The arithmetic nobody shows you, the numbers that actually move it, and how to work out your own bill before you buy anything.
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Ask what it costs to heat a greenhouse through a Canadian winter and you will get one of two answers. Either a cheerful non-answer — “it depends!” — or a confident dollar figure from somebody in Kentucky whose January low is the temperature at which your tomatoes were still ripening.
Here is the thing: it genuinely does depend. A number that is right for a twin-wall greenhouse in Chilliwack is wildly wrong for a single-poly hoop outside Saskatoon, and both are wrong the moment your utility changes its rates. Anyone quoting you one figure for “a greenhouse” is guessing.
So we are going to do something more useful. We are going to hand you the actual arithmetic — the same calculation commercial growers and greenhouse engineers use — with real numbers plugged in, so you can work out your cost, for your structure, at your utility rate, before you spend a dollar. It takes about ten minutes and it is the single most valuable thing you can do before committing to winter growing.
The one formula that runs everything
A greenhouse loses heat four ways at once: conducted straight through the glazing, carried out by air leaking through gaps, vented deliberately, and radiated to the cold night sky. For planning purposes you do not need to model each one separately — published U-values already fold in radiation effects, and the standard equation gets you close enough to size equipment and budget honestly:
Heat loss (watts) = U × A × ΔT
- U is your glazing’s U-value — how leaky the covering is, in watts per square metre per degree. Lower is better.
- A is your total glazed surface area in square metres: roof, sidewalls, and both end walls added together.
- ΔT (“delta T”) is the temperature difference between inside and outside, in degrees Celsius.
That is it. Everything else in this article is a consequence of those three numbers. And notice what the formula tells you immediately: heat loss is proportional to the temperature gap. Double the gap and you double the rate heat pours out. There is no clever trick that escapes this — only better glazing, less surface area, or a smaller gap.
One caveat to hold onto: this equation covers what conducts through your glazing. It does not capture air leaking through gaps around doors, vents and seams — which is a large enough share of real-world loss to deserve its own section further down.
Step 1: find your U-value
| Glazing | Approx. U-value (W/m²·K) | What it means in practice |
|---|---|---|
| Single poly film | ~6.0–6.5 | Cheapest to build, most expensive to heat. Fine for season extension, brutal for winter. |
| Single-pane glass | ~5.6–5.8 | Beautiful light, poor insulation. Classic Victorian glasshouse problem. |
| Single-wall polycarbonate | ~5.0–5.5 | Similar to glass thermally, tougher physically. |
| Inflated double poly | ~4.0 | The commercial standard. Big improvement for modest cost. |
| 4–8 mm twin-wall polycarbonate | ~3.4–3.6 | Roughly 40% less heat loss than single glass. The practical cold-climate default. |
| Multi-wall polycarbonate (16 mm+) | ~2.0–2.3 | Best commonly available. Worth it if you intend to hold real warmth all winter. |
Treat these as planning figures. Real performance drops as glazing ages, dusts, or loses its inflation, and every structure leaks air around doors, vents, and seams on top of what conducts through the panels.
Step 2: measure your glazed area
Add up every surface exposed to the cold — roof, two sidewalls, two end walls. Do not include the floor. For a common 8 × 10 ft hobby greenhouse (about 2.4 × 3 m) with roughly 1.8 m walls and a peaked roof, that comes to something near 30 m². Measure yours properly rather than borrowing that number; surface area is the term most people get wrong, and it scales your entire bill.
Step 3: size the heater with your worst night
To size a heater you use your design temperature — not your average winter, but roughly the coldest night you realistically expect. Take that, subtract it from your target inside temperature, and you have ΔT. For a defensible number rather than a guess, look up your location in Environment and Climate Change Canada’s published climate normals, which give you decades of records for your area instead of your memory of last February.
Work it through for our 30 m² twin-wall greenhouse (U = 3.5), holding just above freezing at 2°C, on a −30°C night:
3.5 × 30 × 32 = 3,360 watts — about 3.4 kW.
The number that stops most people cold
Look at that 3.4 kW figure again, because it contains a nasty surprise that almost no beginner’s guide mentions.
A standard Canadian 120 V, 15-amp circuit is rated for 1,800 watts — but electrical code limits continuous loads to 80% of a circuit’s rating, which puts your real ceiling around 1,440 watts. Your greenhouse, merely holding above freezing on a cold prairie night, wants roughly double that. You cannot heat it from an extension cord. Not because the heater does not exist, but because the circuit will not carry it.
This is the moment a lot of winter greenhouse plans quietly change shape. Your realistic options become a dedicated 240 V circuit run out to the structure, a propane or natural gas heater instead of electric, or — the choice most hobby growers eventually make — reduce the demand rather than chase the supply.
And notice how much the glazing decides here. Run the same greenhouse in single poly (U = 6.5) and the same −30°C night needs 6.2 kW — nearly double, for an identical structure at an identical temperature. The covering is not a cosmetic choice. It is most of your heating bill, decided once, at purchase.
From peak watts to a seasonal bill
Peak load sizes your heater. It does not tell you what you will pay, because your greenhouse does not sit at −30°C all winter. For running costs you need to think in degree-hours: how big the gap is, multiplied by how many hours you spend maintaining it.
The professional method here is heating degree days (or degree hours) — a published climate figure that accumulates how far, and for how long, outdoor temperatures sat below a given threshold across a season. It handles real weather properly, because real weather is not a single average.
For a back-of-envelope planning estimate, though, this will do:
Seasonal kWh ≈ (U × A × effective average heating-season ΔT × heating hours) ÷ 1,000
⚠️ The phrase doing the work there is effective average heating-season ΔT — the average gap across the hours you are actually heating, not the difference between your setpoint and a simple average of winter temperatures. Those are different numbers, and treating them as the same is the most common way this calculation goes wrong. Treat the result as an order-of-magnitude planning figure, and let degree-day data or a season of real metering give you the accurate one.
For our 30 m² twin-wall greenhouse over a five-month heating season (roughly 3,600 hours), the difference between two setpoints is stark:
| Target inside temperature | Rough seasonal energy | Why the jump is so steep |
|---|---|---|
| 2°C (frost protection) | Order of 1,500–2,500 kWh | The heater only fires on genuinely cold nights. Many hours it never runs at all. |
| 12°C (light growth) | Order of 5,000–7,000 kWh | Bigger gap on every hour and far more hours needing heat — it runs nearly continuously from October to April. |
These are illustrative ranges, not a quote — swap in your own area, glazing, and climate and you will get a different answer. How different is worth stating plainly: the same greenhouse might use well under 1,000 kWh in coastal southern British Columbia and comfortably exceed 8,000 kWh in Winnipeg or northern Ontario. Climate is not a rounding error in this calculation; it is one of the biggest terms in it.
But the shape holds everywhere in Canada, and it is the most important thing on this page: going from frost-protection to modest growing warmth does not add a bit to your bill. It multiplies it, typically by three or more.
The reason is that raising your setpoint does two things at once. It widens the gap during every hour you are already heating, and it dramatically increases the number of hours you need to heat at all. At 2°C the heater sleeps through most of autumn. At 12°C it is working from the first cool night in September.
The free heat you have not counted
Everything above assumes your heater is doing all the work. It is not — and this is why real seasonal bills usually land below what peak-load arithmetic suggests.
On a clear winter day, solar gain through the glazing can be substantial. A well-oriented greenhouse will happily sit at 20–35°C inside while it is below freezing outside, needing no supplemental heat at all through a sunny afternoon — and then wanting it again within an hour of sunset. Over a season, those free hours add up to a real dent in your bill.
Two practical consequences. First, do not panic at your peak-load number — that is your worst-case sizing figure, not your typical day. Second, anything that helps you catch and keep that free heat pays back twice: good southern exposure, glazing kept clean, snow cleared off the roof, and thermal mass positioned to soak up the afternoon and give it back after dark. In a Canadian winter, the sun is a short-shift worker — but it does show up.
Which fuel — and the rate on your bill is not the rate you pay
Once you know roughly how many kilowatt-hours of heat you need, the fuel question becomes simple arithmetic. Every fuel converts to a cost per kWh of delivered heat, and comparing them on that basis is the only honest comparison.
| Fuel | Convert it like this | Notes for cold-climate growers |
|---|---|---|
| Electricity | Your all-in rate per kWh, essentially 1:1 — resistance heat is ~100% efficient | Simplest and safest. Cost swings enormously by province: cheap in Quebec and Manitoba, expensive in Alberta, PEI and the territories. |
| Natural gas | 1 GJ ≈ 278 kWh. Divide your $/GJ by 278, then divide by burner efficiency (~0.8–0.95) | Usually the cheapest per unit of heat where it is available — BC, AB, SK, MB, ON, QC. Rarely an option in Atlantic Canada or the North. |
| Propane | 1 litre ≈ 7 kWh. Divide $/litre by 7, then by efficiency | Available anywhere, which is its real advantage. Typically lands near or above electricity in much of the country. |
| Heating oil | 1 litre ≈ 10.6 kWh. Same method | Generally the most expensive of the four, and rarely worth plumbing to a hobby greenhouse. |
| Wood | Highly variable by species, moisture, and stove efficiency | Can be very cheap if you have your own supply — but it needs tending overnight, which is exactly when you need it most. |
⚠️ One trap worth more than the rest of this table combined: the advertised rate is not what you pay. Delivery charges, transmission, regulatory riders, and taxes commonly add 20–50% on top of the energy charge — and in provinces like Ontario and Alberta these add-ons can roughly equal the energy charge itself. A “12 cents per kWh” plan can be a 20-cent reality.
One footnote on electricity, since somebody always asks about heat pumps: an air-source heat pump can deliver two to four units of heat per unit of electricity consumed, which beats resistance heating substantially. The catch is that efficiency falls as it gets colder — exactly when you need it — and most hobby greenhouses are too small to justify the installed cost. Worth considering for a large structure you intend to heat properly for years; overkill for an 8 × 10.
So do not use an advertised rate for this calculation. Take your last bill, divide the total amount owing by the kilowatt-hours consumed, and use that number. It is the only figure that reflects what heating actually costs you.
The levers that actually move your bill
Look back at U × A × ΔT. There are only three terms, so there are only three ways to spend less — and they are not equally easy.
Lower your ΔT — by far the biggest lever. Every degree you drop your target temperature cuts both the rate of loss and the hours of operation. Choosing frost protection over growing warmth is worth more than any equipment upgrade you can buy, which is why the honest recommendation for most hobby growers is to overwinter plants cool rather than push them warm. Our guide to growing citrus in a Canadian greenhouse works through exactly this trade-off with a real crop.
Lower your U — decided at purchase, expensive afterwards. Going from single poly to twin-wall roughly halves your heat loss forever. If you have not bought yet, this is the decision that matters most. If you already own a single-layer structure, adding an interior layer of UV-stabilized greenhouse bubble insulation on Amazon.ca is the cheapest meaningful retrofit there is — it creates a dead air gap and can cut losses substantially for very little money, at some cost in light. ⚠️ Buy the greenhouse-grade UV-stabilized product, not ordinary packing bubble wrap, which goes brittle and yellow after a season of sunlight.
Lower your A — mostly a design decision. You cannot shrink a greenhouse you already own, but you can heat only part of it. Curtaining off a small growing zone with poly sheeting and heating that alone is a genuinely effective trick, and insulating the north wall (which contributes little light but plenty of heat loss) costs almost nothing.
Then there is the term the formula ignores: air leakage. Cold air pouring in through gaps around doors, vents, and glazing seams is commonly cited as accounting for something in the range of 20–40% of a greenhouse’s heating demand — a share big enough that it can outweigh a glazing upgrade. Weather-stripping a door and sealing obvious gaps is the highest-return hour of work in the whole project, and it costs almost nothing.
Equipment that pays for itself
A thermostat controller, before anything else. A heater running all night regardless of need burns money continuously. A thermostat controller on Amazon.ca fires it only when the temperature actually drops to your setpoint, and typically pays for itself within a single season. If you buy one thing from this article, buy this.
A min/max thermometer, to find out what really happened. A min/max thermometer on Amazon.ca tells you the actual low your greenhouse hit at 4 a.m. — which is almost never what you assumed. It also tells you whether your heater is oversized, undersized, or cycling badly. Cheap, and it turns guesswork into data.
Thermal mass, to flatten the swings. Barrels of water along the north wall absorb daytime warmth and release it overnight. Be clear about what this does and does not do: thermal mass mainly reduces short-term temperature swings and delays heater cycling through the night, rather than substantially cutting your seasonal energy use. Its best trick is buying you real hours of frost protection if the power fails.
A small circulation fan. Heat stratifies badly in a greenhouse — warm air pools at the ridge while your plants sit in the cold layer at bench height. A small circulation fan on Amazon.ca pushes that warmth back down where it does some good, letting you hold your target with less fuel.
The right heater, sized to your calculation. Use the peak-load number you worked out above, not a guess. An electric greenhouse heater on Amazon.ca suits small structures and modest setpoints where you have the circuit for it; a propane heater on Amazon.ca covers larger structures or sites without adequate electrical service. ⚠️ Any combustion heater in an enclosed space needs proper ventilation — unvented burners release both carbon monoxide and a great deal of water vapour, and neither belongs in a sealed winter greenhouse. Fit a carbon monoxide detector wherever a combustion heater runs, and treat it as mandatory rather than optional if the greenhouse is attached to your house or is a space you spend time in.
Do this before you buy anything
Ten minutes with a tape measure and your last utility bill:
- Measure your glazed area — roof, sides, ends.
- Look up your glazing’s U-value in the table above.
- Pick your realistic coldest night, and the inside temperature you actually want.
- Multiply: U × A × ΔT = the heater size you need, in watts.
- Divide your last bill’s total by its kWh to get your true all-in rate.
- Estimate seasonal kWh, multiply by that rate, and look honestly at the answer.
Most people who run this calculation do not abandon the idea. They adjust it — they drop the setpoint, curtain off a smaller zone, upgrade the glazing, or decide that frost protection plus a good spring is a better deal than tropical January. All of those are wins, and all of them come from having done the arithmetic first rather than discovering it on a February bill.
The generic advice tells you a greenhouse extends your season. The cold-climate reality is that it extends your season for a price you can calculate in advance — and knowing that number is the difference between a greenhouse you enjoy and a greenhouse you resent.
Common questions
How many BTUs do I need to heat my greenhouse?
Work out your peak load in watts using U × A × ΔT, then multiply by 3.41 to convert to BTU per hour. Our 30 m² twin-wall example at 3,360 watts comes to roughly 11,500 BTU/hr. Buy a heater that meets or modestly exceeds that figure — oversizing slightly is safer than undersizing, but a wildly oversized heater short-cycles and wastes fuel.
Can a greenhouse stay above freezing without any heat?
In most of Canada, not through a real winter. An unheated greenhouse typically runs only a few degrees warmer than outside overnight — enough to shift your effective season by a few weeks at each end, not enough to hold above freezing in January. Thermal mass and an insulated north wall stretch it further, but frost protection through a prairie winter needs a heat source.
Is propane or electricity cheaper for greenhouse heating?
It depends entirely on your province. Convert both to cost per kWh of delivered heat using the table above — propane at $1.00/litre works out near 14 cents per kWh before efficiency losses, which loses to Quebec or Manitoba electricity and beats Alberta or Maritime electricity. Where natural gas is available, it usually beats both.
Does snow on the roof insulate a greenhouse?
Slightly — but it costs you far more in light than it returns in insulation, and a heavy wet load is a genuine structural risk. Clear it. The exception is snow banked against the base of the north wall, which insulates without shading anything worth keeping.
How much does bubble insulation actually help?
Adding an interior layer creates a dead air gap and can cut heat loss through that surface substantially — the single best return per dollar available to someone who already owns a single-layer structure. The trade-off is light transmission, so it suits overwintering far better than active winter growing.
Start with your real season
Every figure above depends on how long your cold season actually runs and when your growing window opens. Our free Zone Calendar is matched to your postal code and built around the two dates that govern a northern growing year — the right starting point before you size a heater or choose a setpoint.
And if you are still choosing a structure, the glazing decision above is the one that locks in most of your future heating bill. We cover it properly in our full guide to cold climate greenhouse design.