LED Grow Light Running Costs: The 2026 UK Guide

The simple formula for working out your LED grow light's running cost is (watts × hours ÷ 1000) × your electricity rate in pence per kWh, and the number that catches most growers out is using the light's marketing "equivalent wattage" instead of its actual power draw. Here's how to calculate your real running cost, and how modern LEDs compare to older HPS and "blurple" lights.

In This Article

• The running cost formula, explained
• A worked example
• What actually drives your electricity bill
• LED vs. HPS vs. older LED running costs
• Practical ways to lower your running costs
• Frequently asked questions

The Running Cost Formula, Explained

The formula is straightforward:

(Watts × Hours ÷ 1000) × Cost per kWh = Daily Running Cost

• Watts: your light's actual power draw from the wall, not a marketing "HPS equivalent" figure. Check the manufacturer's spec sheet for the true wattage.
• Hours: how many hours per day the light runs (commonly 18 hours for vegetative growth, 12 hours for flowering).
• ÷ 1000: converts watts into kilowatts, the unit your energy supplier bills in.
• Cost per kWh: your electricity rate in pence per kWh, found on your latest bill.

A Worked Example

Take a 480W LED fixture running 12 hours a day at an electricity rate of 27p per kWh:

(480 × 12 ÷ 1000) × 27p = 155.5p per day, or about £1.56.

Multiply that out across a month (roughly £47) or a year (roughly £568) to see the real cost of running that fixture on that schedule. Plug in your own light's actual wattage, your schedule, and your own tariff rate from your bill to get your exact figure.

What Actually Drives Your Electricity Bill

Beyond the basic formula, a few things determine your real-world cost:

• Efficacy (μmol/J): the single biggest factor in long-term running costs. A fixture rated at 2.7 μmol/J delivers far more usable light per watt than an older unit rated at 1.5 μmol/J, meaning lower bills for the same results.
• Dimming: young plants don't need full intensity. Dimming a 600W light to 50% power cuts its running cost in half during seedling and early vegetative stages.
• Ancillary equipment: extractor fans (25W-200W+), circulation fans (15W-50W) and water pumps (10W-100W) all add to your total electricity draw, so include them when budgeting your full setup's running cost, not just the light.

LED vs. HPS vs. Older LED Running Costs

A classic 600W HPS light and a modern 480W full-spectrum LED can produce comparable light output for a 1.2m x 1.2m tent, but their running costs are very different. On a 12-hour flowering schedule at 27p per kWh:

• 600W HPS: 0.6kW × 12 hours × 365 days × 27p = roughly £709 per year.
• 480W LED: 0.48kW × 12 hours × 365 days × 27p = roughly £567 per year.

That's a saving of over £140 a year just from switching fixture type, before accounting for the LED's typically longer lifespan. Older "blurple" LEDs sit in between: they're more efficient than HPS but well behind a modern full-spectrum unit, since efficacy has improved substantially in the last few years.

Practical Ways to Lower Your Running Costs

1. Choose the highest efficacy light you can afford. Look for a μmol/J rating of at least 2.5 rather than focusing on wattage alone. See our full LED buyer's guide for real efficacy figures across DiscountHydro's range.
2. Use a timer. A reliable timer stops your light running longer than intended, and lets you align your schedule with any off-peak electricity tariff you're on.
3. Keep your environment stable. A well-sealed, reflective tent means your fans and heaters work less hard to maintain temperature and humidity, which lowers your overall electricity draw, not just the light's.

Frequently Asked Questions

How much electricity does a 600W-class LED grow light use per month?

Running 18 hours a day, a 600W LED uses roughly 324 kWh a month. At a UK average rate of around 27p per kWh, that's approximately £87 a month for the light alone.

Is it cheaper to run an LED than an HPS light?

Yes. A modern LED can match a 600W HPS's light output while drawing significantly less power, typically cutting the electricity cost for the same results by roughly a third to a half.

What's the most energy-efficient LED for a 4x4 (1.2m x 1.2m) tent?

Look at full-spectrum fixtures in the 480W-650W range with an efficacy rating of at least 2.7 μmol/J. See our LED buyer's guide for specific, verified in-stock options.

How do I work out the total electricity cost for my whole tent, not just the light?

Add up the actual wattage of every piece of equipment (light, fans, pumps), multiply each by its daily runtime to get watt-hours, convert to kilowatt-hours, then apply the same formula as above to each and sum the totals.

Does dimming my light actually save meaningful money?

Yes. Dimming to 50% power roughly halves that fixture's electricity draw for as long as it runs at that level, which adds up significantly over a full seedling and early vegetative period.