The light, the fan and whatever else runs — see what one cycle costs you in electricity. Free, no sign-up, in your own currency.
What counts is the actual power draw, not the number in the product name. A light sold as “600 W” often pulls only 100 to 150 watts — check the data sheet or measure it with a plug-in meter. Your price per kWh is on your electricity bill.
| Setup | Light + fan | Power per month |
|---|---|---|
| Small tent (2 × 2 ft) | 100 + 25 W | € 21.60 |
| Medium tent (2.5 × 2.5 ft) | 240 + 40 W | € 47.52 |
| Large tent (3 × 3 ft) | 480 + 75 W | € 93.96 |
Worked out with the price preset above and an exhaust fan running around the clock. Put in your own price and the calculator gets more accurate.
These calculators give you reference values for your own judgement. They replace neither a measurement on site nor expert advice — all figures without guarantee.
The sum is simpler than most people expect: power × time × price. Watts become kilowatts (divide by 1,000), multiplied by hours per day and by days that gives you kilowatt-hours. Times the price per kilowatt-hour, and you have the cost.
An example: a 600-watt light, 18 hours a day, for 60 days. That is 0.6 kW × 18 h × 60 = 648 kilowatt-hours. At 0.35 per kWh that comes to 226.80 — for the light alone.
This is exactly where most rough estimates stop, and exactly where they go wrong. That is why the calculator above has three rows: the exhaust fan runs around the clock, and even at 40 watts it adds up to a noticeable sum across a full cycle.
This is the single most common mistake, and it runs through half the forum threads on the subject: a light advertised as “600 W” usually does not draw 600 watts from the wall. The number often refers to the combined rating of the LED chips, which are deliberately run well below their limit — that is what makes them efficient and long-lived.
A typical “600 W” LED panel actually pulls somewhere between 100 and 150 watts. Anyone budgeting with the number on the box overestimates their bill by a factor of four or five — and then wonders why the real one is so much lower.
Two ways to get the real figure: the data sheet, where the honest manufacturers state “power draw” or “actual wattage” separately, or a plug-in power meter. Those cost very little and settle the question for good — including for the fan, the dehumidifier and everything else you never measured.
The light is the biggest item, but it is not the only one — and the others have one thing in common: they run far longer than the light does.
The exhaust fan is the classic example. It draws little, often 30 to 60 watts, but it runs 24 hours a day. Over 60 days that is more than a small light uses in the same period. A circulation fan behaves the same way at 10 to 25 watts.
A dehumidifier is a different matter: 200 to 400 watts, and in late flower it can run for hours at a time. If you use one, it belongs in the sum — it can rival the light. Heating mats in propagation and water pumps in hydro setups are smaller, but they too run continuously.
The third row of the calculator is there for exactly this. Add up whatever runs besides the light and the fan, and put the total in.
The photoperiod changes halfway through, and with it the cost. In veg the light usually runs 18 hours, in flower 12 — a third less, on the same fixture.
A cycle worked out entirely at 18 hours therefore comes out too high, one worked out at 12 too low. The honest way is to run the calculator twice: once with the veg days at 18 hours, once with the flowering days at 12, then add the two.
With autoflowers the question does not arise — they run 18 or 20 hours throughout and can be worked out in one go.
The obvious lever — running the light less — is the worst one: less light means less yield, roughly one for one. Everything else is better.
Efficiency beats duration. A modern LED delivers around 2.5 to 2.8 µmol per joule, an older HPS about 1.7. For the same light on the plants that is roughly a third less electricity. If you are replacing a fixture anyway, that is where the money is.
Run the light at night where your tariff has cheaper off-peak hours. It costs nothing and changes nothing for the plant — she does not care when her day is, only that it is regular. It also helps in summer, when the room is cooler at night and the fan has less work to do.
Do not throttle the exhaust to save power. It draws little, and if the climate slips, the loss in yield costs far more than the fan ever did. What the climate should look like is what the VPD calculator tells you.
The price is the one figure in the sum you cannot change by growing better — and it varies enormously. Within Europe it runs from roughly 0.10 to 0.40 per kWh; across the world the spread is wider still.
That is why the calculator lets you pick your own currency next to the price. Nothing is converted — you type your own rate and the result comes back in the same money. Take the figure from your bill rather than a national average: standing charges and tariffs differ enough that the average is rarely yours.
What the price does not change is the ranking. A more efficient fixture pays off at any rate, it just pays off faster at a high one.
Electricity is one item, not the whole bill. Soil, nutrients, seeds and the fixture itself all belong in it before the number means anything.
The figure that actually tells you something is cost per gram. It puts electricity in proportion: a cycle that costs a lot in power but yields well can come out cheaper per gram than a frugal one that does not. What you can realistically expect from a given wattage is what the yield calculator estimates.
Working that out by hand across a whole cycle is tedious — which is why CRIS does it for you, with every entry landing on the right grow.
Divide the power draw in watts by 1,000, multiply by hours per day and by the number of days — that gives kilowatt-hours. Multiply by your price per kWh for the cost. Example: 600 W for 18 hours across 60 days is 648 kWh. Add every device that runs, not just the light.
Yes. Pick your currency from the field next to the price — the symbol changes everywhere on the page at once. Nothing is converted: you enter your own price per kWh and the result comes back in the same money. That is deliberate — an exchange rate baked into a page would be wrong within a week, and your electricity bill is in your own currency anyway.
The table above gives three typical setups. As a rough guide, a small tent with a 100-watt light and a fan lands around 20 to 25 per month at 0.35 per kWh, a large one with 480 watts around 100. Your own price and your own hours move that a lot, which is what the calculator is for.
Because that number usually refers to the combined rating of the LED chips, not what the fixture pulls from the wall. The chips are run well below their limit for efficiency and lifespan. A typical “600 W” panel actually draws 100 to 150 watts. Check the data sheet for “power draw”, or measure it with a plug-in meter.
Everything that runs. The exhaust fan matters most — it draws little but runs 24 hours a day, so across a cycle it can use more than a small light. Then circulation fans, a dehumidifier if you use one (200 to 400 watts and a real factor in late flower), heating mats in propagation and water pumps in hydro setups.
Yes, if you want an accurate figure. The light usually runs 18 hours in veg and 12 in flower — a third less. Run the calculator twice and add the results. With autoflowers it does not matter, they run the same photoperiod throughout.
A more efficient fixture. A modern LED delivers around 2.5 to 2.8 µmol per joule against roughly 1.7 for an older HPS — for the same light on the plants, about a third less electricity. After that, running the light during cheaper off-peak hours. Cutting the photoperiod is the worst option: it costs yield roughly one for one.
No. A standing charge is due whether you grow or not, so it does not belong in the cost of a cycle. What belongs in is the price per kilowatt-hour — that is the part that changes because of your grow.
Yes. This calculator uses the same formula as the electricity calculator in the CRIS app. In the app the cost lands on the right grow automatically, alongside soil, nutrients and seeds — and at the end you get what a gram actually cost you.
In the CRIS app electricity lands on the cycle it belongs to, together with soil, nutrients and seeds — and at the end you see what a gram really cost. Offline, no account.