What this calculator works out
This calculator multiplies the number of lights by their wattage and the hours they run each night, giving annual consumption in kilowatt-hours.
Garden lighting is individually small and collectively persistent — it runs every night for years, which is what turns a few watts into a meaningful figure.
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
- Use actual wattage from the fitting, not an equivalent figure. A light described as '50 W equivalent' may draw 5 W.
- Hours per night should reflect real use. Lights on a timer from dusk to midnight run far longer in December than June.
- The calculation assumes the same hours every night of the year, which overstates lights used only in summer.
- Solar lighting draws nothing from the mains, though the batteries have their own lifespan and cost.
A traditional 50 W halogen garden spot running four hours a night uses 73 kWh a year. An LED giving similar light draws around 5 W and uses 7 kWh. Eight of them is the difference between roughly £145 and £15 a year at typical rates — which is why replacing halogen garden lighting pays back very quickly.
How this calculator works
Watts times hours times days, converted to kilowatt-hours:
kWh per year = lights × watts each × hours per night × 365 ÷ 1,000Dividing by 1,000 converts watt-hours to kilowatt-hours, which is the unit electricity is billed in. Multiply the result by your unit rate for an annual cost.
Worked example: eight LED lights
Using the default figures — 8 lights at 5 W each, running 4 hours a night:
- Annual consumption: 58.4 kWh
- At 26.11p per kWh: about £15.25 a year
Around fifteen pounds a year for eight lights every night. The same installation in 50 W halogen would draw 584 kWh and cost around £152 — ten times as much for no more light. Where old halogen fittings are still in place, replacing them is one of the shorter paybacks available in a garden.
Common mistakes
- Using the equivalent wattage. The actual draw is what is billed.
- Assuming the same hours year-round. Dusk-to-midnight is far longer in winter.
- Leaving lights on all night. A timer or sensor cuts consumption without affecting usefulness.
- Using indoor fittings outdoors. They need an appropriate IP rating for wet conditions.
- Running mains cable without protection. Outdoor circuits need RCD protection and proper burial or armoured cable.
Frequently asked questions
Is outdoor electrical work notifiable?
In England and Wales, installing a new outdoor circuit is notifiable under Approved Document P and must be carried out or certified by a competent person. Plugging a low-voltage transformer into an existing protected outdoor socket generally is not. Rules differ in Scotland and Northern Ireland. Any outdoor circuit needs RCD protection.
What is an IP rating?
A two-digit code for protection against solids and water. For garden lighting, IP44 is a minimum for anything exposed to rain, IP65 or above for anything that may be hosed or sit in a wet position, and IP68 for anything submerged such as pond lights. Indoor fittings used outside fail quickly and dangerously.
Are solar lights worth it?
For gentle path and feature lighting, often yes — no wiring, no running cost and easy to reposition. They are dimmer than mains lighting, perform poorly in a shaded position or a wet winter, and the rechargeable batteries typically need replacing every couple of years. They are not a substitute for security or task lighting.
Does garden lighting affect wildlife?
Yes. Artificial light at night disrupts moths and other nocturnal insects, affects bat foraging and can disturb hedgehogs and nesting birds. Using warm-toned rather than blue-white light, directing it downwards, keeping levels low and running lights only when needed all reduce the impact substantially.
Is what I enter stored?
No. Figures are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — Approved Document P and rules on outdoor electrical work
- Health and Safety Executive — electrical safety outdoors, RCD protection and cable burial
Sources are checked at publication and can change — how I choose and check references.