Understanding battery-only autonomy
What this number actually tells you
Battery-only autonomy is how long your system runs if the sun never comes out again after today. A 400Ah, 12V lead-acid bank at 50% depth of discharge holds 2,400Wh of usable energy. Against a 1,500Wh daily load, that's 1.6 days before the battery hits its safe discharge floor. This is the number that decides whether a three-day cloudy stretch in the Pacific Northwest or Ontario leaves you in the dark. Pair this with the federal ITC calculator to see the full picture before you commit.
Why depth of discharge changes everything
Depth of discharge, or DoD, is the percentage of a battery's total rated capacity you can use before recharging without shortening its life. Flooded and AGM lead-acid batteries are commonly limited to 50% DoD. LFP (lithium iron phosphate) batteries handle 80% to 100% DoD safely and hold that capacity over thousands more cycles. Swapping a 400Ah lead-acid bank for a 400Ah LFP bank at 100% DoD nearly doubles your real autonomy without adding a single amp-hour of rated capacity. See the voltage selection tool if you're also deciding between 12V, 24V, and 48V.
How much autonomy is actually enough
Most off-grid builds target 2 to 3 days of pure battery autonomy, using solar to top the bank back up on any day with usable sun. Sizing for a full week of zero-sun runtime usually means a battery bank two to three times larger than needed, since multi-day zero-sun stretches are rare outside extreme northern latitudes in deep winter. The cloudy day reserve calculator helps you land on a target that fits your actual climate instead of a worst-case guess. Much of the cost and performance thinking behind these targets traces back to the National Laboratory of the Rockies Annual Technology Baseline, which tracks how residential battery costs and performance have shifted over the past decade.
Cold weather cuts real capacity below the rated number
Battery capacity drops as temperature drops, and the effect is steeper than most people expect. A lead-acid battery near freezing can lose 20% or more of its rated capacity, and LFP batteries stop accepting charge below freezing entirely unless they carry a built-in heater. Builders in Minnesota, the Canadian prairies, or anywhere with sustained sub-freezing winters should plan around the derated number, not the spec sheet number. The northern latitude winter sizing tool covers this in more depth, and DOE's overview of how battery storage systems get specified is a useful primer if any of the terminology here is new. Panels are only half an off-grid system. The battery storage size calculator in our Battery & Storage section sizes the battery bank your daily load needs to get through cloudy stretches.
What to check once you have this number
If your autonomy figure comes back short of your target, you have three levers: add battery capacity, cut your daily load with the lighting load calculator or similar appliance-specific tools, or add a generator backup to cover the gap without oversizing the bank. For a look at how others weigh autonomy against battery count, the battery count vs autonomy tool in our battery storage tools runs the same math from a different angle. A larger bank isn't purely a cost question either, since the federal ITC calculator in our rebates and incentives tools can shift what that additional capacity actually costs after tax credits. Panel wattage means little if the inverter cannot carry your loads. The quick inverter size estimator in our Inverters & Equipment section can help you size it for the biggest appliance you run.