Battery Bank Sizing for Off-Grid Living: Days of Autonomy, Explained

A grid-tied homeowner sizing a battery is choosing a convenience level — how much of the evening they’d like to run on stored sunshine instead of grid power. An off-grid homeowner sizing a battery bank is choosing something far more consequential: how many consecutive cloudy days can pass before the house goes completely dark. Same hardware, entirely different stakes. That’s why off-grid sizing has its own vocabulary and its own arithmetic, both built around a number that grid-tied design never even mentions — days of autonomy.

Days of autonomy is simply how long your battery bank can carry the house with zero solar input coming in. It’s the central design decision in any off-grid system, it drives more cost than any other single choice you’ll make, and getting it wrong is expensive in both directions. Undersize the bank and you’re firing up a generator every gray week, listening to it drone and burning fuel you hauled in. Oversize it and you’ve sunk thousands of dollars into battery capacity that sits mostly idle 350 days a year, aging whether you use it or not. The whole craft of off-grid sizing is landing that one number in the right place for your climate, your load, and your tolerance for the sound of a generator.

The sizing formula, and every term in it

The standard calculation is compact: bank capacity in kilowatt-hours equals your daily load in kilowatt-hours, times your chosen days of autonomy, divided by the product of depth of discharge and system efficiency. Four terms, four decisions, and each one deserves to be understood rather than plugged in blindly, because small changes in any of them swing the final number hard. Work it with a labeled example throughout — say a cabin drawing 8 kWh a day, which represents modest-but-comfortable off-grid living: an efficient fridge, LED lighting, a water pump, laptops and phones, and no electric heat, since electric heat off-grid is a fast road to an enormous bank.

The daily load has to come from an actual load audit — every appliance, its wattage times its hours of daily use, summed — not from a guess or an optimistic round number. Off-grid design is punishingly unforgiving of optimism here, because unlike a grid-tied home there’s no utility standing by to quietly cover whatever you underestimated. It’s also worth running that audit for your worst season rather than your average one, since the whole point of the bank is to survive the hard stretches, and a load that’s comfortable in September can strain a system sized for the average in the dark, appliance-heavy depths of winter. The full audit process — how to meter loads, what phantom draws to catch, how to account for seasonal appliances — is covered in sizing an off-grid system from your load list, and everything here assumes you’ve done that work and arrived at a defensible daily number.

Days of autonomy is your weather tolerance expressed as a number, and the common choices map to real situations. One to two days suits mild climates or systems paired with a generator that runs regularly anyway, so the bank only has to bridge short gaps. Two to three days is the usual sweet spot for a full-time off-grid home that has backup generation for the rare extended gloom — enough to shrug off an ordinary cloudy patch without touching the generator, not so much that you’re paying for capacity you rarely tap. Four to five days is for genuinely cloudy climates like a Pacific Northwest winter, for remote sites where hauling generator fuel is a serious chore, or for households that simply refuse to listen to a generator and will pay for the silence. Depth of discharge, the third term, is how much of the bank you can actually use without wrecking its lifespan, and it’s where battery chemistry enters the math in a big way — lithium iron phosphate tolerates 80 to 100 percent depth of discharge routinely, while a flooded lead-acid bank lasts dramatically longer if you only ever pull it down to about 50 percent. System efficiency, the fourth term, covers inverter losses and battery round-trip losses together; 0.85 is a reasonable planning figure for a lithium system and a bit lower for lead-acid. Plug the cabin in on lithium: 8 kWh a day times 3 days of autonomy, divided by 0.8 depth of discharge times 0.85 efficiency, works out to roughly 35 kWh of usable lithium capacity. Run the identical cabin on lead-acid at 50 percent depth of discharge and 0.80 efficiency and you need 8 times 3 divided by 0.4, which is 60 kWh of rated capacity to deliver the same three days.

Before trusting any of those four numbers, stress-test the two that people most often get wrong: the daily load and the days of autonomy. Loads have a way of growing after the system is built, because an off-grid home that felt roomy on paper quietly invites the very appliances the audit left out — a second freezer for hunting season, a well-meaning space heater on a cold morning, a partner who starts working from home and runs a desktop and two monitors all day. Sizing with zero headroom means every one of those additions forces either a generator run or an expensive bank expansion, so experienced builders pad the audited load by ten or twenty percent to absorb the creep before it starts. Days of autonomy deserves the opposite scrutiny, because the temptation runs toward a big reassuring number like five days, and each day is enormously expensive. The right figure comes from your actual local weather rather than your anxiety: look up how many consecutive heavily overcast days your site realistically sees in its worst month, remember that even a gray day delivers some solar rather than none, and size the bank to the routine bad stretch while letting a generator cover the rare extreme. Getting these two inputs honest — a load padded for real life and an autonomy figure grounded in weather data — does more for a bank’s real-world performance than any argument about chemistry ever will, because they set the scale of everything downstream. That near-factor-of-two capacity gap between lithium and lead-acid, meanwhile — 35 kWh against 60 for the identical three days — is the heart of the next decision.

Chemistry, generators, and building the bank

Lead-acid ruled off-grid power for decades and still has committed defenders; lithium has taken most of the new-build market, and the honest comparison runs term by term rather than as a verdict.

Factor	LiFePO4 (lithium)	Flooded lead-acid
Usable DoD	80–100%	~50% for good lifespan
Cycle life	3,000–6,000+ cycles	500–1,500 cycles
Upfront cost per rated kWh	Higher	Lower
Cost per delivered kWh over life	Usually much lower	Usually higher
Maintenance	None	Watering, equalization charges, terminal cleaning
Cold-weather charging	Must stay above freezing to charge (heated units exist)	Tolerates cold better, capacity drops
Weight/space	Compact	Heavy, needs ventilated space

The pattern in that table is that lead-acid wins the invoice and lithium wins the decade. Because you can only safely use about half of a lead-acid bank and it survives a fraction of the charge cycles, its cost per kilowatt-hour actually delivered over the system’s life usually comes out higher despite the cheaper sticker price — you buy more of it, you use less of each unit you buy, and you replace it sooner. Lead-acid still makes real sense in a few cases: tight budgets where the up-front number is the binding constraint, rarely-used cabins where calendar aging erodes lithium’s cycle-life advantage because you barely cycle the bank anyway, and owners who already know and don’t mind the maintenance routine of watering cells, running equalization charges, and cleaning terminals. But full-time off-grid households buying today mostly land on lithium iron phosphate, and the sizing math is a big part of why: three days of autonomy costs 35 kWh of lithium instead of 60 kWh of lead-acid, and that smaller physical bank is easier to house, lighter to ship, and maintenance-free once installed. One chemistry-specific trap deserves a flag, because it’s a genuine cost line rather than a footnote: lithium batteries generally cannot accept a charge below freezing without built-in heating, so an off-grid power shed in Montana needs insulated, heated, or self-heating batteries, and that heating draws its own energy in exactly the cold, dark conditions when energy is scarcest.

Now the comparison that separates experienced off-grid designers from catalog shoppers: whether to buy more autonomy at all, or hand the job to a generator. Each additional day of autonomy costs a fixed and substantial amount — roughly 11 to 12 kWh of lithium for our 8-kWh-a-day cabin, plausibly $4,000 to $7,000 installed at recent price ranges, which you should treat as orientation rather than a quote. A quality inverter-generator plus a reasonable fuel store costs a fraction of that, and it covers the tail risk — the fourth, fifth, and sixth consecutive cloudy day — far more cheaply than batteries ever can, precisely because those rare bad stretches are rare. The economics almost always land in the same place: batteries for the routine, generator for the exceptional. Size the bank to comfortably handle the cloudy stretches that occur every month or so, which for most US climates means two to three days, and let the generator handle the once-a-winter week of unbroken gloom. Sizing the bank to survive the single worst week of the year means paying for a large chunk of capacity that sits as empty insurance 97 percent of the time — and unlike a generator that costs nothing while it waits in the shed, that idle battery capacity degrades on the calendar whether you cycle it or not, so you’re paying twice: once to buy it and again as it quietly ages unused. A useful cross-check ties the whole system together: your solar array and your battery bank have to be sized as a pair, because a bank with three days of autonomy paired with an array too weak to refill it in a day or two of decent sun just relocates the problem — you survive the storm, then limp along for a week trying to claw back to full while every cloudy afternoon sets you back. Experienced designers aim for an array that can replace a full day’s load plus meaningfully recharge the bank on an average winter day, not a sunny summer one. This tight coupling between array and bank is the deepest structural difference from grid-tied design, where the grid silently absorbs every sizing error you make, and it’s the point at which the two architectures diverge most sharply.

Capacity in kilowatt-hours gets nearly all the attention in off-grid sizing, but the bank also has to deliver enough instantaneous power in kilowatts, and off-grid that requirement has extra teeth because there’s no grid standing by to catch a surge. It’s the same kWh-versus-kW distinction that runs through every battery spec, but the consequences are sharper when you’re the only source of power for miles. Well pumps are the classic off-grid offender: a one-horsepower pump that draws 1,100 watts while running can demand 3,000 to 4,000 watts for the instant it starts, and that surge stacks on top of whatever else happens to be running at the same moment — the microwave, the fridge compressor kicking in, the water heater element. There are three ratings to check, not one. The bank’s continuous discharge rating tells you the steady power it can sustain. Its surge rating tells you what it can deliver for a brief motor-start spike. And the inverter’s surge capacity — often the real bottleneck, and the one people forget — tells you what the inverter will pass through before it faults. Many a correctly sized 35 kWh bank has been let down by a 3 kW inverter that trips the moment the well pump and the microwave happen to coincide, leaving the owner puzzled that a bank with plenty of stored energy just shut off. Size the inverter and check the surge numbers with the same care you gave the capacity, because a bank that can’t start your pump is a very expensive way to have no water.

Pulling the whole thing together for the 8-kWh-a-day cabin, with every assumption as labeled above, gives a complete and buildable specification. Three days of autonomy, lithium iron phosphate at 80 percent depth of discharge, and 85 percent system efficiency yields a bank of roughly 35 kWh — in practice that’s something like three 12-to-14-kWh batteries, or seven 5-kWh server-rack units wired together. Pair it with a 6-to-8-kW inverter carrying enough surge headroom to start the well pump without faulting. Size the solar array alongside the bank so it can push out something like 10 or more kWh on a poor winter day, enough to both cover the day’s load and claw back into the bank. And add a small generator to cover the tail — the rare extended gloom that would otherwise force you to buy a fourth and fifth day of autonomy at several thousand dollars each. Swap in your own load audit and run the variations through the solar battery calculator; the formula itself is simple enough to do on paper, so the calculator’s real value is showing you how hard the total swings when you nudge days of autonomy up or down by a single day. That one input, more than any equipment choice or chemistry debate, is what sets your budget — which is why the discipline of the whole exercise is deciding how many dark days you truly need to survive on batteries alone, and letting a generator quietly handle the rest.