So a 1000 watt heater running for an hour uses exactly one kilowatt hour, and so does a 100 watt bulb left on for ten hours. The bill does not care which of those you did.

That gives the formula the calculator runs:

Daily kilowatt hours = Watts × Hours per day ÷ 1000

Multiply by your rate per unit and you have the daily cost, which is then scaled to a month using 30.42 days, the average month length across a year, and to a year using 365. The single thing to take from the formula is that watts and hours sit on equal footing. Doubling the wattage and doubling the running time have exactly the same effect on the bill, and that symmetry is the source of the most common mistake people make about their own electricity use.

Filling it in, including that currency box

1. Typical appliance. Choosing from the list fills in a representative wattage automatically. It is a starting estimate rather than a fact about your specific machine.
2. Power consumption in watts. Better to override the default with your own appliance's figure, usually printed on a label on the back or underneath, or in the manual.
3. Hours of use per day. Actual running hours. See the section below before entering anything that is plugged in permanently.
4. Cost per kilowatt hour, with a unit selector.

That last box deserves a word, because it behaves differently depending on what you choose. Pick cent or pence and the calculator assumes you are entering a rate in sub-units and converts the answer up, so a rate of 15 cents returns a result in dollars. Pick rupees or pesos and it assumes the rate is already in main units, so entering 8 returns a result in rupees. Either way the output is labelled with the main currency. Enter the rate in the unit you selected and the answer will be right; the mismatch to watch for is typing a rate in dollars while the selector says cent.

One more note on the wattage. The number printed on an appliance is its maximum draw, not its typical one. Anything with a thermostat or a motor that cycles will average less than its nameplate figure, so treat the result as an upper estimate unless you have measured the device with a plug-in monitor.

The biggest number on your bill is rarely the biggest appliance

Ask most people which appliance costs them the most and they will name the one that feels most powerful. It is usually the wrong answer, and the arithmetic shows why. Take three devices, with electricity at 8 rupees per unit.

• Electric kettle, 2000 watts, 15 minutes a day. 0.50 units a day, costing 4.00 a day and about 1,460 a year.
• Television, 100 watts, 5 hours a day. 0.50 units a day, costing 4.00 a day and about 1,460 a year.
• Refrigerator, 150 watts, running roughly 8 hours a day. 1.20 units a day, costing 9.60 a day and about 3,504 a year.

The kettle draws twenty times the power of the television and costs precisely the same, because it runs for a twentieth of the time. And the appliance that costs most of the three is the one with the second-lowest wattage, because it never really stops.

This is the practical lesson worth carrying away. High-wattage devices used briefly, kettles, toasters, irons, microwaves, are rarely where the money goes. The bill is dominated by whatever combination of wattage and hours is largest, which in most homes means heating, cooling, water heating and refrigeration: things that are either very powerful and used for hours, or modest and used constantly. An air conditioner at 1500 watts for six hours uses 9 units a day, more than seven times the fridge, which is why cooling dominates a summer bill in a way no amount of unplugging phone chargers will offset.

The one appliance you cannot enter honestly at 24 hours

Refrigerators break the calculator if you are too literal with them, and it is worth knowing the fix.

A fridge is plugged in all day, so the instinct is to enter 24 hours. But it does not draw its rated wattage for 24 hours. Its compressor cycles on and off to hold the temperature, running only when needed, and how often depends on the room temperature, the door seals, how often it is opened, and how well it is insulated. Enter 24 hours at the nameplate wattage and you will overstate its consumption by roughly three times.

The US Department of Energy suggests a straightforward rule of thumb for this: to approximate the hours a refrigerator actually operates at maximum wattage, divide the time it is plugged in by three. That is where the 8 hours in the example above came from, and it produces a figure in line with the one to two units a day a typical domestic fridge genuinely consumes.

The same logic applies more loosely to anything thermostatically controlled: air conditioners, water heaters, space heaters and freezers all cycle rather than running flat out. If you want a genuinely accurate figure for one of these rather than an upper bound, a plug-in energy monitor left in place for a day or two will measure the units directly and remove the guesswork entirely.

Why this will not match your bill exactly

This calculator answers the question "what does this appliance cost me", which is the useful question. It does not attempt to reproduce your bill, and it is worth knowing the difference so the gap does not puzzle you.

Real bills are rarely a single rate multiplied by units. Most utilities apply tiered or slab tariffs, where the first block of units is charged at one rate and later blocks at higher ones, so your marginal unit can cost considerably more than your average unit. Many add fixed charges that you pay regardless of consumption: meter rent, standing charges, connection fees. Then there are taxes, duties and surcharges, and in some places time-of-day pricing that makes electricity cheaper at night and dearer at peak hours.

The consequence is worth understanding rather than working around. If you are on a tiered tariff, the saving from switching an appliance off is the rate on your highest block, not your average rate, so cutting consumption is usually worth more than a simple calculation suggests. For the most realistic estimate, take your rate from the marginal band of your latest bill rather than dividing the total amount by the total units.

And there is one form of consumption this tool cannot see at all: standby power. Televisions, computers, set-top boxes and chargers draw a small amount continuously even when switched off, which the Department of Energy calls a phantom load. Each is negligible; a house full of them is not, and none of them appear in an appliance-by-appliance calculation based on hours of use.

Questions people ask

Where do I find my appliance's wattage?

Usually on a label on the back or underneath, or on the nameplate, and otherwise in the manual. Remember it states the maximum draw, so anything that cycles will average less.

How many hours should I enter for a fridge?

Not 24. A common rule of thumb is to divide the plugged-in time by three, giving about 8 hours, because the compressor cycles rather than running continuously.

Which rate should I enter?

The rate per unit from your bill, in the unit you select in the dropdown. If your tariff has tiers, using the rate on your highest band gives the most realistic estimate of what changing your usage would save.

Why is my actual bill higher than these figures?

Because bills include fixed charges, taxes and often tiered rates, and because a household runs many appliances plus standby loads. This tool estimates one appliance at a time on a single rate.

References

The energy an appliance consumes is its wattage multiplied by hours used per day and divided by 1000 to give daily kilowatt hours, which is then multiplied by the utility's rate per kilowatt hour to give the cost. Nameplate wattage represents the maximum power an appliance draws. Because refrigerators cycle on and off rather than running continuously, the US Department of Energy suggests approximating the hours at maximum wattage by dividing the total plugged-in time by three, and notes that many appliances continue to draw standby power, described as phantom loads, when switched off.

1. U.S. Department of Energy, Estimating Appliance and Home Electronic Energy Use. https://www.energy.gov/energysaver/estimating-appliance-and-home-electronic-energy-use
2. U.S. Energy Information Administration, Electricity data and average retail prices. https://www.eia.gov/electricity/