Understanding realistic autonomy sizing
The direct answer for a typical system
For a system with 800 watts of panel and a 2,000Wh daily load targeting 3 days of autonomy, crediting 15% cloudy production reduces the required battery from about 312Ah under a pure zero-sun assumption to roughly 237Ah, a real reduction in both cost and physical space. The panels don't stop working during heavy overcast, they just work at a fraction of their clear-sky output, and that fraction still counts toward covering your load. Current figures live in the National Laboratory of the Rockies Annual Technology Baseline, which updates its residential battery storage cost data each year. Panel wattage means little if the inverter cannot carry your loads. The hybrid inverter sizing calculator in our Inverters & Equipment section can help you size it for the biggest appliance you run.
Why zero-sun sizing tends to overstate what you actually need
A pure battery-only autonomy calculation, useful as a worst-case sanity check, assumes the panels contribute nothing at all during the outage period. In practice, even a dark, overcast sky produces some usable output, typically estimated at 10% to 20% of clear-sky production for planning purposes. Crediting this partial output against your daily load lowers the net deficit the battery actually has to cover, which is the core difference this tool makes versus the simpler battery only autonomy calculator. Once you have that number, the federal ITC calculator shows what it means for the rest of the system. If you want to check this estimate against a second method, the solar panel calculator in our System Sizing section works from your daily load and peak sun hours.
Choosing a conservative cloudy production percentage
10% is a conservative assumption suited to very heavy, sustained overcast or storm conditions, while 20% fits more moderate cloud cover that still lets meaningful diffuse light through. Choosing too optimistic a percentage risks undersizing your battery for a genuinely bad stretch of weather, so when in doubt, lean toward the lower end of this range, especially for systems with limited generator backup.
Pairing this with a generator backup
Rather than sizing the battery bank for an absolute worst-case zero-production scenario that may rarely or never occur, many off-grid builders size around a realistic partial-cloud target like this one and add a generator backup to cover anything beyond it. This combination typically costs less overall than an oversized battery bank built purely around a worst-case assumption. DOE's overview of battery and energy storage technology is a useful primer on how these sizing trade-offs are typically framed.
What to check once you have a battery size
Compare this result against the cloudy day reserve calculator for a related look at short-term reserve planning, and run the battery bank sizing calculator to convert this into a real unit count for your chosen battery size. If you want to see this same trade-off from the battery-count side rather than the day-count side, the battery count vs autonomy tool in our battery storage tools runs it the other direction. Battery cost trends referenced throughout draw on NREL's Storage Futures Study, and a larger bank may also qualify for a tax credit worth checking through the federal ITC calculator in our rebates and incentives tools.