How to Determine Battery Capacity for Off-Grid and Backup Power Systems

To determine battery capacity, calculate your total daily watt-hours, divide by your system voltage, then divide by your allowable Depth of Discharge (DoD) and inverter efficiency. For a standard 5,000Wh daily load on a 48V system using LiFePO4 (80% DoD) and a 90% efficient inverter, you need exactly 144.6 Ah of usable capacity (roughly a 150Ah 48V battery). Sizing strictly to the continuous load without applying the 80% DoD rule or accounting for motor inrush will result in premature BMS low-voltage cut-offs and severe voltage sag.

Getting this number right is the difference between a backup system that runs your house through a 12-hour outage and one that faults out the moment the refrigerator compressor kicks on. Below is the exact framework for mapping your loads, calculating the true amp-hour requirement, and deciding when to split your circuits.

The Load Tally: Mapping Watts, Amps, and Inrush

Before you touch a calculator, you need a data-dense load tally. The most common mistake DIYers make is looking only at the nameplate running watts. Inductive loads—anything with a compressor or motor—draw a massive surge of current for 1 to 3 seconds during startup. If your battery bank cannot deliver this surge without its voltage sagging below the inverter's low-battery cutoff, the system will trip offline long before a DC breaker ever reacts.

What actually "trips" a battery system isn't usually the overcurrent protection; it is the Battery Management System (BMS) opening its MOSFETs because voltage sag pushed the cells below the low-voltage disconnect threshold (typically 2.8V per cell for LiFePO4, or 42V for a 48V nominal pack). High internal resistance at low states of charge exacerbates this.

In this scenario, your absolute peak surge is 2,400W (assuming the well pump and fridge compressor do not start on the exact same millisecond). However, your daily energy consumption is 4,930 Wh. This tally is the foundation for the math that follows.

Sizing the Bank: Chemistry, DoD, and the 80% Rule

With your daily watt-hours established, you must account for system losses and chemistry limits. Inverter efficiency typically hovers around 85% to 93% depending on the load percentage; we use 90% as a conservative baseline. More importantly, you must apply the Depth of Discharge (DoD) limit. Just like the NEC 80% continuous rule for breaker panels, you never size a battery to be drained to absolute zero.

Here is how the chemistry dictates your final amp-hour requirement for our 4,930 Wh load on a 48V system:

The Math for LiFePO4:
4,930 Wh / 48V = 102.7 Ah (Raw energy requirement)
102.7 Ah / 0.90 (Inverter efficiency) = 114.1 Ah
114.1 Ah / 0.80 (80% DoD rule) = 142.6 Ah minimum usable capacity.

Headroom and Future Loads:
Load profiles always grow. You will eventually add a chest freezer, a block heater, or an extra workstation. According to the U.S. Department of Energy's solar planning guidelines, adding a 20% buffer for future expansion is standard practice for off-grid and backup systems. Multiplying our 142.6 Ah requirement by 1.20 gives us 171.1 Ah. The logical, commercially available choice here is a single 48V 200Ah server-rack style LiFePO4 battery (such as those from EG4, SOK, or Trophy Rack), which provides 10.24 kWh of nameplate capacity and roughly 8.19 kWh of usable capacity.

Load Splitting: When to Add Dedicated Inverter Circuits

Sometimes, the total energy capacity is sufficient, but the instantaneous power delivery is not. This brings us to the battery equivalent of adding a dedicated circuit. If your 48V 200Ah battery has a 100A BMS, its maximum continuous output is 4,800W, and its peak surge might be limited to 6,000W for a few seconds.

If your well pump (2,400W surge), refrigerator (1,200W surge), and a microwave (1,500W running) all activate simultaneously, you are pulling 5,100W. While under the continuous limit, the combined inrush of the motors could cause a momentary voltage sag. If your DC interconnect cables are undersized—say, using 4 AWG instead of the required 2/0 AWG welding cable for a 3000W inverter—the resistance in the wire will compound the battery's internal voltage sag. The inverter will read 44V at its terminals, trigger a low-voltage alarm, and shut down. As detailed in Victron Energy's Wiring Unlimited guide, minimizing voltage drop between the battery terminals and the inverter DC bus is just as critical as the battery capacity itself.

When to split the bank and add a dedicated circuit:

• Surge Overload: When your calculated simultaneous inrush exceeds 80% of your BMS peak surge rating or your inverter's peak VA rating.
• Critical vs. Non-Critical Loads: If you want to guarantee that your network router and medical equipment never lose power, but you are willing to shed the water heater or well pump during extended outages.
• Voltage Sag Mitigation: When heavy motor startups cause LED lights to flicker or sensitive electronics to reboot.

The Fix: Do not simply parallel a second battery to fix a surge issue; parallel batteries share continuous current, but the BMS with the lowest instantaneous threshold will still trip first. Instead, install a secondary, smaller inverter (e.g., a 2000W high-frequency unit) dedicated solely to the heavy inductive loads, wired to a separate subpanel. Alternatively, use a smart load-shedding relay (like a Victron ET112 or a standard 24V DC contactor controlled by the battery monitor) to physically disconnect the well pump circuit whenever the battery State of Charge (SoC) drops below 30%, preserving the remaining capacity for critical electronics.

By tallying your true inrush loads, applying the 80% DoD rule, sizing your DC wiring to minimize voltage drop, and splitting heavy motor circuits when necessary, you build a battery bank that delivers its rated capacity without nuisance tripping the BMS.