What Does My Grow Cost in Electricity? An Honest Calculation
The number most people are afraid of – and it almost always turns out lower than feared. Provided you calculate with the right watts. Here are the three figure…
"So what does that actually cost you in electricity?" – the question always comes up sooner or later. And the honest answer is usually lower than people expect. But only if you work with real numbers instead of the ones printed on the box. 🌱
The maths behind it is simple
For every device: watts times hours times days, divided by 1000, gives you kilowatt hours. And kilowatt hours times your rate gives you the cost. That is all there is to it – the calculator just does the adding up across all your devices and both phases.
Here is an example using the default setup the calculator starts with: a 300 W LED, a 20 W exhaust fan, a 40 W circulation fan, 28 days of veg and 63 days of flower, at 35 cents per kilowatt hour.
- LED: 18 hours in veg, 12 in flower – 378 kWh in total
- Exhaust fan: running around the clock – 43.7 kWh
- Circulation fan: running around the clock – 87.4 kWh
That comes to roughly 509 kilowatt hours for the whole run, about 178 euros – spread over a good three months, that is just under 60 a month. And you can already see the important part: the lamp alone accounts for about three quarters of the bill. Whatever you want to save, you save it there or not at all.
The three figures nearly everyone gets wrong
Watts: the label lies in both directions
This is by far the biggest error. What is printed on the lamp is almost never what flows out of the socket.
- LEDs rarely run at 100 %. If your dimmer sits at 70 %, you pay roughly 70 %. Calculate with the label and your bill comes out a third too high.
- HPS pulls more than it says. The ballast draws power too – a 600 W lamp is easily 650 to 680 watts at the wall. Calculate with 600 and you land too low.
- Anything with a motor – dehumidifiers, air conditioners, compressors – pulls several times its rating on startup. That barely matters for the monthly bill, but it matters a lot for what your power strip can take.
The only honest source is a plug-in power meter for ten to fifteen euros – mine is this one (affiliate link, German Amazon store). Plug it in between, read the real value, enter that. It is the best investment in this entire guide.
Hours: half your gear doesn't run continuously
Lamp, exhaust and circulation run on a schedule – you can simply enter those. But dehumidifiers, humidifiers, heaters and air conditioners cycle. They kick in, hit their setpoint and switch off again. Enter 24 hours for a 200 W dehumidifier and you produce a horror figure with no connection to reality.
Two ways to keep this honest: either estimate deliberately low (most dehumidifiers effectively run 4 to 10 hours a day), or hang a meter with a kilowatt hour counter on it for 24 hours and read the real daily consumption. Divide that by the wattage and you have your real hours.
Rate: take the one from your bill
Don't enter the number from an advert, enter the unit rate from your last annual statement , including all taxes and levies. Leave out the standing charge – you pay that whether you grow or not. The calculator starts at 35 cents; depending on your contract you may be well below or above that, and it shifts the final figure more than any single device.
How to fill in the calculator
1. Open the Power Cost Calculator.
2. Enter your days for veg and flower at the top, plus your power rate .
3. Pick your devices from the templates – lamp, exhaust, fan, dehumidifier and so on. For anything not listed there is a field to add your own.
4. Overwrite watts and hours on every device with your real values. The templates are starting points, not truth.
5. Watch the separate hours for veg and flower . That is exactly where the difference sits: your lamp runs six hours less in flower, while your dehumidifier often runs longer.
The calculator covers one complete run , not a year and not a month. Drying is deliberately set to zero hours – it happens after the cycle and would otherwise be counted twice.
Where you actually save
On the lamp – through efficiency, not wattage. The figure that matters is µmol/J: how much usable light a lamp makes per watt. Between a cheap model at 1.8 µmol/J and a good one at 2.8 µmol/J there is over 30 % in running costs for the same light. When buying, that is the only number that counts.
On the dimmer, if you are over target anyway. Measure what actually arrives at the plant first, with the Lux to PPFD and DLI converter. If your PPFD sits above the target range, you are giving money away for light the plant cannot use. Dimming saves immediately and costs no yield.
On the dehumidifier – but at the cause. It is almost always the hidden second biggest item. A dehumidifier running non-stop is not a device problem though, it is a climate problem: it is fighting air that is too humid, which you can often handle more cheaply through extraction and temperature. The VPD Calculator shows you whether your humidity is even too high for the phase you are in.
On waste heat, depending on the season. In winter your lamp's heat is …
What Does My Grow Cost in Electricity? An Honest Calculation