Last updated: 9 October 2026

Ask any grower what keeps them up at night and, after "is that a spider mite?", the answer is usually the electricity bill. Grow lights have a reputation for making the meter spin like a fruit machine on Brighton Pier. That reputation was earned in the days of hot, hungry HPS lamps. A modern LED is a very different animal, and the sums are far simpler than the internet makes out.

Everything here is based on flowering at 12/12 (12 hours on, 12 off), because that's the one schedule every grower goes through, first grow or fortieth. We've done the maths for every light we sell, so there's no need to dig out a calculator. Or your old GCSE maths teacher.

One more thing. Plenty of growers never run a decent LED at full whack from the first day of flower to the last, and the running costs on our product pages are worked out at 75% power. We've done the same here so the numbers match. If you like yours turned up to eleven, the flat-out figures are in the big table too.

What you actually get for your money

Cost on its own tells you very little. A light that costs pennies to run and grows nothing is just a night light with ideas above its station. So here's what the makers say each light can do, next to what it costs to run for a day in flower at 75% power.

• Spider Farmer SF1000 (100W). Spider Farmer quote up to 2.0g per watt, and our product page reckons even a beginner can expect well over 100g every 8 weeks. Running cost: about 24p a day. Find that down the back of the sofa and you'd barely bother picking it up.
• Mars Hydro TS1000 (150W). Our TS1000 page puts its yield potential at well over 2g per watt. About 36p a day. Hardly worth mentioning, really.
• Mars Hydro FC-E3000 (300W). Up to 2.5g per watt, says Mars Hydro. About 71p a day, so under half the price of a can of Red Bull.
• Spider Farmer SE5000 (480W). Up to 2.5g per watt from a light that sips electricity at 2.9 µmol/J. About £1.14 a day, less than a can of Red Bull, which is why our product page reckons it pays for itself very quickly indeed.
• Spider Farmer SE7000 (730W). Spider Farmer say a skilled grower can get over 2.5g per watt. About £1.73 a day, and a whole 8-week flower comes in just under £100. Still cheaper per day than a coffee from the high street.

The yield figures are the manufacturers' own, and real results depend on the grower, the strain, the tent and a fair bit of luck. Treat them as what the light is capable of rather than a promise. The running costs, on the other hand, are plain arithmetic.

How to work out the running cost of any grow light

You only need three numbers:

1. The light's real power draw in watts. Use the "Power" figure in the specifications, not the "HPS equivalent" or "replaces a 600W" claim. Those describe light output, not what comes out of your socket. If you run it dimmed, multiply by the setting (0.75 for 75%).
2. How many hours a day it's on. For flowering that's 12 hours (12/12).
3. What you pay per kWh. Look at your bill or energy app. On a standard variable tariff, the Ofgem cap average from 1 October to 31 December 2026 is 26.32p per kWh. Your own rate may be a little different depending on where you live and how you pay, and fixed or time-of-use deals have rates of their own.

Worked example: a 200W Spider Farmer SF2000 Pro at 75% on 12/12:

• 200W × 0.75 = 150W, or 0.15 kW
• 0.15 kW × 12 hours = 1.8 kWh a day
• 1.8 kWh × £0.2632 = 47p a day. Not bad for a light that fills a 70cm flowering tent.

Leave the standing charge out. You pay it whether the light is on or off, so it makes no difference to what the light costs to run.

Running costs for every light we sell (12/12 flowering)

For those who like the detail, here's the lot. Power figures come from the specifications on each product page. The sums use 26.32p per kWh and 12 hours a day, rounded to the nearest penny, and prices were checked on 9 October 2026. The bold column is the one to look at; the flat-out column is there for anyone who never touches the dimmer. On a phone, swipe the table sideways to see every column.

Most photoperiod plants flower for somewhere between 8 and 10 weeks. Running a longer strain? Add about a quarter to the last column for a 10-week flower.

What about the vegetative stage?

Veg is where every grower does their own thing. Some flip to flower after a couple of weeks, some veg for months, and schedules run anywhere from 16 to 24 hours a day depending on technique. So we won't pretend there's one "right" veg cost. Use the per hour column instead, multiplied by your hours per day and the number of days you veg for. A 200W light costs 5.3p an hour at full power, and young plants rarely need full power anyway. Your plants, your rules.

LED vs HPS: where the savings come from

Here's the bit that surprises people. A watt costs the same whatever kind of light it powers. LED saves you money because you need far fewer watts for the same useful light. Modern fixtures turn much more of each watt into light plants can use (measured in µmol/J) and much less into heat.

The 450W Spider Farmer SF4000 is a straight swap for a traditional 600W HPS. Even with both running flat out, the SF4000 saves about 47p a day, or around £26.53 over an 8-week flower. Plenty of HPS setups run flat out all flower, though. Run the SF4000 at 75% and the saving grows to roughly 83p a day, about £46 a flower. That's before you count the ballast and replacement bulbs, or the extra extraction a hot HPS needs every summer.

Before you swap, a few things matter more than the headline wattage:

• Compare PPF and the PPFD map, not watts. Match the light to your tent size, not to your old bulb.
• Check the footprint. Each product page lists the manufacturer's recommended coverage.
• Payback depends on how much you grow. One grow a year pays back more slowly than running the tent all year round.

Browse the full range of LED Grow Lights, or go straight to Spider Farmer and Mars Hydro. New to all this? Our Ultimate Guide to LED Grow Lights covers the basics.

Five ways to spend even less

1. Dim it when plants don't need full power

Seedlings, cuttings and young plants need far less light than a plant in full flower, and most of the lights above have a dimmer. A dimmed LED draws less power, though not always exactly in line with the dial. As a rough guide, a 200W light at half power on a 12-hour day costs about 32p a day rather than 63p. A plug-in energy monitor will tell you the real figure.

2. Use a timer and stick to 12/12

Leaving a 300W light on for two extra hours a day wastes about 16p a day. It also confuses flowering plants, which take their light schedule very seriously indeed. A Mars Hydro Digital Grow Room Timer (£19.00) or the Mars Hydro iControl Smart Grow Room Controller (£49.00) keeps things exact.

3. Light the space you're actually using

A light far too big for your tent spends good money lighting the walls. A properly matched tent and light almost always beats a big, under-lit space. Our Grow Tent Kits pair each light with a tent that suits its footprint.

4. Check your tariff's time bands

On Economy 7 or a smart off-peak tariff? Put your 12 "on" hours in the cheapest band, ideally overnight, and the cost drops again. Use the off-peak rate from your own bill, because the price cap average doesn't apply to those bands.

5. Use the heat in winter, plan for it in summer

LEDs run much cooler than HPS, but every watt still ends up as heat in the tent. In a chilly British loft in January, that's a free bonus. In a July heatwave your Ventilation & Extraction will be working harder, and fans use a bit of electricity too.

Don't forget the rest of your kit

The light is usually the biggest single cost, but it isn't alone. Work out everything else the same way. A 9W Mars Hydro M6 Clip On Fan running around the clock, for example, costs about 6p a day. Inline fans, heaters, dehumidifiers and pumps all add a little more.

The easiest way to get real figures is a plug-in energy monitor between the wall socket and your extension lead. Leave it for a full day, read off the kWh and multiply by your unit rate. That covers everything on that lead, controllers and fans included.

Small lights for herbs, houseplants and microgreens

Lighting a windowsill of basil rather than a tent? Relax. A 33W Spider Farmer SF300 on a 12-hour day costs about 10p a day flat out. Pocket change, frankly. Run it for longer and the per-hour figure (0.9p) does the rest. Multi-shelf setups from our Grow Shelves & Microgreen Systems work the same way: add up the watts on the shelves you're using and apply the formula.

Frequently asked questions

How much does a 100W LED grow light cost to run per day in the UK?

On a 12/12 flowering schedule at 26.32p per kWh, a 100W LED costs about 24p a day at 75% power, or 32p flat out. A whole 8-week flower at 75% comes to about £13.27.

How much does a 600W grow light cost to run on 12/12?

Lots of growers replacing a 600W HPS go for a 450W to 480W LED such as the Spider Farmer SF4000 or SE5000. At 75% power those cost about £1.07 to £1.14 a day, less than a can of Red Bull. A 600W HPS uses 7.2 kWh a day, about £1.90, and the ballast adds a bit more on top.

How much does it cost to run a grow light during veg?

It depends on your schedule and how long you veg, which varies a lot from grower to grower. Multiply the per-hour cost in our table by your hours per day and your number of veg days. A 100W light costs about 2.6p an hour at full power.

Is an LED grow light cheaper to run than HPS?

Yes. Electricity costs the same per watt whatever it powers, but an LED needs far fewer watts for the same useful light. Swapping a 600W HPS for a 450W SF4000 saves about 47p a day with both flat out, and nearer 83p a day with the LED at 75%.

Do I use the actual wattage or the "equivalent" wattage?

Always the actual power draw (the "Power" spec). "HPS equivalent" figures describe light output. They don't tell you how much electricity the light uses.

Does dimming an LED grow light save electricity?

Yes. A dimmed LED draws less power, so it costs less to run. The saving isn't always exactly in line with the dimmer setting, so measure with a plug-in monitor if you want precise figures.

Should I include the standing charge?

No. You pay it whether the light is on or off, so it doesn't change the cost of running the light.

Is there VAT on top of 26.32p?

No. The government has removed VAT from domestic electricity bills from 1 October 2026 to 31 March 2027, and Ofgem's 26.32p figure already reflects that.

Will these costs change?

Yes. Ofgem reviews the price cap every three months. The next rates (1 January to 31 March 2027) are due by 25 November 2026, and we'll update this guide when they're announced. If you're on a fixed or time-of-use tariff, use your own unit rate in the formula. See the current Ofgem price cap rates.

Is it cheaper to run grow lights at night?

Only if your tariff charges less at night, as Economy 7 and many smart tariffs do. On a standard single-rate tariff every hour costs the same, though running at night can also help keep tent temperatures down in summer.

Electricity price source: Ofgem, Changes to energy price cap between 1 October and 31 December 2026 (published 26 August 2026). Average electricity unit rate 26.32p per kWh for direct debit customers on a standard variable tariff, England, Scotland and Wales. Daily costs are shown at 75% power on a 12/12 schedule, the same basis as our product pages, with full-power figures in the table. Power figures and yield claims come from each light's product page and manufacturer. Everyday comparisons use typical 2026 UK prices: a 250ml can of Red Bull at around £1.50 to £1.95, and a high street flat white at around £3.75 to £4. Product prices were correct on 9 October 2026 and may change.