Electricity Energy and Running Cost Calculator

Energy Formula: Watts, Hours and Kilowatt Hours

Almost every confusion about electricity bills comes from one distinction: power is not energy. Power says how fast energy is being used; energy is the total that has been used. A meter measures the second one, which is why a wattage on a label tells you almost nothing about a bill on its own.

Power is a rate, energy is a total

A watt is a joule per second. Leave a load running and the joules accumulate in proportion to how long you leave it:

The units follow the multiplication, which is the useful thing to hold on to. There is nothing to remember beyond keeping the time unit consistent: seconds give joules, hours give watt hours, and no conversion factor appears anywhere.

What a kilowatt hour actually is

The kilowatt hour is the unit an electricity supplier bills in, and it is exactly what its name says — one kilowatt, for one hour:

It is a unit of energy, not a unit of power, despite having a "watt" in it. A 2 kW heater is not "2 kWh" — it uses 2 kWh every hour it runs. Getting this the wrong way round makes an appliance look either free or ruinous.

Duty cycle: the number that usually decides the bill

Real equipment does not run continuously, so the total splits into hours per day and a number of days. Dividing by a thousand puts it into billing units:

And the money is one more multiplication:

Which is where the intuition usually fails. A 2 kW heater used for twenty minutes a day costs less to run than a 30 W set-top box left on permanently — 0.67 kWh a day against 0.72 kWh. The hours matter at least as much as the watts, and they are the part nobody puts on a label.

Worked example

Worth comparing against the same load left on permanently: 525.6 kWh and 157.68 — three times the energy, for a part that is doing the same job. Nothing about the 60 W changed. This is why "how many watts does it use" is only half of a running-cost question, and the smaller half.

Watts, Watt Hours, Amp Hours and Milliamp Hours

Four units that get used interchangeably and are not interchangeable. Two are energy, one is charge, one is a rate.

• Watt (W) — a rate. How fast energy is being used, right now.
• Watt hour (Wh) and kilowatt hour (kWh) — energy. A total, and what a bill is calculated from.
• Amp hour (Ah) and milliamp hour (mAh) — charge, not energy. Useless for comparing batteries until you know the voltage.

Converting mAh to watt hours

A battery labelled 3000 mAh tells you how much charge it holds, and charge only becomes energy when multiplied by a voltage:

So a 3000 mAh cell at 3.7 V holds 11.1 Wh, and a 3000 mAh pack at 14.8 V holds 44.4 Wh — four times as much energy for the same headline number. Comparing two batteries by mAh alone is comparing them by a number that omits the voltage.

Duty Cycle and Runtime: Why a Fridge Is Not a 120 W Load

A refrigerator’s badge says 120 W, it is plugged in for all 24 hours, and it does not use 2.88 kWh a day. The badge is the power its compressor draws while the compressor is running, and the compressor is not running most of the time — it cycles on to pull the cabinet back down to temperature and then stops.

That is two separate numbers, and collapsing them into one is the commonest way a household estimate comes out wrong by a factor of three:

• Runtime — how many hours a day the appliance is scheduled to be available. For a fridge that is 24; for a kettle it is about ten minutes.
• Duty cycle — the fraction of that time it is actually drawing its rated power. For a fridge that is perhaps 30 to 40 %; for a kettle it is 100 %.

The energy is the product of both

So the fridge is 120 W × 24 h × 0.35 = 1.008 kWh a day, not 2.88. The kettle at 2 kW for ten minutes at 100 % duty is 0.33 kWh a day — a third of the fridge, from an appliance with sixteen times the wattage.

Where to find your own duty cycle

Anything thermostatic cycles: fridges, freezers, air conditioners, immersion heaters, aquarium heaters. The honest figure depends on ambient temperature, insulation and how often the door is opened, so treat 30 to 40 % as a starting point for a fridge in a normal kitchen and check it against a plug-in energy meter if the answer matters. Anything with a motor that runs continuously while switched on — a fan, a pump on a timer, a television — is 100 %, and the runtime column is doing all the work.

A modern inverter appliance blurs this: it does not switch between full power and off, it modulates. For those, the useful figure is the average power over a full cycle at 100 % duty, which is what an energy label’s annual kWh rating is derived from in the first place.

Peak Demand and Energy Are Different Questions

Add up every nameplate in a house and you get a number in kilowatts. Add up every appliance’s daily energy and you get a number in kilowatt hours. Neither can be derived from the other, and they answer different questions.

• Peak power sizes the hardware — the supply, the main breaker, the generator, the inverter, the cable. It is what happens if everything runs at once.
• Energy sizes the bill. It is what happens over a month, and a load that is on for six minutes a day barely appears in it.

A 2 kW kettle is a third of a domestic supply’s capacity and almost nothing on the bill. A 5 W router is invisible to the supply and costs more per year than the kettle. Sizing a battery or a generator from an annual kWh figure gives something that trips the moment two things start together; estimating a bill from a peak figure gives a number several times too high.

Flat, Stepped and Time-of-Use Tariffs

A single price per kilowatt hour is the simplest tariff and increasingly not the one people are on. Three structures cover almost every retail bill.

Flat rate

One price for every unit. Cost is energy times price, and nothing about when or how fast you used it matters. This is what the calculator above works in, so on either of the other two structures enter the rate you actually pay for most of your consumption and read the answer as an estimate.

Stepped blocks

The first so many kilowatt hours are charged at one rate and everything beyond them at another, usually higher. The trap is that the blocks are an allowance per BILLING PERIOD and they reset: a tariff whose first block is 1000 kWh a quarter gives you that block four times a year, so a year of consumption is four trips up the ladder rather than one. Running a year’s energy through the blocks once overstates the bill, often badly.

Time of use

The price depends on the hour. A day is divided into windows — commonly a peak of a few hours in the evening, an off-peak overnight, and a shoulder for the rest — each with its own rate, and the peak rate is often three times the off-peak one.

On a time-of-use tariff the question stops being how much energy you use and becomes when you use it. Moving a dishwasher, a washing machine or an electric vehicle charger from the peak window to the overnight one changes the bill without changing a single kilowatt hour. Loads you cannot move — a fridge, a router, standby draw — spread across the whole day, so they are charged at roughly the average of the windows weighted by how long each one lasts.

Two things this arithmetic deliberately leaves out, because they are fixed costs rather than costs of running something: the daily supply charge, which you pay whether or not you use any energy, and any feed-in credit from solar generation. Add both to the figure here to reconcile against an actual bill.

Standby Power: Small Numbers, Very Long Hours

The clearest demonstration that hours beat watts. A device drawing 2 W in standby sounds like nothing, and it is on for every hour of the year:

One such device is trivial. A household with twenty of them — chargers, set-top boxes, televisions, printers, smart speakers, anything with a clock or a remote receiver — is spending several hundred kilowatt hours a year on equipment that is switched off. That is the arithmetic behind every standby-power regulation.

It also explains why a 0.5 W improvement in a standby circuit is worth engineering effort that would be absurd for a 0.5 W saving in an appliance used ten minutes a day. The multiplier is 8766, and it applies to whichever number is small.

Where the Energy Actually Goes in a Circuit

Every resistor in a circuit is a small heater, and the energy it takes is real energy that was paid for. From the power law:

Two consequences worth carrying around. First, the square: a linear regulator dropping 12 V to 5 V at 1 A turns 7 W into heat, and doubling the current makes that 14 W in the pass element while the load only doubles — which is why switching regulators exist. Second, the resistances you did not design in count too: cable, connectors, fuse holders and switch contacts all take their share, and unlike a resistor they are not on the schematic.

On a long run the wiring can be a genuine line item. A cable dissipating 20 W continuously is 175 kWh a year going into warming a conduit.

Batteries: the Same Sum, Run Backwards

Runtime is this calculation inverted. Instead of multiplying power by time to get energy, divide the energy a battery holds by the power a load takes:

An 11.1 Wh cell driving a 1.5 W load gives 7.4 hours in theory. In practice expect meaningfully less: the usable capacity stops at a cutoff voltage rather than at empty, the cell’s internal resistance turns some of the energy into heat inside the battery, cold weather reduces what is available, and a converter between the two is perhaps 85 to 90 % efficient.

Treat the arithmetic as an upper bound, then take 20 to 30 % off it. That is not pessimism, it is the difference between the energy stored and the energy you can get out at a voltage the load will accept.

Common mistakes

• Treating kWh as a unit of power. It is energy: a 2 kW heater uses 2 kWh per hour of running, and the two numbers being similar is a coincidence of the unit.
• Comparing batteries by mAh without the voltage. mAh is charge; energy is mAh × V, so a 3000 mAh pack at 14.8 V holds four times what a 3000 mAh cell at 3.7 V does.
• Using the power supply’s rating instead of the actual draw. A 90 W laptop charger delivers 90 W at full load and a fraction of that most of the time.
• Forgetting the hours. A high-wattage appliance used briefly often costs less to run than a low-wattage one left on permanently.
• Mixing time units. Watts times hours gives watt hours; watts times seconds gives joules. Mixing them moves the answer by a factor of 3600.
• Ignoring standby. Twenty devices at 2 W each is 350 kWh a year for equipment nobody is using.
• Charging a thermostatic appliance at 100 % duty. A fridge is plugged in for 24 hours and its compressor runs for perhaps eight of them; using the nameplate for all 24 overstates it by nearly three times.
• Running a whole year through a stepped tariff’s blocks once. The blocks reset every billing period, so a quarterly first block is four blocks a year, not one.
• Sizing a supply, an inverter or a generator from an annual kWh figure. That is energy; hardware is sized on peak power, and the two are unrelated numbers.

Frequently asked questions

How do you convert watts to kWh?

Multiply the watts by the number of hours it runs, then divide by 1000. A 60 W load for 8 hours is 480 Wh, which is 0.48 kWh. Watts alone cannot be converted — you need a duration, because watts are a rate and kWh is a total.

What is the difference between a watt and a watt hour?

A watt is how fast energy is being used; a watt hour is how much has been used. The same distinction as speed and distance: 60 km/h tells you nothing about how far you went until you know for how long.

How much does it cost to run a 60 W appliance?

At 8 hours a day for a year that is 175.2 kWh; multiply by your price per kWh. At 0.30 per unit it is about 52.56 a year. Run continuously the same appliance uses 525.6 kWh and costs three times as much.

What is a kilowatt hour in joules?

3.6 million joules. One kilowatt is 1000 joules per second, and an hour is 3600 seconds, so 1 kWh = 1000 × 3600 = 3.6 MJ.

How do I work out my standby power cost?

Multiply the standby watts by 8766 — the hours in a mean year — and divide by 1000 for kWh. A 2 W standby draw is 17.5 kWh a year. Then multiply by your tariff, and by the number of devices doing it.

How do I convert mAh to Wh?

Multiply the mAh by the nominal voltage and divide by 1000. A 3000 mAh cell at 3.7 V is 11.1 Wh. Without the voltage the mAh figure cannot be compared between batteries of different chemistries or pack configurations.

What duty cycle should I use for a refrigerator?

Around 30 to 40 % for a fridge in a normal kitchen, meaning the compressor runs eight to ten hours out of the 24 it is plugged in. A 120 W fridge at 35 % duty is 1.008 kWh a day, not the 2.88 kWh its nameplate would suggest. Hotter rooms, a full freezer compartment and a door opened often all push the figure up.

What is the difference between duty cycle and runtime?

Runtime is how many hours a day the appliance is scheduled to be on; duty cycle is the fraction of those hours it actually draws its rated power. A television is 4 hours at 100 %; a fridge is 24 hours at 35 %. Energy is power times runtime times duty, so both are needed and neither can be inferred from the other.

How does a time-of-use tariff change what I pay?

The rate depends on the hour, so the same kilowatt hour costs perhaps three times as much in the evening peak as it does overnight. Moving a shiftable load — a dishwasher, a washing machine, an EV charger — out of the peak window lowers the bill without changing your consumption at all. Loads you cannot shift are charged at roughly the average of the windows, weighted by how long each one lasts.

Do stepped tariff blocks reset every quarter?

Yes — a block is an allowance per billing period, not per year. If your first block is 1000 kWh a quarter you get it four times a year, so a year of consumption climbs the ladder four separate times. Running a whole year through the blocks once overstates the bill.

What is the difference between peak power and energy consumption?

Peak power is what everything running at once would draw, in kilowatts, and it sizes the supply, the breaker or the generator. Energy is what accumulates over time, in kilowatt hours, and it sizes the bill. A 2 kW kettle is a large share of the peak and almost nothing on the bill; a 5 W router is the reverse.

Does this include the daily supply charge?

No. The figures here are the cost of running the loads you have entered. A retail bill also carries a fixed daily supply charge that you pay regardless of consumption, and may credit solar export separately. Add both to reconcile against an actual bill.

Does a device use energy when it is switched off?

If it has a remote receiver, a clock, a network connection or an indicator, yes — typically 0.3 to 3 W. Only a device disconnected at the wall or behind a hard switch uses nothing. Over a year those small draws are the ones that add up.