Home Backup Battery Sizing & Autonomy Guide

Start by listing the circuits you must keep running, then multiply each load by the hours you want it powered. A fridge, WiFi router, phone charging, lights, fans, a laptop and a small medical device may need far less energy than a whole home with air conditioning, cooking and laundry. After calculating energy in kWh, divide by usable depth of discharge and round-trip efficiency. For many homes, 5–10 kWh works for essential loads, 10–20 kWh is a stronger overnight backup, and 20 kWh or more is usually where partial whole-home backup begins.

A 10 kWh battery can be enough for essential circuits if you manage loads carefully. It may run a refrigerator, lights, internet, fans, TV and device charging for many hours, but it usually will not run an entire home normally for a full day if high-demand appliances are included. Air conditioners, electric ovens, kettles, pumps, dryers and water heaters can drain a battery quickly or exceed the inverter’s surge rating. The better question is not only “is 10 kWh enough?” but “which loads will the 10 kWh battery serve during an outage?”

Battery autonomy means how long your backup system can power selected loads without grid power. It is usually expressed in hours or days. For example, if your critical loads use 500 watts on average and your battery has 10 kWh of usable energy, the rough autonomy is about 20 hours before losses and safety margins. Autonomy changes with real appliance behavior: fridges cycle on and off, fans and lights are predictable, while pumps and air conditioners create heavy short bursts. Good sizing is based on average energy plus peak power.

Critical-load backup is usually the smarter first step because it separates essential circuits from comfort or luxury loads. A critical load panel can include refrigeration, internet, selected lights, security, phone charging, medical equipment, sump pump, gate motor or a few fans. Whole-home backup is more convenient, but it needs more battery capacity, a stronger inverter, correct transfer equipment and careful load control. If budget is limited, prioritize essential circuits and add a path for future expansion.

kW is power: how much load the system can run at a moment. kWh is energy: how long it can run those loads. A battery may have enough kWh for a long outage but still fail to start a pump or air conditioner if the inverter kW or surge rating is too low. Similarly, a powerful inverter with too little kWh may start large appliances but run out quickly. A complete backup design checks both energy capacity and power delivery.

Depth of discharge, or DoD, tells you how much of the battery you plan to use before recharging. LiFePO4 batteries generally allow deeper discharge than lead-acid batteries, but running any battery at extreme limits all the time can reduce comfort margin and may affect long-term performance. Many practical designs use 80–90% DoD for LiFePO4 calculations, unless the battery maker specifies a different usable capacity or warranty condition. Always follow the actual battery datasheet and BMS limits.

Yes, but air conditioning changes the design. You need enough continuous inverter power, enough surge or locked-rotor capability, and enough battery kWh for the runtime you expect. Small inverter AC units are easier to support than large fixed-speed units. A soft starter may help some motors, but it is not a substitute for correct inverter sizing. If AC backup is a priority, size the system around that load first and consider zoning, thermostat setpoints and solar recharge.

A battery can provide backup without solar if it is charged from the grid before an outage, but solar dramatically improves resilience during long outages. Without solar, the battery is like a fuel tank that empties once. With solar, daylight generation can recharge the battery, power daytime loads and extend autonomy. The solar array and inverter must be configured to operate during outages; not every grid-tied solar system can recharge a battery when the grid is down.

For frequent load shedding, calculate each outage block separately. If the power is off for 4 hours twice a day, decide whether the battery must recharge between outages and whether solar is available. A small apartment might use 2–5 kWh for lights, fans, router and fridge support. A larger home may need 10–15 kWh for comfortable evening backup. If pumps, AC or kitchen appliances must run, size both the battery and inverter much higher.

LiFePO4 is usually preferred for modern home backup because it has higher usable capacity, better efficiency, lower maintenance and longer cycle life than typical lead-acid systems. It also holds voltage more steadily under load and is easier to monitor through a BMS. Lead-acid can still work for low-budget or legacy systems, but it needs more oversizing because it should not be deeply discharged regularly. For daily load shedding or solar self-consumption, LiFePO4 is usually the stronger long-term choice.

A critical load panel should include circuits that protect comfort, safety, communication and food storage. Typical examples include refrigerator or freezer, selected lights, WiFi/router, phone charging, security system, gate or garage control, fans, medical equipment and sometimes a small water pump. Avoid adding heavy loads unless the battery and inverter are designed for them. Electric ovens, geysers, dryers and large AC units can reduce runtime dramatically and may require dedicated load management.

Avoid undersizing by measuring real loads instead of guessing, separating essential from optional circuits, accounting for inverter losses, considering battery aging, and checking surge loads. Add a safety margin, especially if the system will be used daily. Confirm the inverter’s battery compatibility, BMS communication, charging current, cable size, fusing and enclosure requirements. Finally, think about future loads such as another fridge, home office equipment, EV charging or a second air conditioner before locking in a non-expandable battery system.