Running cost
Smart plug energy cost calculator
Enter what a device draws while it is working, what it draws while it is pretending to be off, how long it runs each day and the price you actually pay per kWh. You get the daily, monthly and yearly cost, the share of the bill that is pure standby, and how long a smart plug takes to pay for itself once its own power draw is subtracted. The tariff is yours to type, so nothing here goes stale — and all of the arithmetic runs in the browser.
Show the arithmetic behind these numbers
| Period | In-use kWh | Standby kWh | Total kWh | Cost |
|---|
Does the smart plug pay for itself?
| Plug price | Years to pay back | Net saving, first year |
|---|
How the running cost is worked out
There are only a handful of steps, and every one of them is printed back with your own numbers in the “show the arithmetic” panel once you press the button, so you can check it or rebuild it in a spreadsheet:
- In-use energy (kWh a day) = active watts × hours in use ÷ 1,000.
- Standby energy (kWh a day) = standby watts × the remaining hours ÷ 1,000. The remaining hours are simply 24 minus the hours you said the device is in use.
- Cost = total kWh × the tariff you typed. A month is 30.44 days and a year is 365 days, so the monthly figure is an average rather than a calendar month.
- Standby share = standby cost ÷ total cost. This is the number that decides whether a plug is worth buying at all.
- Plug saving = standby watts × the hours a day the plug cuts the power ÷ 1,000 × tariff, less the plug's own draw running for all 24 hours.
- Payback = plug price ÷ net saving per day.
What each result tells you
| Figure | What it is for |
|---|---|
| Cost per day, month and year | The straightforward answer. The yearly figure is the one to compare against the price of a plug, a timer or a newer appliance. |
| Standby share | The percentage of the bill you are paying for a device that is not doing anything. Under about 10% a plug is hard to justify; over 30% it usually is, provided the plug can really cut the supply. |
| Standby cost a year | The standby bill on its own, which is the ceiling on what any plug, timer or switched socket can save you on this device. |
| Net saving per day | The plug's saving after its own consumption. A plug that draws 1 W to remove 1 W of standby saves nothing at all, and the calculator shows that rather than a flattering payback. |
| Payback time | How long the plug has to run before the saved electricity has covered its price. Under a year is usually an easy decision; several years is usually not. |
Getting a standby figure you can trust
The one input most people get wrong is standby power, because the honest number is usually higher than the one on the box:
- Measure it. A plug-in energy monitor or a smart plug that reports power will show the draw after the device has been off overnight. Ten minutes of “off” is not enough — capacitors and quick-start modes take a while to settle.
- Watch for features that keep a device awake. A television with “fast start”, a console in rest mode and a desktop whose USB ports stay live all hold a real load. Switching those features off in the device's own settings costs nothing and often saves more than the plug's own draw.
- Standby is not the same as off. If the device has a physical switch, the switched-off figure is what a plug can deliver — and it is usually lower than standby.
- Check the plug's rating before switching heavy loads. Compact plugs are typically rated 10 A or 13 A at 230 V. Heaters, kettles and anything near the ceiling run the relay hot; use the appliance's own timer instead.
What this calculator cannot tell you
- It does not measure anything. The numbers are only as good as the watts you type, and there is no meter, no API and no utility data behind the page.
- It does not know your tariff structure. Standing charges, day and night rates, peak windows and export payments are all outside the arithmetic — enter the all-in average if you want the yearly figure to match your bill.
- It assumes the device follows the schedule exactly. A plug that is overridden by hand, or a device that wakes itself to update, will drift from the estimate.
- It says nothing about whether a plug is a good idea for a particular appliance. Safety, appliance warranty and the plug's rated load are separate questions from cost.
- Currency is a label, not a conversion. The symbol you type is printed in front of the figures; no exchange rate is applied.
Related guides
If the saving is worth chasing, the next step is usually a schedule rather than a new device: Home Assistant every-5-minutes automation, InfluxDB storage sizing calculator and Zigbee coordinator guide. If you log the plug's power into Home Assistant and keep the history long-term, the storage arithmetic lives in the InfluxDB sizing calculator, which covers long-term history sizing.
Questions people ask
How does this smart plug energy cost calculator work?
Three multiplications. Active energy is active watts times the hours a day the device is switched on, divided by 1,000 to get kWh. Standby energy is standby watts times the remaining hours, divided by 1,000. Both are multiplied by the tariff you type to give cost, and by 365 for the year. The payback figure divides the plug's price by the net saving per day — the standby energy the plug removes, minus the electricity the plug itself uses.
Which standby figure should I enter?
Use a measurement rather than a guess. A plug-in energy monitor or a smart plug that reports power will show the figure after the device has been off overnight — a television that has been unplugged from the wall for ten minutes settles to its true standby draw. Manufacturer numbers are usually taken at the best case, and devices with a quick-start mode, a network connection or a lit LED often draw more than the sticker suggests.
Does the smart plug itself use electricity?
Yes, and the calculator counts it. A Zigbee or Wi-Fi plug typically draws somewhere between 0.3 W and 1 W continuously to keep its radio and relay alive, which is 2.6 to 8.8 kWh a year. The default of 0.5 W is deliberately in that band; if your plug reports its own power draw, put the real number in and watch the net saving fall.
Why do I type the tariff instead of picking a country?
Because a hardcoded price goes stale the moment your supplier changes a rate. Enter the figure from your own bill — the all-in price per kWh including standing charges and taxes if you want the number to match what leaves your account. The page never fetches a tariff, a meter reading or a price from anywhere.
When does a smart plug pay for itself?
Only when the standby power it removes is worth more than the electricity the plug consumes. That happens with devices that draw several watts around the clock — a television, a console in quick-start mode, a desktop with a live USB hub. On a device that draws nothing once it is off, or on a tariff of a few cents, the payback figure is never reached and the calculator says so instead of inventing one.
Can I switch a heater or another high-draw appliance with a plug?
Only inside the plug's rating. Most compact smart plugs are rated 10 A or 13 A at 230 V, roughly 2,300 to 3,000 W, and some are 16 A. A heater near that ceiling runs the relay hot and shortens its life; check the rating printed on the plug before you switch anything heavy with it. The appliance presets in this tool describe running cost only — they are not a statement that a given plug is safe for that load.
Does anything I type leave my device?
No. The watts, hours and tariff stay in the page's memory while you use it and are discarded when you close the tab. The only files the page produces are the ones you ask for: the copied summary and the CSV you download, both built in the browser from the same numbers.