Grow Room Economics: Tutorial + Calculator
What do your lights actually cost?
Everyone throws around numbers. "My 650W runs me forty bucks a month." That number is useless to you, because it was calculated with someone else's power rate, someone else's photoperiod, and a wattage printed on a box instead of measured at the wall. Here's how to get your number, and how to compare two lights honestly.
How this works
Find your real kWh rate
A kilowatt-hour (kWh) is one thousand watts running for one hour. A 1000W light on for one hour burns 1 kWh. That same light on for 12 hours burns 12 kWh. That's the entire unit of measure. Utilities bill you per kWh, so once you know your price per kWh, everything else is multiplication.
Here's the part almost everyone gets wrong. Your bill lists a rate, something like $0.089/kWh, and growers plug that in. But that's usually just the supply (generation) charge. Underneath it sit delivery charges, distribution charges, transmission, fuel cost adjustments, riders, and a fixed customer charge. Those are real dollars and most of them scale with usage.
Pick a rate: three levels of honest
Start anywhere. Just know what you're giving up as you move up the list.
The national average
Fine for a ballpark, and better than a number you made up. The U.S. residential average is 18.8¢/kWh (EIA, April 2026). Know that this is a blend of a country where North Dakota pays 12.4¢ and Hawaii pays 46.6¢, a 3.8x spread. If you use this, treat the answer as a rough order of magnitude, not a budget.
Your state's average
Closes most of the gap in about four seconds. It's still an average, since your specific utility, your rate plan, and your usage tier all move it, but it puts you in the right neighborhood.
EIA residential averages, April 2026. Rates move every year, so treat these as a starting point, not a quote.
Your actual bill
Go get it. Take the total dollars you owe and divide by the total kWh you used. Both are printed on the same bill. That's your all-in blended rate, and it's the only number that's actually yours. It takes thirty seconds and it's usually 30 to 60% higher than the headline rate on that same bill.
Plenty of utilities now run time-of-use pricing: power costs more during peak demand hours (typically late afternoon into evening) and less overnight. The gaps aren't small. On some plans, peak power costs two to three times what off-peak costs.
Your lights don't care what time it is. Your plants don't either. They care about the length of the dark period, not the clock. If you're on a TOU plan, running your lights-on period during off-peak hours is one of the biggest savings available to a home grower, and it costs you nothing but changing a timer. Call your utility or pull up your rate plan and find out. This calculator can't know your peak hours, which is exactly why you need your bill.
Two more things that can wreck your estimate
- Tiered rates. Some utilities charge more per kWh once you cross a monthly usage threshold. A grow can push you entirely into the expensive tier, meaning the marginal cost of your lights is higher than your blended average. If your bill shows tiers, use the top tier's rate for a worst-case number.
- Seasonal rates. Plenty of utilities charge a different summer rate than winter. Your December run and your July run genuinely cost different amounts, even before you account for the air conditioner.
Find your real wattage
The number on the box is a rating, not a measurement. Actual draw at the wall depends on the driver's efficiency, the input voltage, and above all, where you've got the dimmer set. A "650W" fixture at 70% power isn't drawing 650W, and a cheap fixture rated "1000W" might pull 480W and be honest about neither.
If you can't measure it, estimate in this order of trust:
| Method | How to do it | Trust |
|---|---|---|
| Plug-in meter | Read actual watts at your current dim level | Exact |
| Manufacturer spec | Find the spec sheet's rated wattage at 120V input, at full power. Use that as your max. | Good |
| Dimmer math | Assume draw scales roughly with the dial: 50% is about half the max wattage. Close enough to plan with. | Rough |
| The name of the light | Don't. "1000W" in a product title is marketing, not a measurement. | None |
Run the numbers
Two rooms, because most growers run two photoperiods. Veg defaults to 18 hours, flower to 12. Change them to whatever you actually run. Leave a room at zero if you don't use it.
Leave at 30 unless you run a schedule with days off.
Effective draw: 240 W
Fans, pumps, heat mats, anything running 24/7.
Effective draw: 650 W
Fans, pumps, dehu, anything running 24/7.
"Other gear" is treated as running 24 hours a day, since fans and pumps usually do. If yours is on the light timer, just add its watts into the fixture wattage instead.
Advanced: compare two lights fairly
This is the whole reason growers get burned by cost comparisons. A cheaper light to run is not a cheaper light. If light A costs 30% less per month and puts out 40% less usable light, you didn't save money. You bought a smaller light and shrunk your harvest.
The honest unit is efficacy: µmol/J (micromoles of photons per joule of electricity). It's on every reputable spec sheet. It answers the only question that matters: how much light do I get per dollar of power? A 2.9 µmol/J fixture and a 1.9 µmol/J fixture running the same wattage cost you exactly the same on the bill. One just gives you roughly 50% more photons for that money.
Using your flower room's effective wattage, hours, and rate from above.
Fine print worth knowing: efficacy is measured at full power. Most LEDs get more efficient when you dim them, because you're driving the diodes softer, so a big fixture run at 60% often beats a small fixture run at 100% for the same wattage. That's the actual case for buying more light than you think you need and dimming it.
Yield
Grams per watt, and why it lies to you
Most growers benchmark on grams per watt, and 1.0 is a great result for a home grower. Hit it and you're doing well.
What 1.0 isn't is the ceiling. Some growers pull 2.0 grams per watt or better, and that comes down to genetics and style far more than it comes down to gear. It's also a lot harder to get there than the people posting those numbers make it sound, especially in your first few runs. So don't take someone else's 2.0 as proof you're doing something wrong, and don't take it as proof they bought a better light.
The reason any of this matters is the question underneath it: is growing your own actually worth it for you? Run your yield and your power cost through the calculator below and you'll get a cost per gram. Put that next to what you'd otherwise pay, and you'll have a real answer instead of a feeling. For most home growers the power cost turns out to be a small fraction of retail, but your rate, your yield, and your genetics decide that, not me.
Before you go comparing your number to anybody else's, here's what grams per watt hides.
- It ignores time completely. Grams per watt says nothing about how long you vegged. Two growers can both hit 1.0, and one did it in nine weeks while the other took sixteen. Those are not the same result. You can inflate the number just by vegging bigger plants for longer, which costs you power, time, and a turn of the room.
- Plant count and plant size trade off. A higher plant count with a short veg usually gets you there faster than a few large plants vegged out for months. Same yield, less calendar. But the second approach makes the grams per watt look identical, so the metric can't tell you which grower was more efficient.
- Harder is not always better. You can absolutely run lower PPFD, take a longer, gentler ride, and get excellent yield per watt along with better quality. Cranking the light to chase the number is not the same thing as growing well. It's a balance, and figuring out where your room and your plants want to sit is the fun part.
- Genetics is the biggest variable of all. Before your light, before your environment, before your soil, the cultivar decides most of what's possible. Two cuts under the exact same watts in the exact same room will not turn in the same number. Any grams per watt comparison across different genetics is close to meaningless.
Work out your grams per watt and cost per gram
Uses your flower room's light wattage, hours, and rate from Step 3. Enter what you pulled and how long the flower cycle ran.
Two things to be straight about. This counts your flower room lights only, not veg power, not the A/C, not the dehu, so your true cost per gram is higher than what this shows. And grams per watt here is measured against your actual dimmed wattage, not the fixture's max rating, which is the honest way to do it and will give you a different number than the guy quoting his fixture's box.
This calculator gives you an estimate based on the numbers you enter. Real bills include fixed customer charges, taxes, and utility-specific fees that don't scale with usage. Rates vary enormously by region and by season. Measure your own draw and read your own bill. That's the whole point of this page.
BuildASoil. We don't make claims we can't back with data. That goes for soil, and it goes for power bills.
Built for growers who'd rather measure than argue.
