Solar for a greenhouse

Ventilation is the everyday load. An exhaust fan for a hobby greenhouse draws 100–300 watts and a commercial one 500 watts or more, running whenever the house overheats, which in Colorado’s sun is most afternoons from March through October. Circulation fans are smaller but run longer. Over a summer month, fans alone can add 50–150 kWh to a bill.

Heat is the load that matters in winter, and it’s usually not electric. Most Colorado growers heat with propane or natural gas because electric resistance heat at 1.5–5 kW running through a January night costs too much. If you do heat electrically, or with a heat pump, a greenhouse can use more power than the house. Grow lights add up too: a bank of LED fixtures at 100–600 watts each, run 12–16 hours a day for seedlings, is a serious load in February and March.

Water is usually small. A pump for drip or hydroponics draws tens to a few hundred watts, though a well pump feeding the greenhouse is 1 kW or more when it runs. Add it all up honestly before sizing; a greenhouse that’s “just a few fans” and one that grows tomatoes under lights through the winter are very different customers.

Greenhouse glazing is glass or polycarbonate on a light frame. It isn’t designed to carry panels plus snow, there’s nothing solid to anchor to, and every panel you put on it is a patch of shade in the one building on your property that exists to gather light. A few specialty semi-transparent panels exist for large commercial greenhouses; they’re not something we install, and for a residential grower they don’t make sense.

The right hosts are a south- or west-facing house roof, a garage or shed roof, or a ground mount near the greenhouse. A ground mount is often the natural choice on a lot with a greenhouse, because you already have open space and the array can be set at the tilt that maximizes winter production, when the lights and any heat are running. It also keeps the array out of the snow-shed zone of nearby roofs.

The greenhouse itself just needs a proper circuit from the house or a subpanel, buried in conduit, with GFCI protection because it’s a wet location. If we’re trenching for an array anyway, we run the greenhouse feeder in the same trench.

A greenhouse in Colorado buys you weeks on both ends of the season and, with heat, year-round growing. The same low winter sun that makes a greenhouse worth having makes solar output lowest in December and January, exactly when heat and lights peak. A grid-tied array handles that mismatch through net metering: you bank summer surplus as credits and draw them down in winter. Off-grid, that mismatch would take an enormous battery, which is why we don’t design greenhouse solar off-grid.

Hail deserves a mention. Front Range hail that dents a car will shred single-wall polycarbonate and break glass. Panels are tested against roughly one-inch hail and often outlast the greenhouse glazing in a storm. If you’re building new, twin-wall polycarbonate with a hail-rated warranty is worth the money, and it holds heat better too.

If your greenhouse is a 6-by-8 kit with a single vent fan and no wiring, a self-contained solar attic-fan style unit or a 50–100 watt panel with a small battery running a 12-volt fan is a reasonable purchase, roughly $100–$300 at retail, prices vary, and needs no permit. Wax-cylinder vent openers need no power at all and are the simplest way to keep a small greenhouse from cooking. We don’t sell any of that, but it’s the honest answer for a small hobby house.

If the greenhouse has a circuit, lights, a heater or a pump, treat it as part of your house load and size a grid-tied array for the total. That’s a job we install. Growers with commercial-scale houses, especially with electric heat, lights or refrigeration on a commercial rate, should look at our commercial and agricultural solar pages; the federal business credit under §48E remains available for solar placed in service by the end of 2027 as of September 2026, which the residential side no longer has.