Appliance Running Cost Calculator

What this calculator does

It turns one appliance’s rating plate and your usage pattern into energy, money and carbon — per day, week, month and year. Two inputs make it more honest than the usual watts × hours arithmetic:

• Standby power, counted on all seven days of the week, because equipment left plugged in draws it whether you used it that day or not.
• Duty cycle, because a fridge is “on” 24 hours a day and its compressor is not.

The tariff and the grid carbon intensity are yours to supply. Nothing is hardcoded, so the tool works on any tariff, in any currency, on any grid.

The formula

E_active  = P × h × δ / 1000            kWh per day of use
E_standby = P_sb × h_sb / 1000          kWh per calendar day

E_week    = E_active × D + E_standby × 7
E_year    = E_week × 52
E_month   = E_year / 12

cost      = E × t
CO₂e (kg) = E_year × I / 1000

• P — rated input power from the plate, W. If the plate gives amps and volts, multiply them.
• h — hours the appliance is switched on, on a day it is used
• δ — duty cycle, the fraction of switched-on time it draws rated power
• P_sb, h_sb — standby power and the hours per day spent in standby
• D — days used per week
• t — your tariff per kWh; I — your grid’s carbon intensity in g CO₂e/kWh

The year is taken as exactly 52 weeks, because usage is entered per week; the month is a twelfth of that year rather than a calendar month.

Reading the result

Nameplate power is a ceiling, not a reading. The plate gives the maximum the appliance may draw — what the circuit and the plug have to be sized for. Very few appliances sit at it. A 2000 W hair dryer on a low setting draws 800 W. A “1500 W” microwave draws that from the wall to deliver about 1000 W of cooking power, and only while the magnetron is on. A gaming PC rated at 750 W idles near 60. Where you know the appliance modulates, either lower the rated power or use the duty cycle — but not both, or you will halve it twice.

Duty cycle is the dominant term for anything thermostatic. Fridges, freezers, air conditioners, heat pumps and heaters run to hold a setpoint, then stop. A domestic fridge typically runs its compressor 15–40% of the time; the figure rises with a warm kitchen, a full-open door habit, or a failing door seal. Enter such an appliance at 100% duty and you will overstate its cost by a factor of two to six — which is exactly why fridges appear catastrophically expensive on calculators that omit the term. This tool warns when a load of 100 W or more is entered as running at full rated power for 12 hours a day or longer, because that combination is nearly always a thermostatic appliance entered wrongly.

Standby is small, continuous and therefore easy to miss. Watts multiplied by 8760 hours a year is the whole trick: a 10 W standby draw is 87.6 kWh a year, more than many appliances use in active operation. Modern equipment sold under ecodesign-style rules is usually under 1 W, but set-top boxes, games consoles in “instant on” mode, AV receivers, older printers and anything with a network connection it keeps alive can sit at 5–20 W indefinitely. The tool flags standby above 10% of the appliance’s annual energy, because at that point a switched socket is the cheapest efficiency measure available to you.

Typical values

Rough figures to sanity-check what you enter — measure yours rather than trusting these:

Grid carbon intensity spans roughly 20 g CO₂e/kWh on a hydro- or nuclear-dominated system to over 700 g on a coal-heavy one, and moves hour by hour. Take the figure from your own grid operator or national inventory.

Worked example

A television and set-top box: 100 W combined while watching, on 4 hours a day, 7 days a week, at a 100% duty cycle. The pair draw 10 W in standby for the other 20 hours a day. The tariff is 0.30 per kWh and the grid runs at 400 g CO₂e/kWh.

active/day  = 100 × 4 × 1.00 / 1000 = 0.4 kWh
standby/day = 10 × 20 / 1000        = 0.2 kWh
per week    = 0.4 × 7 + 0.2 × 7     = 2.8 + 1.4 = 4.2 kWh
per year    = 4.2 × 52              = 218.4 kWh

At 0.30 per kWh that is 65.52 a year, or 5.46 a month.

Now the part that is worth knowing: the standby portion is 0.2 × 7 × 52 = 72.8 kWh, which is 21.84 a year — 33% of the total — spent while the equipment is switched off. Annual emissions are 218.4 × 400 / 1000 = 87.36 kg CO₂e, of which about 29 kg is standby.

A switched socket, or a smart plug on a schedule, removes that third of the bill outright.

FAQ

How do I measure real consumption instead of estimating it? Use a plug-in energy meter. Leave it in place for a full week — not an hour — so it captures whole thermostat cycles, wash programmes and idle periods, then read the accumulated kWh. Divide by 7 for a daily figure and enter it as power = kWh/day × 1000 / 24 with 24 hours and a 100% duty cycle. For standby, put the appliance in standby and read the wattage directly; if the meter reads 0 W it may simply be below its resolution, so use the kWh accumulator over a day instead.

What duty cycle should I use for a fridge? Start at 30% and adjust. A fridge in a cool room, well filled, with good seals, runs nearer 15–20%; one in a hot kitchen or with a failing seal can exceed 50%. If the appliance has an energy label quoting annual kWh, work backwards: duty = label kWh × 1000 / (rated W × 8760).

Why is the year 52 weeks rather than 365 days? Because usage is entered per week, and 52 whole weeks is 364 days. The difference is 0.3% — far smaller than the uncertainty in your duty cycle — and it keeps the weekly, monthly and annual figures exactly consistent with each other.

Does this handle time-of-use or block tariffs? Not directly. Enter the rate that actually applies to when the appliance runs — the peak rate for an evening-only appliance, the off-peak rate for an overnight one, or a usage-weighted average for something that runs all day. For a block tariff, use the marginal rate of the block you are in, since extra consumption is charged there.

Should the standby hours and the hours in use add up to 24? Usually, yes — an appliance is either in use or in standby. If it is unplugged or switched off at the wall, set standby hours to 0. The tool warns if the two together exceed 24, which means one of them is being counted twice.

Indicative figures based on the ratings and usage you enter. Actual consumption varies with load, ambient temperature, settings and appliance condition; a plug-in energy meter left in place for a week is always more accurate than any calculation.