Solar Battery Sizing Calculator

Solar battery sizing answers how much stored energy you need to run loads when the sun is not shining. The inputs are daily watt-hours (your energy budget), days of autonomy (how long you want to operate without meaningful solar recharge), and depth of discharge (how much of the battery pack you plan to use safely).

Unlike grid-tied systems that export surplus power, off-grid and backup designs must carry night loads, cloudy days, and inverter conversion losses inside the battery bank. This calculator estimates nameplate watt-hours and equivalent amp-hours at 24 V and 48 V so you can compare lithium packs, lead-acid banks, and commercial module sizes.

Use the result alongside solar panel sizing and inverter selection. Batteries are often the most expensive part of an off-grid system—right-sizing them avoids both comfort shortfalls and overspending on capacity you will rarely cycle.

How this calculator works

1. Enter daily energy in watt-hours (Wh)—from a load list, utility data, or the output of our off-grid load calculator.
2. Set days of autonomy: how many consecutive days the bank should support your loads with little or no solar input. Weekend cabins often use 1–2 days; full-time off-grid homes commonly plan 2–3.
3. Adjust depth of discharge (DoD): the fraction of rated capacity you will actually use. Lithium (LiFePO₄) often allows ~90% usable energy; many lead-acid designs stay near 50% for longevity.
4. The tool multiplies daily Wh by days and a 1.1 efficiency buffer, then divides by DoD to get required battery watt-hours.
5. Amp-hours at 24 V and 48 V are derived by dividing battery Wh by nominal system voltage—useful when shopping for batteries rated in Ah at a specific voltage.

Core sizing relationships

Battery Wh = (Daily Wh × days × 1.1) ÷ DoD | Ah = Battery Wh ÷ system voltage

Daily Wh is your baseline demand. Multiplying by days of autonomy scales storage for outages or cloudy stretches. The 1.1 factor builds in round-trip losses through the inverter and charge controller—not a substitute for a full loss budget on large systems, but a practical planning margin. Dividing by DoD converts usable energy needs into nameplate battery capacity: a 50% DoD means you need twice the rated Wh to deliver the same usable energy as a 90% lithium design.

Formulas used by this tool

• Battery Wh = (daily Wh × days × 1.1) ÷ depth of discharge.
• 1.1 is an efficiency / safety buffer on daily energy.
• Ah at 24 V and 48 V = Wh ÷ nominal system voltage.

Worked example: 3 kWh per day, 2 days backup

A cabin uses 3,000 Wh per day, you want 2 days of autonomy, and you plan to cycle a lithium bank to 90% depth of discharge.

1. Battery Wh = (3,000 × 2 × 1.1) ÷ 0.9 = 6,600 ÷ 0.9 ≈ 7,333 Wh nameplate.
2. At 24 V: Ah ≈ 7,333 ÷ 24 ≈ 306 Ah—look for a 24 V pack or 12 V modules wired for 24 V nominal.
3. At 48 V: Ah ≈ 7,333 ÷ 48 ≈ 153 Ah—half the current for the same power, which can reduce cable size on larger systems.
4. If you switch to lead-acid at 50% DoD, rerun with 0.5 DoD—the required Wh roughly doubles for the same comfort level.

Practical tips

• Size for the loads and season you actually live with, not a best-case summer weekend.
• Match DoD to your battery chemistry and manufacturer warranty—deeper cycling shortens lead-acid life.
• Higher system voltage (24 V or 48 V) reduces current for the same power and can shrink wiring costs.
• Leave headroom for inverter standby draw and battery aging—banks lose usable capacity over years.
• Pair battery sizing with solar array sizing so the bank can recharge within your target days after an outage.

Frequently asked questions

Related guides

• How to Size a Battery Bank for Solar