One and a half kilowatt-hours a day. That's what our basement seed shelf uses, according to the plug-in power meter Nate brought up from his workbench last weekend. He estimates electrical jobs for a living, so of course he owns one, and of course he was delighted to be asked.
I've been starting seeds under lights since 2023, when the wire shelf went in, and for two springs I assumed the electricity cost "maybe a dollar a month." I never checked. With a written garden budget this year, it felt silly to track every bag of compost and ignore the thing humming in the basement 16 hours a day.
The setup
The shelf is a 4-tier, 48-inch wire unit. Two 4-foot LED shop lights hang on chains over the top two tiers. I bought them at Home Depot in 2023 for $24.98 each. The seedling heat mat under the pepper tray cost $21.99 the same year. The lights run on a plug-in timer from 6 a.m. to 10 p.m., and the mat has no thermostat, so it's simply on, day and night.
Why 16 hours? Seedlings under shop lights get far less light than they would outdoors, even on a gray day, so I make up for it with a long day. When I tried 12 hours that first spring, the tomatoes stretched toward the tubes like kids reaching for a high shelf. At 16 hours they stay short and stocky, and they still get eight hours of dark.
Everything plugs into one power strip. Nate put the meter between the strip and the wall outlet, and we left it alone for exactly seven days.
The numbers
After a week, the meter read 10.5 kilowatt-hours. Before that, we'd checked each piece on its own for a minute: the two lights together pulled about 64 watts, 32 apiece, and the mat about 20 watts. The math lines up. The lights use roughly 1 kilowatt-hour over their 16 hours, and the mat adds about half a kilowatt-hour over 24.
For the rate, I took our January electric bill, divided the total dollars by the kilowatt-hours used and got about 27 cents a kilowatt-hour once delivery charges and fees are folded in. Here's how it shakes out over a month:
- Lights: about 31 kilowatt-hours, or roughly $8.30.
- Heat mat: about 14.4 kilowatt-hours, or roughly $3.90.
- Together: about 45 kilowatt-hours, or a little over $12.
So about $12 a month, not a dollar. I was off by a factor of twelve, which I would circle in red if one of my students handed it in.
The shelf runs from mid-February, when the onions go in, to the middle of May, when the tomatoes and peppers go out. Call it three months and change, or about $40 for the season. If I count only the 40 or so tomato seedlings I'm planning and the two dozen peppers sown last Saturday, that's a bit over 60 cents a plant. Add the onions, basil and flowers and it drops well below that.
Is it worth it?
For us, yes, but I want to be fair about it. The electricity is a real cost, and it isn't in my $450 garden budget. I've given it its own line in the notebook, next to the water bill, because both depend on the weather and the utility's rate as much as on my choices. Someone could reasonably argue it belongs in the garden total, and if I add it, last year's tidy story of the garden coming out ahead gets a little less tidy.
What that $40 buys is varieties I can't find as plants anywhere nearby: ají dulce from my mom's seed, Cherokee Purple, Black Cherry, real storage onions. A six-pack of whatever tomato the garden center happens to stock wouldn't give us those.
The meter also answered a question about the future. A third shop light would pull another 32 watts for 16 hours a day, about $4 more a month. That's roughly a third more on the bill for half again as much lit shelf space, which is a better trade than I expected. I'm not buying one this year, but into the notebook it goes.
The part I hadn't thought about is the mat. At $3.90 a month it's about a third of the cost, and it only really earns its keep during germination. Right now it's warming a tray of peppers that, seven days in, show no sign of life at all. The meter is back on Nate's workbench. The basement glows a cool blue-white every evening, the color of a dentist's waiting room, and somewhere under that plastic dome the peppers are taking their sweet time.
