Days of Autonomy Calculator

Frequently Asked Questions

How big a battery do I need for N days of autonomy?

Required Wh = daily load × autonomy days ÷ usable depth-of-discharge. For 5 kWh/day, 3 days autonomy, and 80% usable DOD (LiFePO₄): 5,000 × 3 ÷ 0.8 = 18,750 Wh.

What DOD should I use by chemistry?

Flooded lead-acid: 50% usable. AGM: 50–60%. LiFePO₄: 80–90% usable. Lithium's deeper DOD means it needs about half the nameplate capacity of lead-acid for the same usable energy.

How many days of autonomy is typical?

Grid-tied with battery backup: 1 day. Weekend cabin: 2 days. Full-time off-grid with backup generator: 2–3 days. Pure solar with no generator: 4–5 days, sized for the worst week of the year.

Does temperature change battery sizing?

Yes - lead-acid loses ≈ 20% capacity at 0 °C; lithium derates charge current near 0 °C and can be damaged charging below freezing. Size up by 10–25% for cold sites or insulate/heat the battery enclosure.

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Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

Days of autonomy is the number of days an off-grid battery bank can power the loads with zero solar input - the answer to "how many cloudy days will my system survive?" It is the single most important off-grid sizing parameter after the load audit, because it determines both battery capacity (and cost) and how often the backup generator must run. The standard recommendation is 3 days of autonomy for a typical off-grid home; high-latitude or weather-prone sites may need 4–5 days; a generator-paired hybrid system may only need 1–2 days. This tool computes either days of autonomy from a given bank size or required bank capacity (Wh and Ah) for a target number of autonomy days.

Formula / Method

Usable energy = nominal bank Wh × (DOD ÷ 100), where DOD is the maximum depth of discharge before damage or warranty loss. Days = usable Wh ÷ daily Wh. Inverted, required nominal bank Wh = (daily Wh × target days) ÷ (DOD ÷ 100). Convert to Ah by dividing nominal Wh by bus voltage (12, 24, or 48 V). DOD limits depend on chemistry: lead-acid prefers ≤ 50% DOD for long life (so a 200 Ah bank only delivers 100 Ah usefully); LiFePO₄ tolerates 80–100% DOD daily. Round bank size up to whole battery modules (typically 100 Ah or 200 Ah units in 12 V, or 5 kWh stack modules in 48 V).

Worked Example

A 10 kWh LiFePO₄ bank at 80% allowed DOD with a 4 kWh/day load: usable = 10,000 × 0.80 = 8,000 Wh; days = 8,000 ÷ 4,000 = 2.0 days. Adequate for an overnight outage but on the marginal side for a 3-day cloudy stretch - consider another 5 kWh module. Inverting for design: same 4 kWh/day load, 3-day target, 80% DOD: required nominal = (4,000 × 3) ÷ 0.80 = 15,000 Wh = 15 kWh; at 48 V bus that is 15,000 ÷ 48 ≈ 313 Ah. Lead-acid at 50% DOD for the same target needs (4,000 × 3) ÷ 0.50 = 24 kWh nominal - 60% more battery for the same workable energy.

Common Mistakes

Using nameplate Ah without applying DOD - a "200 Ah" lead bank only delivers about 100 Ah of useful daily energy.

Mixing battery chemistries or ages in parallel - weakest cell dictates the bank's capacity and lifespan.

Forgetting temperature derating - lead-acid loses 30%+ usable capacity at 0°C; lithium also drops in the cold (and must never be charged below 0°C without a heater).

Sizing batteries only for autonomy and ignoring charge-rate limits - a huge bank with too-small a solar array can never fully recharge after a deep discharge.

Confusing nominal Wh (or Ah) with usable Wh in marketing materials - some "10 kWh" products are only 8 kWh usable.

Tips & FAQ

How many days is enough? 1–2 days for a grid-tied battery with backup generator, 3 days for a typical off-grid home, 4–5 days for a winter-heavy or cloudy site, 7+ days only for remote installations where generator service is impractical. Lead-acid vs. LiFePO₄? Lithium is now cheaper per delivered kWh over a 10-year life despite higher up-front cost - it doubles usable capacity (80% vs 50% DOD), lasts 3,000–6,000 cycles vs 500–1,200 for FLA, and weighs ¼. Lead-acid still wins on tolerance of abuse and field repairability. Battery autonomy vs. generator backup: a smaller bank (1–2 days) plus a propane genset that runs ≈ 100 hours/year is often cheaper and lower-maintenance than a huge bank sized to ride out every cloudy week. Round trip efficiency: lead-acid is 80–85% round-trip, lithium 92–97% - over 10 years that 10-point gap is significant. Self-discharge: lithium loses 1–3% per month; lead 3–15% per month - matters for seasonal cabins. BMS: always use a battery management system for lithium; cell imbalance and over-discharge will brick a $4,000 bank in one episode. Sizing math: always size in Wh first, then convert to Ah at your bus voltage - comparing 12 V Ah to 48 V Ah without converting gives 4× the wrong answer. These are planning estimates; consult your battery manufacturer's spec sheet for warrantied DOD and cycle-life under your operating conditions.

Days of Autonomy Calculator

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