You have found a light you like. Then the thought arrives: what is this going to do to my electricity bill?
It is the question that stops more indoor grows than any other, and it usually gets answered badly — with a scary guess, or with a number somebody read on a forum years ago. So here is the honest version. You can work out exactly what a grow light costs you, on your tariff, in about thirty seconds.
The arithmetic, once
Electricity is sold in units — kilowatt-hours. One unit is a thousand watts running for one hour. That is the whole of it.
Watts ÷ 1000 × hours on = units per day.
An 85 watt light running 18 hours a day: 85 ÷ 1000 = 0.085, multiplied by 18, is about one and a half units a day.
Now find your unit rate. It is on your bill, or in your energy app, listed as a rate per kWh. Multiply the two together. That is your number — right for your house, on your tariff, this month.
You will notice we have not printed a figure for you. That is deliberate. Unit rates move every few months, and a running cost written into an article is correct on the day it is written and quietly becomes wrong afterwards. Yours is on your bill and it takes ten seconds to find.
Three things that make the real number smaller than you fear
1. The watts on the box are not always the watts it draws
A lot of marketplace lights are advertised at an "equivalent" wattage — a number describing what they claim to replace rather than what they pull from the wall. A light sold as 2000W that actually draws 200W is not a bargain, it is a badly labelled 200W light.
Check for a stated actual power draw. If a listing will not tell you what the thing pulls from the socket, you cannot work out its running cost — and that is reason enough to be careful.
2. Dimmable means you choose the bill
This is the part most people miss. A dimmable light is not one running cost, it is a dial. Seedlings and young plants do not want full power anyway, so turning the light down early in the grow is better for the plants and cheaper, which is a rare combination.
A board with 5-channel dimming takes that further — you can adjust the channels separately, so you are not paying to run parts of the spectrum your plants are not using at that stage.
3. What matters is light per watt, not watts
Two lights drawing the same power can put very different amounts of usable light on your plants. That efficiency is the whole reason LM301H EVO diodes cost more to manufacture than the cheapest ones available: you get more light out of the same electricity.
Which means the cheapest light to run is very often not the cheapest light to buy. Over a few grows the efficient one pays back the difference and then keeps going.
Matching the light to the space
Buying more light than your space needs is the most common way people end up with a bill they did not expect. Roughly:
- A windowsill, a shelf, a few herbs or seedlings. The Mini Full Spectrum LED Grow Light is the right size of tool for this. Small draw, two modes, nothing wasted.
- A small tent, a cupboard, one or two plants. The LM301H EVO 85W Quantum Board. Dimmable, so you can run it gently while everything is young.
- A bigger tent, or you want real control. The LM301H EVO 120W board, dimmed down whenever you do not need all of it.
Two habits that cut it further
Run your lights at night. If you are on a tariff with cheaper overnight hours, your plants genuinely do not mind which hours are their day. Set the timer so their day is your night, and you are buying the same light at the cheaper rate.
Do not pay for the dark hours. Eighteen hours on and six off while plants are growing, twelve and twelve when they are flowering. That is not a money-saving trick — it is what the plants want — but it does mean a good chunk of every day costs you nothing at all.
The short answer
A grow light is a known, calculable and surprisingly modest cost, and you can work yours out from the wattage and your own unit rate before you buy anything at all.
The genuine mistakes are buying a light that will not tell you its real draw, and buying more light than your space can use. Neither is expensive to avoid. Both are expensive to get wrong.
Read next
Looking for the kit? Start here, or browse the full grow range.