Battery & storage
Enter a heating-focused conservation load, storm length, and weak winter solar offset. Get a realistic battery size for a cold-weather outage.
How it works
Step 1
Running the furnace blower only, rather than full heating, keeps this figure manageable, similar to the conservation approach in the storm season planning tool.
Step 2
Short days, low sun angle, and potential snow cover all reduce production well below summer levels.
Step 3
The storm season planning tool shows how much the season itself changes the math.
Winter storm backup planning combines two factors that both work against available battery capacity at once. Heating-related loads, a furnace blower for circulation, a small space heater, or a well pump kept from freezing, add to the conservation-mode wattage beyond what a summer scenario typically requires, a combination Ready.gov's winter storm preparedness guidance specifically calls out as a higher-risk outage scenario. At the same time, short winter days, a lower sun angle reducing panel efficiency, and the real possibility of snow accumulating on panels all reduce available solar offset well below what the same system would produce in summer. Together, these factors mean winter storm planning often lands on a noticeably larger battery figure than a similar summer storm scenario with the same conservation approach.
Snow accumulating directly on solar panels can significantly reduce or temporarily eliminate production entirely until the snow melts or slides off the panel surface. Panels installed at a steeper tilt angle tend to shed snow more readily than those mounted at a shallow angle, a pattern documented by NREL's solar resource research, worth considering alongside the charge time from solar tool for a new installation. This makes winter solar offset inherently less predictable than other seasons, and a conservative offset estimate is generally the safer assumption for winter storm planning.
Many furnace systems can run just the blower motor to circulate existing warm air through the home's ductwork without running the full heating cycle, using significantly less power than complete furnace operation while still preventing the coldest spots in the home. Combining blower-only circulation with a small space heater focused on one main living area, rather than heating the whole home to a normal comfortable temperature, can meaningfully reduce the battery capacity a winter outage actually requires, and confirming your inverter can handle the blower's startup surge is covered by the hybrid inverter sizing calculator in our Inverters & Equipment category. The HVAC backup tool covers full heating and cooling scenarios if that level of comfort is the goal instead.
If your home relies on a well pump that needs protection from freezing during a winter outage, the well pump backup tool covers that specific load in more detail alongside this broader winter storm planning figure. Cold weather also affects the battery's own chemistry-level capacity; the cold weather battery capacity loss calculator in our Battery Chemistry & Storage category models that directly.
Questions
A household running a 900 watt heating-focused conservation load for 3 days, with only 1.5 kWh of weak daily solar offset typical of short winter days, needs about 73 kWh of nameplate capacity, roughly 6 batteries, notably more than a comparable summer storm scenario.
Winter storms combine two factors working against battery capacity: heating-related loads like furnace blowers and space heaters add to the conservation-mode wattage, while short winter days, low sun angle, and potential snow cover on panels all reduce available solar offset compared to summer conditions.
Snow covering panels can significantly reduce or temporarily eliminate production until it melts or slides off, though panels mounted at a steeper tilt angle often shed snow faster than those installed at a shallow angle. This makes winter solar offset inherently less reliable than other seasons.
Many furnace systems can run just the blower to circulate existing warm air, or a smaller space heater in one room, using significantly less power than running the full heating system continuously, which can meaningfully reduce the battery capacity needed during a winter storm outage.
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