LED Grow Light Running Cost UK: what you'll actually pay
We run grow lights for a living. Here's the honest maths — no inflated wattage, no scare stories.
Running cost calculator
Live monthly & annual cost — edit any field.
100W ÷ 1000 × 16h × 30 days × 24.5p = £11.76/month
Estimate only. Real draw can run 5–15% higher than the spec sheet once driver and heat losses are counted.
LED grow light running cost UK: the quick answer
Every running-cost question is really one sum. You pay for energy by the kilowatt-hour (kWh), so the only inputs that matter are how much power your light truly draws, how long it's on, and what you pay per unit. Get those three right and the cost is exact — there's no mystery in it.
The mistake we see most is trusting the headline wattage on a cheap listing. A light sold as "1000W" might pull 100–150W at the wall. Always calculate from true power draw (the figure on the driver or a plug-in energy monitor), not the marketing number.
The formula (and how to use it)
This is the whole thing. It's the same maths our grow-lights guide uses for a full rack — just scaled up.
Prefer to skip the arithmetic? The calculator above does all four steps live as you type. The unit rate defaults to the Ofgem energy price cap (~24.5p/kWh, Q1 2025); swap in your own tariff for an exact figure.
Wattage scenarios at 24.5p/kWh
Here's the full grid — the four wattages people ask about most, at both a 12-hour and a 16-hour photoperiod, at the Ofgem cap. Monthly assumes 30 days.
| Light | 12h · month | 12h · year | 16h · month | 16h · year |
|---|---|---|---|---|
| 20W seedling | £1.76 | £21.17 | £2.35 | £28.22 |
| 100W shelf / tent | £8.82 | £105.84 | £11.76 | £141.12 |
| 300W small grow | £26.46 | £317.52 | £35.28 | £423.36 |
| 600W full tent | £52.92 | £635.04 | £70.56 | £846.72 |
Figures are direct energy cost only. Add VAT-inclusive standing charges (already part of your bill, not per-light) and any losses below.
The honest cons: why your bill may beat the calculator
We'd rather you trust these numbers than be surprised. Three things make real-world cost drift above the clean maths:
Spec vs real draw
Cheap lights quote an "equivalent" or peak wattage. The driver's real pull can sit 5–15% above the rating, and unbranded units are the worst offenders. Measure with a £10 plug-in monitor.
Heat & cooling
Every watt a light wastes becomes heat. In a sealed tent that means fans — and in summer, the temptation to cool the room — quietly adding to the total energy you draw.
Cheap-driver loss
A poor driver wastes power as heat instead of photons. You pay for watts that never reach the plant. A good fixture's efficiency (µmol/J) is the figure that actually protects your bill.
People also ask
How much does it cost to run LED grow lights?
At the Q1 2025 Ofgem cap (~24.5p/kWh), a 100W LED run 16h/day costs about £11.76/month (£141/year). A 300W light is about £35/month, and a 600W about £70/month. Use the calculator with your own wattage and tariff for an exact figure — and expect real-world cost a touch higher once heat and driver losses are counted.
Do grow lights use a lot of electricity in the UK?
Modern LEDs are fairly economical. A typical 100–300W panel uses roughly 48–144 kWh a month at 16h/day — comparable to a fridge-freezer or a couple of gaming PCs. Older HID lamps and cheap unbranded units draw far more for the same usable light, which is where alarming bills come from.
How much does it cost to run a grow light 12 hours a day?
At 12h/day and 24.5p/kWh: a 100W light is about £8.82/month, 300W about £26.46, and 600W about £52.92. Twelve hours suits flowering and most houseplants; seedlings and leafy greens often run 16–18h, raising cost proportionally.
Do grow lights take a lot of electricity?
Not on a single-light scale. What matters is true power draw, not the inflated "equivalent" wattage on cheap listings. A quality 100–300W LED on a sensible photoperiod adds a few pounds to tens of pounds a month. Costs climb when you run many fixtures, run them 18–24h, or buy lights with inefficient drivers.
Run the numbers, then run the right light
The cheapest light to run is the efficient one you measured before you bought it. We build indoor farms for a living — the kit on our shelves is the kit in this guide. Honest reviews, not sales.