Battery Bank Sizing Calculator

For grid-tied home backup, 1 day is usually enough, since most US utility outages last under 8 hours. The bank carries you through evening and overnight; solar refills it the next day.

For off-grid cabins and full-time setups, 2–3 days is standard. It covers a long cloudy stretch without forcing a generator start. Bank size and cost grow in step with every day you add (a 4-day bank costs twice a 2-day bank), and the extra days rarely earn their cost, so most builders pair a 2–3 day bank with a backup generator.

DoD is the percentage of nameplate capacity you can use without damaging cycle life. A 10 kWh battery rated for 50% DoD gives you 5 kWh of real, usable energy; the rest is reserve to protect the cells.

LiFePO4 tolerates 80% DoD cycle after cycle. AGM and Gel lose cycle life rapidly past 50% DoD: a lead-acid bank discharged to 80% routinely will last roughly half as long. The calculator above uses 80%/50%/50% by chemistry.

Per kWh nameplate, lithium (LiFePO4) prices have fallen to about 1.5–1.7x lead-acid. For that premium you get 3–6x the cycles (3,000+ vs 500–1,000) and 1.6x the usable capacity per kWh (80% DoD vs 50%). And because lead-acid needs a much bigger bank to deliver the same usable energy, the upfront cost gap for a whole bank is now small. On a per-usable-kWh-cycle basis, lithium is typically 3–5x cheaper over the bank's life.

Lead-acid still has a place: short-duration emergency backup that rarely cycles, or projects where upfront cash is the tight constraint. For any system that cycles daily, LiFePO4 wins on total cost of ownership.

Avoid it. A bank performs at the level of its weakest cell. Adding a fresh battery to an aged pack forces the new one to age prematurely while the old ones get overworked. Total bank life typically drops 30–50%.

If one battery in a series string fails, the practical call is usually to replace the whole bank. If you have parallel strings, you can sometimes pull a bad string and run on the remaining matched units while you decide.

Yes, especially for lead-acid. At 32°F (0°C), AGM/Gel batteries lose roughly 20% of usable capacity; at 0°F (-18°C), they lose ~40%. LiFePO4 holds up better (10–15% loss at freezing), but most LiFePO4 batteries stop accepting charge below 32°F to prevent permanent plating damage.

If your bank lives in an unheated space in a cold climate, derate 10–20% in the calculator and budget for an insulated battery box with low-wattage heat (a $30 reptile heating pad on a thermostat works well).

12V is standard for RVs and small van builds, where most accessories, fridges, and lights are already 12V native.

24V is a middle ground for cabins around 3–5 kW. 48V is the modern default for whole-house and off-grid homes. Higher voltage means lower current for the same power, which cuts wire size, charge-controller cost, and resistive losses. Most pre-built lithium server-rack batteries (EG4, SOK, Pytes) ship 48V for this reason.