What this calculator works out
This calculator separates active and idle power, applies the hours spent in each, and returns the daily energy along with monthly and annual cost.
Separating the two states is what makes the figure realistic. Most machines spend far more time idle than working, and idle draw is what a always-on system actually costs.
Enter your details
Default unit rates are the Ofgem price cap average for Great Britain, Direct Debit, 1 July to 30 September 2026, including VAT. The cap is reviewed quarterly — use the rate from your own bill where it differs.
Before you calculate
- Measure with a plug-in energy monitor rather than using the power supply's rating. A 750 W supply does not draw 750 W.
- Active power means under real load, not synthetic maximum. A desktop gaming under load and the same machine editing a document differ enormously.
- Include the monitor, which is frequently 20 to 50 W and is often left out.
- Sleep and hibernate draw a small amount that is not zero — commonly 1 to 5 W.
A machine drawing 250 W for four hours and 60 W for twenty hours uses 1.0 kWh actively and 1.2 kWh idling — more energy doing nothing than doing something. For a server or a machine left on continuously, reducing idle draw or enabling sleep saves more than any change to how it is used.
How this calculator works
Two states summed, then priced:
Daily kWh = (active watts × active hours + idle watts × idle hours) ÷ 1,000
Daily cost = kWh × price in pence ÷ 100
Monthly = daily × 30 · Annual = daily × 365The calculation covers the hours entered. Where the two do not total 24, the remaining hours are treated as drawing nothing — which is correct for a machine switched off and wrong for one asleep.
Worked example: a desktop
Using the default figures — 250 W active for 4 hours, 60 W idle for 4 hours, at 26.11p per kWh:
- Daily energy: 1.24 kWh
- Monthly cost: £9.85
- Annual cost: £118.18
£118 a year for eight hours a day. Leave the same machine idling the other sixteen hours and it adds 0.96 kWh daily — another £91 a year for a computer nobody is using. Setting a sleep timer is a two-minute configuration change worth roughly what a mid-range component costs.
Common mistakes
- Using the power supply rating as the draw. It is a maximum capability, not consumption.
- Forgetting the monitor. It can be a third of the total.
- Leaving a machine on all night unnecessarily. Idle hours accumulate quickly.
- Assuming sleep draws nothing. It is small but not zero.
- Using a national average unit rate. Take it from a recent bill.
Frequently asked questions
How much does a computer cost to run?
It varies by an order of magnitude. A laptop drawing 20 to 45 W costs a few pounds a year in typical use; a gaming desktop drawing 400 W under load can cost well over £200 if used heavily. The state a machine spends most of its time in matters more than its peak capability.
Is it better to sleep or shut down?
For overnight, either is fine — sleep draws a few watts and resumes instantly, shutdown draws nothing and takes longer to start. The old concern about wear from power cycling is not a meaningful factor on modern hardware. For anything longer than a night, shutting down is the clearer choice.
Do screensavers save power?
No, and they never did on modern displays — a screensaver keeps the screen on and the graphics hardware working. Turning the display off after a period of inactivity is what saves power, and it is a separate setting from the screensaver in most operating systems.
What about leaving a machine on for updates or backups?
Wake timers let a machine sleep and wake at a scheduled time to run them, which is the best of both. Most operating systems support this and it is worth configuring rather than leaving a machine running continuously for the sake of a nightly job that takes twenty minutes.
Is what I enter stored?
No. Figures are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — energy labelling, efficiency standards and help with energy costs
- MoneyHelper — guidance on reducing household energy bills
Sources are checked at publication and can change — how I choose and check references.