How to Heat a Greenhouse in Winter: kWh, Cost, and Cheaper Options

Q: What is the cheapest way to heat a greenhouse?

Where the crop allows, not heating actively at all: thermal mass like water barrels plus bubble insulation and a cold frame can carry hardy plants through winter at zero running cost. When heat is needed, a thermostat-controlled electric heater in a small insulated zone is cheapest at backyard scale.

Q: What temperature should I keep my greenhouse in winter?

To keep frost off and overwinter plants, hold above 35-45F, which is far cheaper. To actively grow tender crops you need roughly 55-65F, and that higher target is what sends the heating bill up steeply, so decide which you actually need.

Updated July 2026 · Editorial team · Topic: greenhouses / winter heating

How to Heat a Greenhouse in Winter: kWh, Cost, and Cheaper Options — Greenhouses

The greenhouse was the fun part. The electric bill in February was the education. A gardener in Ohio plugged a 1,500-watt heater into a little 6×8 to keep tomatoes cozy through winter, then opened a power bill that had jumped by the price of a nice dinner out — every week. Heating a greenhouse is not hard. Heating one affordably is the whole skill, and it starts with admitting that a thin box in a cold yard leaks heat about as fast as you can pour it in.

Short answer: A single 1,500-watt electric heater draws 1.5 kWh per hour, and in a small uninsulated greenhouse in real cold it can run the equivalent of 8–16 hours a day — that is roughly 360–720 kWh a month, or about $60–125 at $0.17/kWh. You cut that bill far more by keeping heat than by making it: bubble-wrap insulation, a thermostat, and heating only a small zone can halve it. And for cold-hardy crops, an unheated cold frame costing $50–150 skips the bill entirely.

Reviewed by the BackyardStead Lab editorial team. We publish real ROI, plain numbers and USDA/extension data so you can judge for yourself — we run the math, not a farm. Educational information only: backyard-chicken and livestock rules vary by city, home canning must follow USDA/NCHFP-tested methods (botulism risk), and mushrooms should be grown only from a known-species kit — never foraged on our word.

First, do the kWh math for your own greenhouse

Every heating estimate online is wrong for your greenhouse, because the number depends on your size, your insulation, your target temperature, and how cold your nights get. But the structure of the calculation is fixed, and once you see it you can size your own.

Wattage to kWh: a heater's watts ÷ 1,000 = kWh per hour at full output. A 1,500 W heater = 1.5 kWh/hr.
Duty cycle: a thermostatic heater only runs part of the time. Multiply kWh/hr by the hours per day it actually fires.
Monthly cost: kWh/hr × hours/day × 30 × your electricity rate.

Heater runs (equivalent)	kWh/day (1.5 kW)	kWh/month	Cost/mo @ $0.17
4 hrs/day (mild)	6	180	~$31
8 hrs/day (moderate)	12	360	~$61
12 hrs/day (cold)	18	540	~$92
16 hrs/day (deep cold)	24	720	~$122

Your rate matters as much as your climate — electricity runs from around $0.11 to over $0.30 per kWh depending on the state, so the same greenhouse costs three times as much to heat in one region as another.

Cutting the bill: keep heat, do not just make it

The cheapest kilowatt-hour is the one you never spend. Every trick below reduces how hard the heater works.

Move	Cost	Effect
Line interior with bubble insulation	$20–50	Cuts heat loss ~20–40%
Thermostatic controller on the heater	$15–35	Stops heating empty air; big saver
Heat a small zone, not the whole house	$0	Partition off a bench with plastic sheeting
Seal gaps around doors and vents	$5–15	Stops the draft that undoes everything
Thermal mass (water barrels)	$0–40	Soaks day heat, releases it at night

Field note: Black-painted water barrels are the closest thing to free heat a greenhouse has. Fifty-five gallons of water stores an enormous amount of daytime solar warmth and bleeds it back out overnight, flattening the temperature swing that stresses plants. A few drums along the sunny wall can lift the coldest pre-dawn hour by several degrees at zero running cost — thermal mass is passive, silent, and never shows up on a bill.

The cold frame: the option that skips the meter entirely

There is no heated greenhouse humming in a backyard behind this site — the kilowatt math above is drawn from utility rates and heater specifications, not a meter anyone here read — but the honest recommendation for many growers is to not heat at all. A cold frame is a low, glazed box over a garden bed: passive, unheated, and built for $50–150 from old windows or a kit. It holds only a few degrees over outside, but for spinach, kale, mache, and other cold-hardy greens, a few degrees plus wind protection is the whole game. It grows through winter in mild zones on zero electricity.

The rule of thumb: heat a greenhouse only for tender plants that cannot survive cold at all. For hardy crops, a cold frame or an unheated structure does the job for free — and the crop-by-crop temperature needs are laid out in what to grow in a greenhouse by season.

Common mistakes

Heating an uninsulated greenhouse. Insulating first can halve the bill; heating a bare box pours money through the glazing all winter.
Running a heater with no thermostat. A heater that never cycles off wastes most of what it burns. A $20 controller is the highest-return purchase in the whole project.
Heating the whole greenhouse for a few plants. Partition a small bench with sheeting and heat that. You do not need 80 square feet warm to overwinter a dozen pots.
Using an unrated indoor heater in a damp greenhouse. Moisture plus mains electricity is a real hazard. Use a heater rated for wet or greenhouse use, on a GFCI outlet, never a bare household space heater.
Aiming for summer temperatures. Most overwintering just needs to stay above freezing (35–45°F), not 70°F. Every extra degree of target multiplies the bill.

FAQ

How much does it cost to heat a greenhouse in winter?

For a small hobby greenhouse with a 1,500-watt heater, expect roughly $30–125 a month depending on how cold your nights get, how well the structure is insulated, and your local electricity rate. Insulating with bubble wrap and adding a thermostat can cut the low end of that range substantially.

What is the cheapest way to heat a greenhouse?

Not heating actively at all, where the crop allows it — thermal mass like water barrels plus bubble insulation and a cold frame can carry hardy plants through winter on zero running cost. When you must add heat, a thermostatically controlled electric heater in a small insulated zone is the cheapest reliable method for a backyard scale.

Can a greenhouse stay warm without electricity?

It can hold a few degrees over outside passively through solar gain and thermal mass, which is enough for cold-hardy greens in mild climates. It cannot keep tender, frost-sensitive plants alive through a hard freeze without some heat source. Match the ambition to the crop and the climate.

What temperature should I keep my greenhouse in winter?

For simply overwintering plants and keeping frost off, aim to hold above 35–45°F, which is far cheaper than a growing temperature. To actively grow tender crops through winter you need roughly 55–65°F, and that target is what sends the heating bill up steeply, so decide which you actually need.

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General educational information, not professional horticultural advice. Prices, energy costs, plant hardiness and local climate vary by region and season; check figures against current listings and your local extension office before spending.