Watt My Roof / Off-grid solar in winter: sizing for low sun, cold and snow

Off-grid planning

Off-grid solar in winter: sizing for low sun, cold and snow

An off-grid system has to work in its worst month, not its average one. Here is how winter changes panels, batteries and backup power.

Why winter is the hard month

• Fewer sun hours. Shorter days and a low sun angle can cut daily solar energy to a fraction of summer.
• Storms come in runs. Several cloudy days in a row drain the battery with little recharge.
• Snow blocks panels. A covered panel makes almost nothing until it clears.
• Loads go up. Lights run longer, and furnace blowers, heat tape and engine heaters add new draws.
• Cold limits batteries. Lithium batteries can't be charged below freezing, and lead-acid loses usable capacity.

Size the array for your worst month

The array size depends directly on peak sun hours. Here is the same small cabin (the off-grid planner's essential loads: a fridge, lights, internet and a water pump, 2.55 kWh a day) sized with 400 W panels at different sun-hour values.

A cabin sized on a typical 5-hour annual average gets 2 panels. If December only delivers 2.5 hours, it needs 4. Find your monthly values with NREL's PVWatts calculator, then enter the lowest month you need to run on solar into the planner's "Peak sun hours" setting.

Add battery days for winter storms

Days of autonomy is how long the battery can run your loads with no sun. Summer cabins often get by with one or two; winter use usually needs more, or a generator. For the same cabin:

Battery cost climbs quickly. Past two or three days, a generator is usually the cheaper way to cover the rare long storm. See the battery sizing guide for the full calculation.

Keep batteries warm enough to charge

• LiFePO4: don't charge below about 32°F (0°C). Choose batteries with low-temperature charge protection, and either self-heating or a place that stays above freezing.
• Insulated enclosure: an insulated box inside the cabin or a utility room is simpler than heating batteries outdoors. Never enclose batteries with a fuel-burning heater.
• Lead-acid: can charge in the cold but delivers noticeably less capacity, and a discharged lead-acid battery can freeze.

Check panel voltage on the coldest morning

Panels produce higher voltage when they are cold. A typical panel's open-circuit voltage (Voc) rises about 0.25–0.3% for every °C below 25°C, so at -20°C it can be roughly 11–14% above the label value. That combined string voltage must stay under your charge controller's maximum PV input, or the controller can be damaged on a clear, frigid morning.

Enter the coefficient exactly as the datasheet lists it, as a negative number such as −0.28%/°C (−0.0028). With a record low below 25°C, the two negatives multiply to a positive, so the result is higher than the label Voc, as it should be. If your answer comes out lower than the label Voc, a sign is wrong.

Example: two panels with a 40 V Voc in series, a −0.28%/°C coefficient, and a −25°C record low: 80 V × (1 + (−0.0028 × (−25 − 25))) = 80 V × 1.14 = 91.2 V. That fits a 100 V charge controller, but three panels in series (136.8 V) would need a 150 V controller. Many controller makers also publish string-sizing calculators.

Plan for snow and tilt

• Tilt steeply. Around latitude + 15° favors the low winter sun and helps snow slide off.
• Leave clearance below the array. Snow sliding off needs somewhere to go; ground mounts should sit above typical snow depth.
• Clear panels gently. Use a soft roof rake from the ground. Never chip ice or climb onto a snowy roof.
• Mind snow load. Racking must be rated for your area's snow load. This is a structural question for a professional.

Reduce winter loads

• Use propane or wood for heat, cooking and hot water. Electric resistance heat is rarely practical on off-grid solar.
• Choose an efficient fridge or chest freezer. A cold cabin also helps them run less.
• Watch hidden winter loads: furnace blowers, heat tape on pipes and well-pump heaters. Check them in the appliance wattage chart.

Plan generator backup

An inverter/charger lets a generator recharge the batteries and run loads at the same time. Size the generator to cover your charger's input plus any loads running while it charges, and keep it outdoors, away from windows, with a working carbon monoxide alarm inside. Connecting a generator to a building's wiring requires a transfer switch installed by an electrician.

FAQ

Next, check that your inverter can start winter loads like a well pump with the inverter sizing guide.