Whole-House Battery Backup: Capacity, Runtime, and Buying Checks

Updated September 29, 2026

A whole-house battery backup can keep selected circuits or, in a larger design, most household loads operating during an outage. Runtime is not determined by battery capacity alone. Load profile, inverter output, motor starting surge, battery limits, temperature, conversion losses, solar production, and reserve all matter.

Start with critical loads

List the appliances that must operate, their running watts, startup watts, and daily hours. Refrigeration, medical equipment, a well pump, communications, lighting, and furnace controls often take priority. Large electric water heaters, resistance heat, ovens, dryers, and air conditioners can consume a battery quickly.

Daily energy in kWh = watts × hours ÷ 1,000. Add inverter losses and reserve, then model the worst realistic outage day.

Battery energy and inverter power are different

Kilowatt-hours describe stored energy; kilowatts describe how much power can be delivered at once. A battery may hold enough energy for a day but still be unable to start a large compressor. The inverter must supply continuous load, startup surge, voltage, and split-phase requirements while staying within the battery’s discharge limit.

Current Humless components

The Humless IP65 5kWh LiFePO4 battery and Humless 6kW universal inverter are active components. The Humless 6kW inverter and IP65 battery bundle combines compatible equipment. Confirm usable capacity, output, surge, environmental rating, expansion rules, communications, and included hardware for the exact configuration.

For larger off-grid designs, compare the active AIMS Power 22.1kWh master battery cabinet with a compatible inverter such as the AIMS 12kW split-phase inverter/charger. Compatibility must be verified; similar voltage labels do not prove that arbitrary equipment can communicate or operate safely together.

Grid-tied backup needs isolation

A grid-connected battery system must prevent power from energizing utility lines during an outage. Use listed transfer or multimode inverter equipment and a permitted design installed by qualified professionals. Decide whether backup covers a critical-load panel or the main service and how oversized loads will be shed.

Solar changes recharge, not initial runtime

Solar can extend an outage only when the array, weather, inverter, and battery leave enough surplus after daytime loads. Model several cloudy days and the seasonal worst case. Keep a generator integration plan if long outages or critical medical loads exceed the practical battery and solar budget.

Installation and fire safety

Follow the manufacturer’s spacing, temperature, mounting, ventilation, disconnect, overcurrent, grounding, and enclosure requirements. Battery systems can produce dangerous fault current even when the grid is off. Permits, utility approval, and fire-code requirements vary by jurisdiction.

Test the outage plan

Perform controlled outage tests, record state of charge and load, verify automatic transfer, and confirm every household member knows which appliances must remain off. Review firmware, alarms, filters, terminals, and emergency shutdown procedures on the prescribed schedule.

The practical buying rule

Size a whole-house battery backup from a measured load profile and desired outage duration. A smaller system that reliably supports critical circuits is often more useful than an expensive “whole-house” label with no documented runtime.