Battery & storage
Enter a conservation-mode load, days of outage, and any partial solar recharge from cloudy days. Get a realistic multi-day battery size.
How it works
Step 1
Multi-day planning needs true essentials, not full comfort loads, to stay on a manageable battery size, the same approach used in the critical loads calculator.
Step 2
Even heavy storm clouds allow some panel output, meaningfully reducing net battery drain over several days.
Step 3
The reserve percentage tool covers adjusting your setting ahead of storm season.
Running full household comfort loads, central HVAC, full lighting throughout the home, and every appliance continuously across several days would require an enormous battery bank, often well beyond what most residential systems can practically provide. A conservation-mode approach, running only true essentials like refrigeration, minimal lighting, and phone charging, keeps a multi-day outage manageable on a realistic, affordable battery size, an approach consistent with Ready.gov's power outage preparedness guidance, which is why this calculator defaults to that framing rather than a full comfort load requiring a much larger inverter, a difference the hybrid inverter sizing calculator in our Inverters & Equipment category illustrates directly.
Solar panels continue producing electricity under heavy cloud cover, typically somewhere between 10 and 25% of clear-sky output depending on cloud density, which meaningfully offsets net battery drain across a multi-day storm event even without full sun, a production range documented by NREL's solar resource research. This partial production is a key reason why a pure battery-only worst-case estimate often overstates the real capacity needed for storm season, since some solar recharge continues even on the stormiest days.
Because most battery systems allow adjusting the backup reserve percentage through an app, many households raise this setting specifically heading into hurricane or storm season, ensuring maximum capacity is locked away and unavailable for daily cycling right when outage risk peaks. The battery reserve percentage tool covers exactly how this seasonal adjustment works and what tradeoffs come with it. The hybrid inverter sizing calculator checks whether your inverter can handle this result.
If your region faces a specific storm risk like wildfire-related planned outages or winter storms, the wildfire PSPS planning tool and winter storm planning tool cover those region-specific scenarios in more detail. If financing affects how much storm-season capacity you can justify, the financing for battery addition calculator in our Financing category covers that cost question.
Questions
A household running a 400 watt conservation-mode load for 5 days, with a modest 3 kWh of daily solar offset from cloudy-day production, needs about 40 kWh of nameplate capacity, roughly 3 batteries, far less than running full comfort loads continuously would require.
Running full household comfort loads, HVAC, full lighting, and every appliance, continuously across multiple days requires enormous battery capacity, often more than most home systems can practically provide. A conservation-mode approach, running only true essentials, keeps multi-day outages manageable on a realistic battery size.
Yes, solar panels still produce some power under heavy cloud cover, typically 10 to 25% of clear-sky output, which meaningfully reduces net battery drain over a multi-day event even without full sun. This partial production is why battery-only worst-case estimates often overstate real storm season need.
Many households do raise their reserve percentage heading into storm season specifically to make sure maximum capacity is available if an outage strikes, then lower it again during calmer months. The battery reserve percentage setting tool covers this seasonal adjustment in detail.