The whole answer is one formula plus three honest inputs — your daily watt-hours, your days of autonomy, and your depth of discharge. Everything else is arithmetic.
Ah = (Wh/day × days of autonomy × 1.25) ÷ system voltage ÷ DoD
Wh/day = everything you'll run, summed · 1.25 = safety margin for inverter losses and aging cells · DoD = 0.80 for LiFePO4, 0.50 for AGM
Run a real load list — don't guess. A nameplate wattage is not a daily watt-hour total until you multiply by the hours that load actually runs. Our battery bank sizer directory keeps the full quick-sizing table, and the math below is the whole method. The same arithmetic as six numbered steps, with this cabin’s 1,000 Wh example and the van’s 1,500 Wh example, is how to calculate battery bank size. A diesel heater sized from published current, not from a guessed wattage, is the diesel heater battery page.
The worked example uses 1,000 Wh/day. That is the weekend-cabin input on the sizing table below, not a measured appliance list.
Now the formula, with 2 days of autonomy and LiFePO4 at 80% DoD:
That's the 2-day result. A third day is (1,000 × 3 × 1.25) ÷ 12 ÷ 0.80 = 390.6 Ah, about 391 Ah at 12V, or about 98 Ah at 48V.
| Daily use | 12V bank | 24V bank | 48V bank | Est. cost* |
|---|---|---|---|---|
| Weekend cabin (1.0 kWh/day) | 260 Ah | 130 Ah | 65 Ah | $700–2,100 |
| Van life light (1.5 kWh/day) | 391 Ah | 195 Ah | 98 Ah | $1,050–3,100 |
| Van life full (3.0 kWh/day) | 781 Ah | 391 Ah | 195 Ah | $2,100–6,250 |
| Small homestead (5.0 kWh/day) | 1,302 Ah | 651 Ah | 326 Ah | $3,500–10,400 |
| Full off-grid home (10 kWh/day) | 2,604 Ah | 1,302 Ah | 651 Ah | $7,050–20,800 |
*Battery cost uses the 12V Ah in the row, because the 24V and 48V figures store the same watt-hours. Cost = (Ah × 12 ÷ 1,200) × the 12V 100Ah street price, rounded to the nearest $50. Low is Redodo at $269.99; high is Battle Born BB10012 at $799.00. Checked 2 Oct 2026. LiTime $302.39 and SOK $369.00 sit inside that span. Renogy lists $356.99 and was unavailable, so it is not an endpoint. Same 2-day autonomy assumption throughout.
Induction cooktops, well pumps, and electric heat are the loads that blow past a weekend cabin's watt-hour budget. Cabins that heat with electricity stop being battery problems and become generator or wood-stove problems. Anything that cycles (a fridge, a freezer, a heater) runs only part of the clock, so use hours actually on, not a nameplate times 24.
Every extra day multiplies the bank linearly, and it's the most expensive knob you turn. Two days is the worked example when some charging backup exists (solar, a generator, or a charger). If the bank is the only source through a stretch of storms, add days and an honest load-shedding plan. Sizing for a single day and then hoping to conserve can leave the bank empty on the second night.
Flooded lead-acid and AGM are typically limited to about 50% depth of discharge (Battle Born chemistry guide). The Battle Born 100 Ah manual lists that lithium battery at 100% usable depth of discharge and 3,000–5,000 deep discharge cycles; this page still plans LiFePO4 at 80% DoD. That manual does not publish an AGM cycle count. If you're sizing an AGM bank, use 0.50 instead of 0.80 as the DoD divisor. The 1 kWh/day, 2-day, 12V bank is then (1,000 × 2 × 1.25) ÷ 12 ÷ 0.50 = 416.7 Ah, about 417 Ah of AGM, against about 260 Ah of LiFePO4.
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Three 100 Ah 12V LiFePO4 batteries cover the 2-day, 1 kWh weekend example (about 260 Ah).
100 Ah LiFePO4 batteries on Amazon → 2,000W pure-sine inverters on Amazon →
The 260 Ah result is energy for two days. It does not subtract storage loss, and it does not know the battery is too cold to charge. On the Battle Born 100 Ah spec table, self-discharge is 2–3% per month. One month, applied once to the printed 260 Ah, is 5.2 Ah at 2% and 7.8 Ah at 3%. The storage section of that same manual separately says batteries lose 4–5% capacity per month. Both statements, and the 10 mA Bluetooth draw that is not the same as either percent, are on how long a battery holds a charge.
The BMS chapter says the BMS will not allow a charging current under 32°F (0°C). The heading on that paragraph says low-temperature charging below 25°F (−3.9°C). Charge voltages and the heat-enabled model's draw are on the lithium charger page. How those batteries connect — parallel for the 260 Ah at 12 V, series for the 65 Ah at 48 V, and not past 48 V — is series and parallel wiring. Terminal torque is maintenance, connections, and terminals. The 416.7 Ah AGM figure in the paragraph above, counted as four 105 Ah GPL-31T batteries, is the AGM guide.
Ah sizing covers energy; the inverter covers power. Add up every motor you might start at once, and the inverter's surge rating (not its continuous rating) must clear the biggest starter. Sizing the bank without checking surge leaves energy the inverter will not deliver.
Sizing one inverter on its own: what size battery bank for a 3000W inverter.
Battery banks store; panels refill. If your cabin has an array, the bank and the array are sized against each other: the array should replace a full day's use in your worst month of sun, and the bank covers the nights between. That is why autonomy days matter more for a solar cabin than for a cabin with a generator.
After the bank is sized, size solar panels for an off-grid cabin so the array can replace a day of use in your worst month of sun.
Using Ah = (Wh/day × autonomy days × 1.25) ÷ voltage ÷ 0.80, a 1 kWh/day cabin is about 130 Ah at 12V for one day (33 Ah at 48V) and about 260 Ah at 12V for two days (65 Ah at 48V). A 5 kWh/day cabin at two days is about 1,302 Ah at 12V or 326 Ah at 48V.
Two days with any charging backup; three if the bank stands alone through storms. Each extra day multiplies the bank linearly — and it's the most expensive knob in the formula, so pair autonomy days with an honest load-shedding plan.
The same daily watt-hours are fewer amp-hours at a higher voltage. On the 1 kWh/day, 2-day row, 12 V is about 260 Ah and 48 V is about 65 Ah. This page does not publish an inverter wattage that requires 48 V.
The formula gives about 130 Ah at 12V for one day of autonomy and about 260 Ah for two days. Two 100 Ah batteries cover the one-day result (200 Ah). Three 100 Ah batteries cover the two-day result (300 Ah).
LiFePO4. Flooded lead-acid and AGM are typically limited to about 50% depth of discharge, so a 260 Ah LiFePO4 cabin bank (1 kWh/day, 2 days, 12V) becomes about 417 Ah of AGM for the same energy. The Battle Born 100 Ah manual lists 3,000–5,000 deep discharge cycles and 100% usable capacity for that lithium battery; this page still plans LiFePO4 at 80% DoD. That manual does not publish an AGM cycle count.
Backup power comparison: the generator sizing calculator covers what size generator you need when a battery bank won't carry the load.
Next steps: the battery bank sizer directory has the full quick-sizing table and voltage comparison, and the van-life math lives in how many batteries you need for van life. Charging-side math: GeneratorSizer's generator size calculator ↗.
Sources for the 2 Oct 2026 price checks and for depth of discharge and cycle life:
Yes — free to use, no signup, no email required.
Estimates follow industry-standard sizing rules and common safety margins. Always confirm with a licensed professional for final sizing.
Yes — the page is fully responsive and works on phones, tablets, and desktop.