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Solar Battery Bank Sizing Calculator

Free solar battery bank sizing calculator. Size your battery bank in amp-hours and kWh based on daily load, days of autonomy, system voltage, and chemistry.

Solar Battery Bank Sizing Calculator

Usable energy needed
10,000 Wh
Gross capacity needed
12,713 Wh
Bank capacity (Ah)
530 Ah
Bank capacity (kWh)
12.7 kWh

How to use this calculator

Enter your average daily load in watt-hours (Wh), choose your system DC voltage (12V, 24V, or 48V), set the number of days of autonomy you want without solar input, then pick your battery chemistry. The calculator returns the bank capacity in both amp-hours and kWh.

If you don’t know your daily load, take your monthly utility kWh, multiply by 1000, and divide by 30. For backup-only sizing, use only the loads that will run during an outage (fridge, lights, internet, well pump) — typically 3–8 kWh/day, not the whole-house figure of 28 kWh/day from the EIA average.

The formula

The calculation is straightforward once you account for losses:

Usable energy needed (Wh) = Daily load × Days of autonomy
Gross capacity (Wh)       = Usable / (DoD × Inverter eff × Battery eff)
Bank capacity (Ah)        = Gross capacity / System voltage

Where DoD is the safe depth of discharge (90% for LiFePO4, 50% for lead-acid), inverter efficiency is typically 92% for a quality pure-sine inverter under load, and battery round-trip efficiency is 95% for lithium and 80–85% for lead-acid.

Worked example — 5 kWh/day cabin

A grid-isolated cabin with 5,000 Wh of daily DC-equivalent load, 2 days of autonomy, on a 24V LiFePO4 system:

  • Usable energy: 5,000 × 2 = 10,000 Wh
  • Gross capacity: 10,000 / (0.90 × 0.92 × 0.95) = 12,710 Wh
  • Bank size: 12,710 / 24 = 530 Ah at 24V (or 12.7 kWh)

That maps to two 280 Ah 24V LiFePO4 server-rack batteries in parallel, or four 100 Ah 12V LiFePO4 in series-parallel. With AGM lead-acid (50% DoD, 85% efficiency) the same load needs ~1,070 Ah at 24V — roughly 2× the cells and 3× the floor space.

Battery chemistry — the dominant cost driver

The chemistry choice changes the system cost by 30–50% before you ever add panels. Per the National Renewable Energy Laboratory (NREL) and EnergySage 2025 cost reports, the typical 2026 installed costs are:

  • LiFePO4 lithium: $800–1,200 per kWh installed (Battle Born, EG4, Victron)
  • AGM lead-acid: $400–600 per kWh installed, but you need 1.8–2× the kWh to match usable energy — net cost similar or higher than lithium
  • Flooded lead-acid: cheapest per nameplate kWh ($250–400) but requires ventilation, watering, and only 1,000–1,500 cycles versus 4,000–6,000 for lithium

Most US off-grid installers in 2025–2026 are quoting LiFePO4 by default. The 10-year total cost of ownership crossed below lead-acid around 2022 once price-per-kWh fell below the $1,000 threshold.

NEC 706 compliance — what the calculator doesn’t show

The calculator sizes the bank energetically. NEC 706 (Energy Storage Systems) and NEC 690.71 add code requirements you must layer on top:

  • Battery enclosure ventilation (lead-acid: hydrogen accumulation; lithium: thermal runaway containment)
  • Disconnect switches sized per Article 240
  • Maximum continuous current per battery cable; see the Solar Panel Wire Size Calculator for sizing
  • Listed equipment (UL 9540 / UL 1973 for the cells, UL 1741 for inverters)
  • Overcurrent protection and short-circuit ratings

For systems above 1 kWh in a dwelling, IRC R328 requires a minimum 3-foot clearance from doorways and a 50-foot maximum aggregate of 20 kWh per dwelling unit unless a fire watch is established. Most utilities also require AHJ (Authority Having Jurisdiction) sign-off before the system is energized.

How to estimate daily load

If you’re sizing for the first time, build a load table:

  • Refrigerator (Energy Star 18 cu ft): 1.2 kWh/day
  • LED lighting (8 fixtures × 5 hours × 10W): 0.4 kWh/day
  • Laptop + router + modem: 0.5 kWh/day
  • Well pump (1/2 HP × 1 hour): 0.4 kWh/day
  • Microwave / coffee / induction (intermittent): 0.5–1.5 kWh/day
  • Heating/cooling: wildly variable — usually a separate system (mini-split, propane, wood)

Add 15% for inverter standby, ghost loads, and BMS self-consumption. Real-world cabin baselines run 3–8 kWh/day; full-house off-grid in the US is rare below 15 kWh/day.

Sources cited

  • NREL Energy Storage Cost Report (2025)
  • NEC 706, 690.71, IRC R328 (2023 cycle, adopted by most US states 2025–2026)
  • US DOE Office of Energy Efficiency & Renewable Energy: Off-grid sizing methodology
  • EnergySage 2025 Battery Marketplace pricing report
  • Battle Born / Victron / EG4 published cycle-life curves and efficiency specs

Cross-check your final design with a licensed electrician (state license + NABCEP PV Installer Certification) and your local AHJ before purchasing — code interpretations vary by jurisdiction. Use this calculator for planning and quoting; do not energize a battery system without inspection.

Frequently asked questions

How big a battery bank do I need for an off-grid cabin?
For a typical small cabin running 3–5 kWh/day with two days of autonomy, you usually need 250–400 Ah at 24V (or 125–200 Ah at 48V) of usable lithium capacity. With lead-acid, you roughly double that because you can only safely use 50% of nameplate capacity. The exact number depends on inverter and battery efficiencies, which the calculator handles.
Why does lead-acid need twice the capacity of lithium?
Lead-acid batteries are damaged if discharged below ~50% of their nameplate Ah, while LiFePO4 lithium tolerates 80–90% depth of discharge without significant cycle-life loss. Lead-acid round-trip efficiency is also ~80–85% versus 95% for lithium. Combined, lithium delivers nearly 2× the usable energy per nameplate Ah.
Should I pick 12V, 24V, or 48V?
Use 12V only for systems below ~1500W of inverter load (RVs, small sheds). Use 24V for typical 3–5 kWh/day cabins. Use 48V for whole-house systems above 5 kWh/day — it cuts wire current by 4× compared to 12V, reducing cable size, voltage drop, and fire risk per NEC 690.
How many days of autonomy should I plan for?
Two days is the most common figure for grid-tie backup. Off-grid in cloudy regions usually plans for 3–5 days. Going above 5 days makes batteries the dominant system cost, and a small backup generator becomes more economical than additional storage.
Does the calculator account for battery temperature derating?
No — assume the battery sits at 20–25°C. Below 0°C, lithium charge acceptance drops sharply (most BMS units block charging below 0°C entirely), and lead-acid loses ~1% capacity per °C below 25°C. For unheated outdoor enclosures in cold climates, oversize the bank by 20–30% or specify heated batteries.

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