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Sprout & Spectrum

Running Cost and Efficiency

A grow light costs less to run than almost anyone expects, and the exact figure takes one line of arithmetic. Here it is, with your own tariff.

A plug-in energy meter beside a grow light power supply on a shelf indoors

What a grow light actually adds to the bill

Almost everyone overestimates this, and it puts people off buying a light that would cost them a few dollars a month. The arithmetic is one line, it needs no assumptions beyond your own tariff, and it settles the question in about ten seconds.

kWh per month = watts × hours per day × 30 ÷ 1000
Cost per month = kWh per month × your rate ($/kWh)
A typical houseplant setup, a whole winter

A 36 W grow bulb running 12 hours a day, at the US residential average of 18.34 cents per kWh:

36 × 12 ÷ 1000 = 0.432 kWh a day.

0.432 × 30 = 12.96 kWh a month → $2.38 a month.

Over five months of winter, about $11.90. That is the entire cost of keeping a plant growing through a dark season.

The rate above is the US residential average from the EIA’s Electric Power Monthly for June 2026. Your own rate is on your bill, and it varies enormously by state — which is exactly why the cost calculator takes it as an input rather than assuming it.

How this category divides

Monthly cost by fixture class, at 12 hours a day and 18.34 cents per kWh
Fixture classTypical drawkWh/monthCost/month
Clip light10 W3.6$0.66
Grow bulb36 W13.0$2.38
Pendant36 W13.0$2.38
Linked bars (4 × 20 W)80 W28.8$5.28
LED panel100 W36.0$6.60
Large LED panel150 W54.0$9.90
Full shelf rack (12 bars)240 W86.4$15.85

What decides the cost

Two things, and only two: how many watts the fixture draws, and how many hours you run it. Efficacy — photons per joule — does not change the bill directly. What it changes is how much light you get for those watts, which means an efficient fixture lets you hit the same target with fewer watts, or run for fewer hours.

The efficacy spread across the fixtures we have specs for is 1.39 to 2.7 µmol/J. To deliver the same 250 µmol/s, the efficient one draws about 93 W and the inefficient one about 179 W — which over a five-month winter at 14 hours a day works out to roughly $36 versus $69. Real, and worth thinking about at purchase, but not the difference between affordable and unaffordable.

The mistake nearly everyone makes

Using the “equivalent” wattage from the box. A fixture marketed as “1000 W equivalent” that actually draws 160 W costs you the 160, not the 1000. Always compute from actual draw. If a product page will not tell you actual draw, that is a meaningful signal about the rest of its specifications.

Picks in this category

Each of these names a top pick and links straight out to it. Prices come from the live price layer, which is why they read “check price” rather than showing a figure we cannot verify at the moment you load the page.

Tools

Put your own numbers in. The formula and a worked example sit under each one.

Everything else here

Questions people actually ask

+How much does it cost to run a grow light per month?
Multiply watts by hours per day, multiply by 30, divide by 1000, then multiply by your electricity rate. A 100 W panel run 12 hours a day is 36 kWh a month — about $6.60 at the US residential average of 18.34 cents per kWh in June 2026.
+Do grow lights use a lot of electricity?
For a home setup, no. A typical grow bulb costs a couple of dollars a month and a 100 W panel under seven. A full three-shelf seed-starting rack is around $16 a month while it is running, and it only runs for part of the year.
+Does a more efficient grow light save money?
Indirectly. Efficacy does not change what a watt costs; it changes how much light you get per watt, so an efficient fixture hits the same target with less power. Across common fixtures the spread is about 1.39 to 2.7 µmol per joule, which over a five-month winter is roughly $36 against $69 for the same delivered light.

Sources