Battery capacity calculator

Key takeaways

• Battery Wh = daily Wh × days of autonomy ÷ depth of discharge.
• Usable capacity matters more than nameplate capacity.
• Temperature and inverter losses reduce real-world capacity.
• System voltage affects cable size and efficiency.

Formula

Battery Wh = Daily Wh × Days of autonomy ÷ Depth of discharge

Example: 2,000Wh × 1 day ÷ 0.8 = 2,500Wh

After you calculate Wh, convert to amp-hours (Ah) using system voltage: Ah = Wh ÷ Voltage. Use the voltage guidance in choosing system voltage.

Choosing days of autonomy

Autonomy is how long you want to run without sun. Off-grid systems often target 2–3 days, while backup systems may use less than one day because they only cover critical loads. More autonomy means more batteries and higher cost.

How to use this calculator

1. Estimate daily energy use in Wh.
2. Choose how many days of autonomy you want.
3. Select a depth of discharge based on battery type.
4. Convert Wh to Ah using your system voltage.

If you still need daily Wh, start with the panel output calculator or the sizing guide.

Depth of discharge (DoD)

DoD is the portion of a battery’s capacity you can safely use. Lead-acid batteries often use 50 percent or less. Lithium batteries can often use 80 percent or more, depending on the manufacturer.

System voltage and amp-hours

Higher system voltage means lower current for the same power, which reduces wire size and losses. A 2,500Wh battery bank at 12V is about 208Ah; at 24V it is about 104Ah; at 48V it is about 52Ah.

Use the voltage selection guide to choose a system voltage.

Ah conversion example

Suppose you need 4,000Wh of usable storage and plan a 24V system. Ah = 4,000 ÷ 24 ≈ 167Ah. If your battery bank allows 80 percent usable depth of discharge, total capacity should be about 167 ÷ 0.8 ≈ 209Ah.

Example scenarios

Small off-grid cabin: 1,500Wh/day, 2 days autonomy, 50% DoD.

1,500 × 2 ÷ 0.5 = 6,000Wh (6 kWh)

RV backup: 1,000Wh/day, 1 day autonomy, 80% DoD.

1,000 × 1 ÷ 0.8 = 1,250Wh (1.25 kWh)

Inverter and temperature losses

Inverters are not 100 percent efficient. If your loads are AC, include an efficiency factor (often 85–95 percent). Cold temperatures can reduce usable capacity, especially for lead-acid batteries. Plan extra capacity if you expect cold conditions.

Charging rate and solar input

Battery size must match the amount of solar input available. A very large battery bank with too few panels can stay undercharged, which shortens battery life. As a rough rule, ensure your array can provide enough daily energy to fully recharge the bank within a reasonable number of sun hours.

A small controller or array can limit recharge speed. Plan for full recharge regularly when possible.

If charging stays low, see solar battery not charging.

Battery aging and replacement planning

Batteries lose capacity over time. Lead-acid banks may need replacement sooner than lithium. When sizing, consider a small buffer so your system still meets needs as capacity declines. This is especially important for off-grid systems that rely on storage for multiple days.

Expect changes over time.

Use the solar system cost breakdown to budget replacements.

Assumptions and limitations

• Daily Wh estimates are based on average usage, not peak loads.
• Battery capacity declines over time with cycling.
• Temperature and charge settings affect real usable capacity.
• Improper charging can shorten battery life.

Common mistakes

• Using nameplate capacity instead of usable capacity.
• Ignoring inverter losses for AC loads.
• Choosing autonomy days without checking weather patterns.
• Underestimating surge loads or nightly usage.

Quick reality check

If your system relies on high-surge loads, add extra capacity so voltage does not sag under load.

Battery placement and ventilation

Batteries should be installed in a dry, well-ventilated area with stable temperatures. Lead-acid batteries can release gases during charging and need ventilation. Lithium batteries should be kept within the manufacturer’s temperature range to avoid capacity loss and safety issues.

Safety note

Batteries can deliver very high current. Use proper fusing and cable sizing, and follow manufacturer guidance. If you are unsure about wiring or ventilation requirements, consult a licensed electrician.

Keep terminals covered, remove jewelry when working nearby, and keep a fire extinguisher rated for electrical fires available.

FAQ

Should I size for one day or more?

Off-grid systems often use two or more days. Backup systems may use one day or less.

Is bigger always better?

Larger batteries reduce cycling depth but increase cost and charging requirements.

What if my loads change?

Consider leaving room for expansion or plan to add batteries later.

Do I need a battery for a grid-tied system?

Not always. Batteries are optional for grid-tied systems unless you want backup power.

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