The basic battery sizing formula is simple, but each input matters. Start with the energy you need in kWh. Multiply by the number of days you want the battery to cover. Then divide by usable depth of discharge and efficiency so the battery is not forced to deliver more than it safely can.

For example, if you use 12kWh per day, want one day of autonomy, use 85% depth of discharge and 95% efficiency, the calculation before margin is 12 ÷ 0.85 ÷ 0.95 = about 14.85kWh. Add a 10% safety factor and the recommended battery becomes about 16.34kWh. At 48V, that is about 340Ah.

Why usable capacity is lower than label capacity

A battery labeled 10kWh may not deliver the full 10kWh in normal operation. The BMS may reserve energy to protect cells, the inverter loses energy during conversion, and a backup reserve may be kept for outages. Temperature, battery age and high load current can also reduce available energy.

Daily energy versus backup energy

If you are sizing an off-grid system, daily energy use may include everything. If you are sizing emergency backup, do not use full monthly home consumption unless you truly want whole-home backup. Instead, add only essential loads such as refrigerator, lights, fans, router, security, medical equipment and selected outlets.

Battery size should be matched with solar generation. A 20kWh battery is not useful if your solar array can only produce a few spare kWh after daytime loads. For solar systems, check how much surplus energy is available after the house is powered during the day. That surplus determines how often the battery can be charged.

For off-grid systems, autonomy days matter because cloudy weather can reduce solar output. A cabin that uses 6kWh per day and needs two days of autonomy may need more than 14kWh of battery after DoD and efficiency are included. If a generator is available, the battery bank can be smaller because the generator acts as a backup energy source.

For grid-tied homes, the best battery size is usually the amount you can fill during the day and use at night. Oversizing the battery can make payback slower because unused capacity does not save money.

For a home backup system, the first decision is whether you want essential-load backup or whole-home backup. Essential-load backup is usually more affordable and reliable because it separates important circuits from heavy appliances. Common essential loads include LED lights, fans, refrigerator, Wi-Fi router, laptop charging, CCTV, security system, garage door, medical devices and a few selected sockets.

Whole-home backup sounds attractive, but it can require a much larger battery, higher inverter rating, bigger transfer equipment and stricter load management. Electric ovens, geysers, kettles, irons, pool pumps, bore pumps, EV chargers and large air conditioners can drain or overload a battery quickly. In many homes, the smartest approach is to back up essentials and leave heavy loads on grid or generator.

When entering daily energy use into the calculator, use the correct load group. If you want whole-home backup, use full daily kWh. If you only want outage backup, add the essential appliances and expected hours. This avoids buying too much battery or expecting too much from a small one.

Off-grid and RV systems need more conservative sizing because there may be no grid available when solar output is poor. In these systems, autonomy days are more important. A grid-tied home may only need a few backup hours, but an off-grid cabin may need enough stored energy for two cloudy days plus a reserve.

Start by listing every load: lights, water pump, fridge, laptop, Starlink or router, fans, tools, microwave, induction cooking, heater controls and any DC loads. Then estimate how many hours each load runs per day. Add the kWh total and multiply by the autonomy days. Finally, account for depth of discharge, inverter efficiency and safety margin.

For RV and marine systems, pay extra attention to alternator charging, DC-DC charger limits, cable size and low-temperature charging protection. A battery may be large enough in kWh but still limited by how fast it can safely charge from solar, alternator or shore power.

After using the calculator, save the result and share it with your installer along with your appliance list. Ask the installer to confirm the battery’s usable capacity, continuous discharge rating, peak discharge rating, inverter size, cable size, protection devices, backup circuit plan and communication protocol.

For LiFePO4 systems, ask whether the battery communicates with the inverter through CAN or RS485. Closed-loop communication helps the inverter follow the battery’s safe charge current, discharge current, temperature warnings and state of charge. This is especially important for larger home batteries and hybrid solar inverters.

Also ask about future expansion. If the calculator suggests you need 10kWh today but you may add an air conditioner, EV charger or larger solar array later, choose a battery family that supports parallel modules. Expansion should be planned before installation because mixing brands, ages or capacities later can create BMS and warranty problems. For best results, ask the installer to run a short backup test after commissioning so you can see the battery percentage, inverter load, recharge behavior and circuit transfer under real conditions before handover.