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Apartment Botany

Grow light calculator: size, distance, and what it costs to run

A grow light is usually cheaper to run than people fear, and hung further away than it should be. Both of those are settled by numbers rather than opinions, and neither appears on the box. This calculator handles the second half of the decision: not which lamp, but what size, how high, how long, and what it adds to the bill.

If you have already run the window estimator, it starts from your window and works out how much of the gap the lamp has to close. If you have not, it assumes an average one and says so.

No saved window yet, so this assumes a middling one. For a real answer, estimate your window first and come back. It takes about a minute.
What do you want from the plant?
How big is the area to light?

The shelf, sill, or patch of floor the plants sit on.

Hours per day

University of Minnesota puts foliage plants at 12 to 14 hours and flowering plants at 14 to 16. Illinois Extension caps it at 16.

The lamp you are considering

Wattage is on the box or the listing. Your electricity rate is on your bill, per kilowatt-hour.

What it costs to run

$2.39per month

14 kWh a month, about $28.64 a year at 36 W for 13 hours a day.

The gap
Your window supplies under half the floor of the medium band, so the lamp is doing most of the work. Mounting distance will matter more than the price of the lamp.
Wattage to look for
45 to 60 W for 3sq ft, scaled from UMN’s per-plant figures for the medium band. At 3 sq ft one lamp will light the middle well and the corners poorly. Two smaller lamps beat one bright one.
Mounting distance
12 to 24 inches above the leaves, per UMN. This is the setting that decides whether the lamp works, and it is free.
Check before buying
Does the maker publish PPFD at several distances, or only a single PPF figure? Only the first can be checked against what your plant needs.

The running cost is arithmetic and is exact if your inputs are. Everything else is a starting range: the bands, hours, and distances come from University of Minnesota and Illinois Extension, scaled by area, which is our extrapolation. No tool can predict the light at your shelf without the beam angle of a specific lamp.

The running cost, in full

The formula is short enough to check by hand, which is the point:

watts × hours per day × 30 ÷ 1000 = kilowatt-hours a month, then multiply by your rate per kilowatt-hour.

A 36 watt lamp on for 13 hours a day is about 14 kilowatt-hours a month. At 17 cents that is roughly $2.39, or under $30 a year. Set against the cost of replacing a plant, or of the lamp itself, it is almost never the deciding number, and yet the vague fear of it stops more purchases than the price tag does.

The rate field defaults to a placeholder. Replace it with the number from your own bill, because rates vary enough between regions and tariffs that any average printed here would be wrong for most readers.

Why the mounting distance matters more than the price

Light falls off with distance faster than intuition suggests, and manufacturers who publish honest figures show it plainly. Soltech lists its 36 watt Aspect Gen 2 at roughly 50 µmol/m²/s at one meter, about 34.9 at four feet, and about 15.5 at six feet.

Put that against University of Minnesota’s floor for low-light plants, which is 50 µmol/m²/s. At one meter that lamp is at the bottom of the lowest band. At six feet it is delivering under a third of it. Nothing is wrong with the lamp. The distance is doing that, and the distance is the one variable that costs nothing to change.

What this calculator does not do

It does not predict the PPFD at your shelf. Doing that honestly requires the beam angle and efficacy of a specific lamp, and inventing a general formula for it would be false precision dressed up as a result.

It also does not convert foot-candles into micromoles. That conversion depends on the spectrum of the light source, and a figure valid for daylight is not valid for a magenta LED. The window estimate and the plant categories here are both expressed in foot-candles, using UMN’s own table, which publishes a foot-candle equivalent beside each photon figure. Staying in one unit end to end is the reason no conversion is buried in here.

The wattage range is UMN’s per-plant figure scaled by the area you enter, which is our extrapolation rather than their recommendation. Treat it as a starting bracket for shopping, not a specification.

Common questions

How much does a grow light cost to run per month?
Multiply the wattage by the hours per day, multiply by 30, and divide by 1,000 to get kilowatt-hours a month, then multiply by your electricity rate. A 36 watt lamp running 13 hours a day uses about 14 kWh a month, which at 17 cents a kilowatt-hour is around $2.39. Grow lights are usually far cheaper to run than people expect, and the fear of the bill stops more purchases than the price of the lamp does.
How many watts do I need for a grow light?
University of Minnesota Extension gives rough per-plant figures by category: 10 to 15 watts for low-light plants, 15 to 20 for medium, and over 20 for high-light plants. Scaling that by the area you are lighting gives a starting range. Wattage is a crude proxy though, because efficiency varies between lamps, and the distance you hang it at changes the result more than the wattage does.
How far above plants should a grow light be?
University of Minnesota puts foliage houseplants at 12 to 24 inches below the lamp, flowering plants at 6 to 12, and seedlings at 4 to 6. Light falls off sharply with distance: one manufacturer publishes about 50 micromoles per square meter per second at one meter from a 36 watt lamp, dropping to roughly 15.5 at six feet. The mounting height is free and it matters more than the difference between an $85 lamp and a $200 one.
How many hours a day should a grow light be on?
University of Minnesota puts foliage houseplants at 12 to 14 hours a day and flowering plants at 14 to 16. Illinois Extension sets the ceiling, advising that 14 to 16 hours is sufficient and not to exceed 16. Running a lamp around the clock does not speed growth, because plants run processes in the dark that they cannot run in the light.
What is the difference between PPF and PPFD?
PPF measures everything a lamp emits, in micromoles per second, and is a property of the lamp alone. PPFD measures what actually lands on a surface, in micromoles per square meter per second, and depends on the lamp, the distance, and the beam angle. PPFD is the one that has to clear the plant's requirement. PPF is always the larger, friendlier number, which is why a lamp that publishes only PPF cannot be checked against what your plant needs.
Can a grow light replace a window?
For light, yes, and that is the one thing about a dim room you can genuinely fix. A lamp lets you choose a light level rather than accept the one your window gives. It does not fix the other things a window is doing, and a plant under a lamp in a cold corner still has a temperature problem.

Next

Once you know the size and the running cost, the remaining question is which lamp, and what the specification sheet is hiding. That is in grow lights for houseplants. If you are not yet sure a lamp is necessary, what a north-facing window actually gives covers what grows without one, and houseplant winter care explains why autumn is when this question usually arrives.