How Much Does an Indoor Grow Light Actually Cost to Run?

“How much will this add to my electricity bill?” is the question that stops a lot of people from buying a grow light at all — usually because nobody gives them an actual number. Here’s the math, done with real rates and real wattages, so you can check it against your own electricity bill in under a minute.

The formula

What a real grow light actually costs

Using the same four lights from our clip-on grow light comparison, run 13 hours a day (the middle of the 12–14 hour range recommended for herbs and greens):

Light	kWh / month	At US average (18¢/kWh)	At a high-rate state (35¢/kWh)
SANSI 10W Pot Clip	3.9	$0.70	$1.36
SANSI 20W 2-Head Most common setup	7.8	$1.40	$2.73
GooingTop Clip-On (≈10W)	3.9	$0.70	$1.36
Good Earth Lighting 18" Strip	6.2	$1.12	$2.17

Calculated from each light's rated power (see the grow light comparison for sourcing). National average and 'high-rate state' figures are from the US Energy Information Administration's Electric Power Monthly, mid-2026 — the national residential average was approximately 18¢/kWh; California was around 35¢/kWh. Rates vary by utility and change over time; check your own bill for the number that actually applies to you.

For almost everyone, that’s one to three dollars a month — closer to the cost of a coffee than a genuine budget concern.

Where you actually land depends on your state

Even at Hawaii’s rates — the most expensive in the country by a wide margin — a single 20W grow light running 13 hours a day comes out to roughly $4 a month. A multi-light setup for a serious windowsill garden might run $5–15 a month even in an expensive state. That’s real money, but it’s a long way from the “your electricity bill will explode” reaction a lot of people expect.

What actually moves the number

Wattage matters most. Doubling the wattage doubles the cost — the SANSI 20W costs exactly twice what the SANSI 10W does to run, because the maths is linear on power draw.

Hours per day matters second. Running a light 16 hours instead of 12 adds about a third to the cost — and doesn’t help your plants, since they need a dark period regardless. This is one more reason a built-in timer pays for itself: it stops a light being left on longer than it needs to be, by accident.

Your local rate matters most of all, proportionally. The exact same light costs roughly three times as much to run in California as it does in Washington state, purely because of the price per kWh — nothing about the light itself changes.

Putting it in context

Compare the running cost to what you’re replacing. A bag of pre-washed salad greens or a small pack of basil from a supermarket regularly costs more than a month of grow-light electricity for the plant that would have produced it. The light isn’t free, but for a small herb-and-greens setup, it’s rarely the expense that should decide whether you start.

If you’re still deciding whether a grow light makes sense for your window in the first place, the beginner setup checklist walks through that decision before you spend anything.