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What a grow light costs to run in the UK
A houseplant lamp is one of the cheaper things you can leave on. The bill still deserves the actual watts and the actual hours, not the "300 W equivalent" printed on the box.
The rate used on this page
From 1 October to 31 December 2026, Ofgem's price cap average for a default tariff paid by Direct Debit in England, Scotland and Wales is 26.32 pence per kWh for electricity. The daily standing charge on that same average is 54.83 pence. Ofgem states that electricity on the cap has no VAT from 1 October 2026 to 31 March 2027, and that the figures already reflect that. Gas is still at 5% VAT. Source: Ofgem, changes to the cap for 1 October to 31 December 2026.
Use 26.32 p only as that average. Your region, a prepayment meter, a standard-credit tariff, or any fixed deal can be different. Read the unit rate on your bill and substitute it. The standing charge is paid whether the lamp is on or off, so it is not part of the lamp's running cost.
This is a domestic electricity sum. It is not a quote, and it is not financial advice.
The sum
Kilowatt-hours = watts ÷ 1,000 × hours
Cost in pence = kilowatt-hours × pence per kWh
A month here means 30 days of the same schedule.
Worked examples at 26.32 p/kWh
- 10 W gooseneck, 12 hours: 0.12 kWh, 3.2 p a day, about £0.95 over 30 days.
- 15 W bulb, 12 hours: 0.18 kWh, 4.7 p a day, about £1.42 over 30 days.
- 20 W bar, 14 hours: 0.28 kWh, 7.4 p a day, about £2.21 over 30 days.
- 40 W of bars (for example four 10 W tubes, or a brighter bulb), 14 hours: 0.56 kWh, 14.7 p a day, about £4.42 over 30 days.
The hours are the ones from hours and a dark period. If your window already does half the job, use fewer lamp hours and the cost falls in proportion. Leaving a lamp on for 24 hours is not a saving strategy. Plants need a night, and you would pay for light you should not be giving them.
Check the watts on the rating label or the listing, not the "equivalent" number. A 15 W LED sold as a "200 W equivalent" costs the same to run as any other 15 W load. That distinction is spelled out in what listings exaggerate.
Mats and whole systems
A heat mat with a thermostat is not on for a fixed number of hours, so you cannot multiply watts by 14 and call it exact. You can still put a ceiling on it: watts ÷ 1,000 × 24 × 26.32 p is the cost if it never switched off. The propagation page does that for one 108 W mat listed with a controller. Real use should be lower. It should not be zero, either, in a cold room.
Pumps, extractors, dehumidifiers and a high-wattage panel are a different total. Hydroponics Guide covers system-scale running costs. This page is only the lamp, or a small mat, in a house.
The shelf tool uses the same 26.32 p rate and shows a rough daily and 30-day cost for the wattage it assumed. Swap in the watts from the box once you have chosen a model.