Battery Capacity Calculator
Size Ah and Wh from load, hours, and days of autonomy.
Battery Capacity Calculator
Battery capacity sizing starts from how you actually use power: a load in watts, how many hours per day it runs, how many days you want to operate without recharge, and the battery chemistry you plan to buy. The calculator turns that into required watt-hours and amp-hours at your chosen system voltage.
This is the load-first approach—ideal when you know one or more appliances and their duty cycle but have not yet summed a full daily energy budget. It uses the same core relationships as solar battery sizing but breaks daily Wh into load watts × hours per day, and outputs Ah at 12 V, 24 V, or 48 V.
Use the result to shop for battery modules, compare lithium versus lead-acid bank sizes, and check whether your voltage architecture keeps amp-hours reasonable. Pair with battery runtime to see how long that bank runs a specific appliance, and with solar panel sizing to ensure you can recharge after an autonomy event.
How this calculator works
- Enter load watts—the power draw of the equipment or combined loads you are sizing for.
- Enter hours per day—the average time that load runs each day (not always 24 h for fridges, pumps, or workshop tools).
- Select chemistry: lithium (LiFePO₄) models ~90% depth of discharge; lead-acid models ~50% for typical cycle-life planning.
- Choose system voltage (12, 24, or 48 V) and days of autonomy—how many days the bank should carry the load with little or no charging.
- Daily Wh = load W × hours/day. Required Wh = (daily Wh × days × 1.1) ÷ DoD. Required Ah = required Wh ÷ system voltage.
Core capacity relationships
Daily Wh = load W × h/day | Required Wh = (daily Wh × days × 1.1) ÷ DoD | Required Ah = required Wh ÷ system V
Multiplying watts by hours gives daily energy for that load. Scaling by days of autonomy sizes storage for outages or cloudy stretches. The 1.1 factor adds a small margin for inverter and wiring losses. Dividing by DoD converts usable energy needs into nameplate battery capacity—lead-acid needs a larger rated bank than lithium for the same usable kilowatt-hours. Dividing by system voltage expresses the result in amp-hours, which many battery datasheets use.
Formulas used by this tool
- Daily Wh = load watts × hours per day.
- Required Wh = (daily Wh × days × 1.1) ÷ DoD.
- Required Ah = required Wh ÷ system voltage.
Worked example: 500 W for 5 hours
A workshop load averages 500 W for 5 hours per day. You want 2 days of autonomy on a 24 V lithium bank at 90% DoD.
- Daily Wh = 500 W × 5 h = 2,500 Wh per day.
- Required Wh = (2,500 × 2 × 1.1) ÷ 0.9 ≈ 6,111 Wh nameplate.
- Required Ah at 24 V = 6,111 ÷ 24 ≈ 255 Ah—compare to available 24 V packs or 12 V modules in series/parallel.
- At 48 V the same energy is ≈ 127 Ah—half the current, often preferred for larger continuous loads.
Practical tips
- Use measured or metered watts; nameplate labels on motors and compressors overstate average draw.
- If several loads run at different times, size for the peak simultaneous draw or sum separate daily Wh values.
- Weekend cabins often use 1 day of autonomy; full-time off-grid homes commonly plan 2–3 days.
- Higher system voltage reduces Ah for the same Wh—helpful for long cable runs and large inverters.
- Re-run with lead-acid chemistry to see how much larger the bank must be for the same comfort level.
Frequently asked questions
How is this different from solar battery sizing?
Both use (daily Wh × days × 1.1) ÷ DoD. This calculator derives daily Wh from load watts × hours per day and lets you pick 12/24/48 V for Ah output. Solar battery sizing takes total daily Wh directly and shows Ah at 24 V and 48 V side by side. Use whichever matches the inputs you have.
What hours per day should I enter?
Enter average runtime, not always-on unless the load truly runs 24 h. A fridge might average 8–12 h of compressor time spread across the day. A water pump might run 30 minutes total. A laptop might run 4 h. Use a plug-in meter or manufacturer duty-cycle data when possible.
Why does chemistry change the result?
Different chemistries allow different safe depth of discharge. Lithium in this model uses 90% DoD; lead-acid uses 50%. The same usable energy therefore requires a larger nameplate lead-acid bank. Always confirm against your battery warranty and BMS settings.
What does the 1.1 efficiency factor cover?
It is a planning buffer for inverter conversion, cable losses, and charge-controller round-trip inefficiency between storage and loads. It is not a full loss diagram—on critical designs, add margin for temperature and aging as well.
Should I size for 12 V, 24 V, or 48 V?
Choose the voltage your inverter, charge controller, and battery modules support. 12 V suits small RV and cabin loads. 24 V is common for mid-size off-grid homes. 48 V reduces current for the same power and is typical once continuous loads exceed roughly 2–3 kW.
Does this include solar recharge sizing?
No. This tool sizes the battery bank only. You still need enough solar or generator input to refill the bank after using your autonomy days. Use the solar panel sizing or battery charging time calculators for generation-side planning.
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