If you’re growing indoors, one of the most common questions is:
“How much does it actually cost to run a grow light?”
This calculator gives you a clear, realistic answer based on your own setup — not marketing claims, inflated “equivalent wattages”, or average electricity prices that go out of date.
You enter:
- Your grow light’s actual power draw
- How many hours it runs each day
- What you pay per unit of electricity
No assumptions. No sales pitch. Just the maths.
If you’re new to indoor lighting, our Grow Lights Explained: A Complete Guide walks through how grow lights work, the different types available, and what actually matters when choosing one.
Grow Light Running Cost Calculator (UK)
Enter your grow light’s actual wattage, how many hours you run it, and your electricity unit rate. You’ll get an instant estimate of the running cost per day, month, and year.
UK tip: Check your latest bill or energy app and look for “Unit rate” or “Price per kWh”.
This calculator works regardless of how energy prices shift in the future.
Standing charges aren’t included because they don’t change when you switch a grow light on or off.
Estimated running cost
Show the maths (plain English)
Note: This estimate is for the grow light only. Fans, heaters, dehumidifiers and pumps can add extra usage.
How to reduce running costs (without buying anything)
- Use a plug-in timer so you’re not “donating” electricity because you forgot to switch off.
- Don’t over-light — extra hours rarely help once plants have enough light.
- Dial in distance — too far away wastes power without improving growth.
- Adjust seasonally — your setup may not need the same runtime all year.
How Grow Light Running Costs Are Calculated
The cost of running a grow light is based on energy use, not grow area size or the headline wattage printed on the box.
Here’s how it works in plain English:
- Grow lights draw power in watts (W)
This is how much electricity the light actually uses while switched on. - Electricity is billed in kilowatt-hours (kWh)
1,000 watts equals 1 kilowatt. - Time matters just as much as wattage
A low-watt light run for very long hours can cost more than a higher-watt light used efficiently.
The core calculation is:
(Watts ÷ 1000) × hours per day × electricity cost = daily running cost
Monthly and yearly costs are simply scaled from that daily figure.
Ignore “1000W Equivalent” — Use Real Power Draw
Many grow lights are marketed as “600W”, “1000W”, or even higher — despite drawing far less power from the wall.
If the box or listing says “1000W” but the specifications show “Power draw: 100W”, use 100W in the calculator.
Your electricity supplier doesn’t care about marketing terms — they only charge you for what actually travels through the plug.
If you can’t find the real figure:
- Look for phrases like “Actual power draw” or “Wattage at the wall”
- Check the customer Q&A section — there’s often a grower who’s tested the light with a plug-in power meter and shared the real number
If this part feels confusing, our Grow Light Wattage Explained guide breaks it down clearly.
Efficiency Still Matters (Just Not How You Think)
More efficient grow lights don’t magically make electricity cheaper — but they do make better use of each watt.
That usually means:
- More usable light per unit of electricity
- Shorter required runtimes
- Less wasted energy as heat
The “Secret Heater” Effect
Grow lights don’t just produce light — they also produce heat.
Less efficient lights turn more electricity into heat.
In winter, that might slightly offset heating.
In summer, it can mean spending more on fans or ventilation.
A more efficient light tends to run cooler, making the growing environment easier — and often cheaper — to manage overall.
For a deeper explanation of how efficiency affects real-world setups, see Grow Light Efficiency Explained.
How to Reduce Grow Light Running Costs (Without Buying Anything)
You don’t need new equipment to keep electricity use sensible. Small adjustments add up over time:
- Avoid over-lighting
Once plants have enough light, extra hours don’t improve growth — they just waste energy. - Match hours to plant type
Leafy greens usually need fewer hours than fruiting crops. - Use a simple plug-in timer
It costs less than a fiver and is the single best way to avoid “donating” money to your energy company because you forgot to switch the light off at 11pm. - Position lights correctly
Lights that are too far away waste energy without improving results. - Adjust seasonally
Winter and summer setups don’t always need identical runtimes.
These changes reduce running costs without compromising plant health.
Related Reading
If you’re comparing running costs across different setups, it can help to see how wattage and efficiency vary between common grow light types. Our Top 10 LED Grow Lights UK guide gives a practical overview, while Best Grow Lights for Beginners and Best Cheap Grow Lights UK are useful if you’re working to a tighter budget or just getting started.
If you’re growing in a tent or using a frame-based setup, Best Grow Lights for Every Tent Size and Top 10 Grow Light Stand Systems may also be helpful reference points.
A Neutral Tool You Can Reuse Anytime
This calculator is designed to be:
- Evergreen
- Informational
- UK-relevant
- Independent of buying advice
Buying guides on HomeGrower may link to or embed this calculator where helpful — but the calculator itself will remain a standalone reference tool you can rely on over time.
If you want to browse all our grow light guides and comparisons in one place, see our Grow Lights UK hub.