Standby electricity cost:
formula and worked example

What standby power is and why it adds up

Many devices keep drawing a little power after you have turned them off: a television waiting for a remote signal, a set-top box, a charger left plugged in, a microwave clock. Each draw is a few watts at most, which is why it escapes notice, but it runs around the clock.

This calculation turns the idle wattage into a yearly cost. The method is the same as for an active appliance, but the hours input is nearly the whole day, and that continuous operation makes small loads cost more than people expect.

It is a tool for perspective. A single device on standby will rarely change a bill, but a house full of them can account for a measurable share, and some of that is avoidable with a switch.

The formula and its inputs

• Standby watts is the idle draw, ideally measured with a plug-in meter that resolves fractions of a watt. Nameplates seldom list it.
• Hours daily is how long the device is in standby each day, which is up to 24 for something never unplugged.
• Rate is the tariff per kilowatt-hour in your currency.

Use only the hours actually spent in standby. If a device is active for three hours, its standby time is 21 hours, not 24.

Worked example: an 8 W device left on standby all day

Take a set-top box that draws 8 W when not in use and is never unplugged, at a tariff of 8 per kWh.

1. 1Convert to kilowatts: 8 / 1,000 = 0.008 kW.
2. 2Daily energy: 0.008 x 24 = 0.192 kWh.
3. 3Annual energy: 0.192 x 365 = 70.08 kWh.
4. 4Annual cost: 70.08 x 8 = 560.64 per year.

560 a year from one box is modest, but five such devices bring it to about 2,800.

How much does standby really matter?

Judge the figure against your total bill and against what it costs to remove. A device that costs 560 a year in standby, and which you can switch off at the wall without any loss, is a free saving. A refrigerator or a modem you need running is not a standby problem.

Some devices do useful things in standby, such as keeping a clock, an alarm or a network connection alive, or preserving a hard disk's cache. The real question is whether you need that function while you are away or asleep.

• Power strips with a switch make it easy to cut a whole group of devices at once.
• Newer devices tend to draw less in standby than older ones, so measure rather than assume.

What a 10% higher standby draw does

If the device idles at 8.8 W rather than 8 W, the annual energy goes from 70.08 to 77.088 kWh and the cost from 560.64 to 616.704, a change of exactly 10%.

The relationship is linear. Doubling the idle draw doubles the cost, so a poorly designed device at 16 W costs twice a good one at 8 W over the same year.

Mistakes when estimating standby cost

• Assuming standby is always 24 hours when the device is in use for part of the day, which overstates the cost slightly.
• Using the rated active power instead of the idle power, which can inflate the figure many times over.
• Reading a meter that cannot resolve below a watt and getting zero for a device that draws half a watt.
• Ignoring the number of devices: the cost of one is small, and the argument is about the total in the house.

A related trap is to count devices that are meant to be on, such as a fridge, as standby loads.

Putting the numbers to work

Walk through the house and list devices that stay plugged in. Measure each with a meter for a few minutes, record the watts, and add them. The total tells you the base load, the electricity used when nothing is apparently on.

Then decide which devices can be switched off at the wall each night, and which need to stay on. Small habits applied to the biggest items bring most of the saving.