How to Size an Off-Grid Battery Bank (2026)
The short answer: Start with the loads that must run, the hours they must run, and the backup plan. Then compare those needs with the exact battery's usable energy and power limits. Daily use alone does not determine a safe or reliable battery size. Have a qualified installer review the complete design.
Start with the load and backup plan
List the circuits that must stay on, how much energy each uses over time, the largest loads that may run together, and any motor-starting surge. Decide how long the system must support those loads, what can be switched off, and whether a generator or grid connection is part of the plan. Daily energy hides timing and peak power.
For solar-backed storage, test battery charging and discharging across local weather sequences. NREL's System Advisor Model battery documentation describes dispatch options and models that account for voltage, temperature, degradation, and replacement assumptions. An annual energy average cannot establish how a bank performs through a low-sun period.
Use the exact battery's usable-energy limits
Nameplate energy and energy delivered to AC loads are different quantities. Start with the manufacturer's stated usable capacity, discharge limits, operating temperatures, and inverter compatibility. Current, temperature, state of charge, and aging can change what a product delivers. The Victron Lithium Smart Battery manual is one product-specific example: its capacity data and operating limits depend on stated test conditions. Those values are not defaults for other batteries or chemistries.
- Nominal-energy estimate: nominal kWh ≈ nominal voltage × nameplate Ah ÷ 1,000.
- Approximate AC energy: stated usable battery kWh × inverter efficiency, before other system losses.
- Power check: compare simultaneous and surge loads with the battery, BMS, inverter, and charger ratings.
Hypothetical arithmetic only: a product rated for 10 kWh usable and an assumed 90% inverter efficiency would deliver about 9 kWh AC before standby, wiring, temperature, reserve, and other losses. This is not a recommended bank size. It says nothing about the load timing, the system's peak output, or how quickly solar can recharge it.
Choose a voltage the equipment supports
12 V, 24 V, and 48 V are system options, not daily-energy tiers. Match the battery voltage to the inverter, charger, controller, and battery configuration allowed by their manuals. For the same ideal DC power, current is approximately power divided by voltage. Actual battery-side current also depends on conversion losses and the battery's operating voltage. Check continuous and short-duration ratings; this estimate is not a method for selecting conductors or protection.
Series and parallel connections are product-specific
In simple arithmetic, series connections add nominal voltage while amp-hours stay the same; parallel connections at the same voltage add amp-hours. Real batteries can only be connected that way when the manufacturer allows the exact model and configuration. Connection limits, BMS requirements, protection, and compatibility differ by product. Follow the complete manual rather than combining batteries from a generic diagram.
Safety and local installation rules
Do not use an article to choose a fuse, breaker, wire size, spacing, or bank layout. These depend on exact products and local code. In the United States, NFPA 855 covers stationary energy storage installation; the local authority decides which rules apply. UL summarizes code-specific fire testing. Use equipment approved for its intended location and have a qualified local professional review the design before it is energized.
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Frequently asked
How many kWh of battery do I need for a 10 kWh/day home?
Define required loads, backup duration, recharge source, and allowed backup; test those conditions against the product's usable capacity and power limits.
How do I convert battery kWh to amp-hours?
Nominal kWh ≈ nominal volts × nameplate Ah ÷ 1,000. Use stated usable kWh and current limits when planning a real system.
Is 48 V always better than 12 V or 24 V?
No. Match all equipment. Higher voltage usually lowers current for the same power; model and local code determine a safe system.
Can I connect batteries in series or parallel?
Only when the manufacturer permits the exact models and arrangement; follow its BMS and protection instructions.