Grow light running cost and DLI calculator
Work out the hours of light a daily light integral needs, or the DLI a photoperiod delivers, then price the lights at every start time on your electricity tariff. It runs in your browser, and nothing you enter is stored or sent.
How to use the calculator
- Choose what you know: the target DLI from your crop recipe, or the hours the lights already run.
- Enter the canopy PPFD, as an average across the crop rather than one reading under a fixture.
- Enter the lighting load, either as kilowatts or as the watts per fixture and the number of fixtures.
- To see the cost, choose your currency and how your tariff is priced: one flat rate, or a day rate and a night rate with the hours the night rate runs. The rates and standing charge start at one published business tariff for the currency, so replace them with your own.
- Choose Calculate. Change any figure and calculate again; nothing is kept between visits.
How the calculation works
The hours a DLI target needs follow from the light the canopy receives:
hours = DLI × 1,000,000 ÷ (PPFD × 3,600)
DLI is in mol/m²/day and PPFD in µmol/m²/s, so the million turns moles into micromoles and the 3,600 turns seconds into hours. Working the other way, DLI = PPFD × hours × 3,600 ÷ 1,000,000. Both assume a steady light level through the photoperiod.
The hours are then capped at 24 and rounded to the nearest whole tariff price period, the step a tariff prices electricity in, which is also how the energy module in BeeGrow AI rounds. Rounding can land a little above or below the target, so the result always shows the DLI the rounded hours actually deliver.
Energy per day is the lighting load in kilowatts times the priced hours. A flat rate or a day and night pair becomes a rate for every price period of the day, and the photoperiod is priced at every possible start time, including windows that run past midnight onto the same day’s rates. The cheapest start is shown, and on a tie the earliest start wins. Costs are added up in millionths of a penny, cent or kuruş and converted once at the end, as the platform does, so a day of additions builds in no rounding drift.
Rates are entered in pence for pounds, in cents for dollars and euros, and in lira for Turkish lira, as tariffs in those currencies are usually quoted. Nothing is converted between currencies.
Default tariffs
Each currency starts with one published business tariff, so its rates and standing charge come from the same price list and fixed charges are never counted twice. They are examples, not national prices: your own tariff will differ, so use your bill where you can.
- Pound sterling (£): Octopus Energy for Business 12M Fixed, for sites using over 27,543 kWh a year, London region, from 19 October 2026. 31.36p/kWh on a single-rate meter; 32.31p by day and 25.03p at night on an Economy 7 meter, with night priced from 00:00 to 05:00; standing charge 50.78p a day; excluding VAT. For comparison, DESNZ puts the average paid by UK businesses using 20 to 1,999 MWh a year at 27.67p to 28.59p/kWh in April to June 2026, standing charges included.
- US dollar ($): CoServ, a Texas electric cooperative, small commercial under 35 kW, October 2026. 12.50¢/kWh on the standard rate with $30.00 a month; 19.20¢ on-peak (15:00 to 20:00, May to October) and 9.86¢ off-peak on time-of-use with $32.50 a month; taxes not stated. For comparison, EIA puts the US commercial average at 13.85¢/kWh over the 12 months to July 2026, fixed charges included.
- Euro (€): France’s regulated Tarif Bleu for businesses (non-résidentiel), 36 kVA, from 1 August 2026. 13.18c/kWh on the single-rate option with €498.60 a year; 14.07c peak and 9.99c off-peak on the off-peak option with €513.72 a year; excluding VAT, excise and CTA. For comparison, Eurostat puts the euro area average for businesses using 20 to 1,999 MWh a year at 18.17c to 21.83c/kWh in January to June 2026, excluding VAT and recoverable taxes.
- Turkish lira (₺): EPDK’s regulated low-voltage commercial tariff, from 4 April 2026. 5.93 TL/kWh single rate; 5.98 TL by day and 4.02 TL from 22:00 to 06:00 on the multi-time option, whose 8.45 TL peak from 17:00 to 22:00 is priced here at the day rate; no standing charge; energy plus distribution, excluding taxes.
The figures were checked on 22 October 2026. Prices change, and the tax treatment differs between them as stated.
The standing charge is added once a day and shown separately, because it is the same whenever the lights start. A monthly or yearly fixed charge is turned into a daily one, a month as 12 ÷ 365 of a year. Where a tariff’s single-rate and two-rate options have different fixed charges, the standing charge follows the option you choose. The 30-day figure is 30 times the daily cost: an estimate that assumes every day repeats the same rates.
A worked example
These inputs are illustrative, not advice for any crop: a target DLI of 14.4 mol/m²/day, a canopy PPFD of 250 µmol/m²/s, a 6 kW lighting load, and a tariff in pounds charging 28p/kWh by day and 9p/kWh from 00:00 to 07:00.
- Hours: 14.4 × 1,000,000 ÷ (250 × 3,600) = 16.0 hours. That needs no rounding, so the lights deliver the full 14.4 mol/m²/day.
- Energy: 6 kW × 16 hours = 96 kWh a day.
- Cheapest start: any start from 15:00 to 00:00 keeps all seven night hours inside the photoperiod, buying 42 kWh at 9p and 54 kWh at 28p, which is £18.90 a day. Those starts tie, so the calculator shows the earliest in the day, 00:00.
- With a standing charge of 60p a day, 30 days at the cheapest start cost 30 × (£18.90 + £0.60) = £585.00, if every day repeats the same rates.
What the calculator leaves out
- Heating, cooling and dehumidification. Lights add heat, so moving them changes when that heat arrives, and those costs can rise or fall with the schedule.
- Network and capacity charges on business supply contracts, unless they are already inside the rates you enter.
- VAT differences. Enter every rate as you pay it, all with VAT or all without.
- Fixture ageing. LED output falls over the years, so a PPFD measured some time ago may be higher than today’s.
- Uneven light. One PPFD figure stands in for the whole canopy, while edges and the gaps between fixtures receive less.
- Sunlight in glasshouses. Daylight changes by the hour and the season. Enter only the shortfall the lights have to cover, and treat the hours as an average.
- Days when the clocks change, which are an hour shorter or longer than the day the calculator prices.
- Tariffs that change price many times a day. The calculator takes one flat rate or one day and night pair. The platform’s energy module prices the full rate profile of a supported tariff.
What the energy module adds
The energy and tariff optimisation module in BeeGrow AI does this arithmetic on data it keeps for you. It captures the day’s rates on your intelligent electricity tariff for your grid supply point on a schedule, so the record survives after the rates roll over. Saved comparisons keep the light power, DLI target and start time behind each figure, so a comparison can be re-read months later and still mean something. Tariff products are compared side by side on the cost of running your actual lighting load. It proposes a schedule, and the grower decides whether to run it.
The reasoning behind pricing light this way is in buying light when it is cheap, and pricing a whole season is covered in working out vertical farm lighting cost over a winter.
Common questions
How do I convert PPFD to DLI?
Multiply the PPFD by the hours of light and by 3,600, the seconds in an hour, then divide by 1,000,000 to turn micromoles into moles. A canopy PPFD of 250 µmol/m²/s for 16 hours gives 250 × 16 × 3,600 ÷ 1,000,000 = 14.4 mol/m²/day. The formula assumes a steady light level; where output changes through the day, or sunlight adds to it in a glasshouse, the light has to be added up over time instead.
How many hours a day should grow lights run?
As many as it takes to reach the crop’s daily light integral at the PPFD the lights deliver: hours = DLI × 1,000,000 ÷ (PPFD × 3,600). A target of 17.28 mol/m²/day at 300 µmol/m²/s needs 16 hours. The DLI target itself comes from your crop recipe, and intensity and the length of the dark period matter too, so check the crop before changing its hours.
How much does it cost to run a grow light for a month?
Multiply the lighting load in kW by the hours a day to get kWh, multiply that by the rate in pence per kWh, then by 30 days, and add the standing charge for the whole bill line. A 6 kW load running 16 hours a day at a flat 24p/kWh uses 96 kWh and costs £23.04 a day, or £691.20 over 30 days before the standing charge. On an intelligent electricity tariff the rate changes through the day, so the answer depends on when the lights come on.
Why price every start time?
On an intelligent electricity tariff the same hours of light cost different amounts depending on when they start, and the cheapest window is not always overnight. The calculator prices every possible start time in the day, including windows that run past midnight, and shows the cheapest. On a tie it shows the earliest.
Does it include heating and dehumidification?
No. It prices the electricity the lights use and nothing else. Moving the lights moves when their heat arrives, so heating, cooling and dehumidification costs can rise or fall with the schedule, and only a metered trial shows the net effect.
Price your lighting against your own tariff
A 30-minute walkthrough against your zones, crops and sensors. No slides unless you ask for them.
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