Whole-Home Battery Backup Sizing

Define what whole-home backup means

Whole-home backup describes a proposed scope of protected circuits. It does not guarantee unrestricted simultaneous appliance use. When properly designed, a system can support the selected home loads within equipment power limits and while sufficient stored energy is available. A clear load plan should identify loads that remain on, loads managed automatically and loads intentionally excluded.

Manufacturer material describes both energy storage and load-management features. Enphase's battery overview, for example, explains turning power-hungry appliances on and off through supported controls. This is a design tool, not a substitute for measuring actual demand or checking the installed product generation.

Understand kW and kWh

Power, measured in kilowatts (kW), describes the rate of supplying electricity. Energy, measured in kilowatt-hours (kWh), describes an amount used or stored over time. A project must satisfy both requirements. A battery may store enough energy for the intended period but be unable to supply a large appliance's starting demand; a powerful inverter may have too little available energy for an overnight load plan.

Equipment names are not enough. Obtain the actual continuous-power, starting-load and usable-energy data for the exact battery/inverter configuration. Do not compare a manufacturer's product name with another product's usable-energy rating as though they were the same measurement.

Build a circuit-level load schedule

Start with a list of loads, their expected operating times and measured or documented demand. Review refrigeration, communications, lighting, pumps, cooking, heating/cooling and any medical equipment separately. Electric resistance heat, water heating and EV charging deserve particular attention because their use can dominate the outage energy budget.

Record simultaneous-use assumptions. Starting loads, cycling behavior and weather can change demand. Medical equipment requires an individualized continuity plan; a residential battery article should never promise uninterrupted life-safety power.

Use an illustrative energy budget

As a mathematical example only, an average protected demand of 0.5 kW for 12 hours uses 6 kWh before accounting for losses, reserve settings or changing loads. At 2 kW for the same 12 hours, the load uses 24 kWh. These numbers are not a manufacturer's capacity recommendation, a site design or a prediction of your runtime.

The design must then account for the usable operating window, any reserved charge, conversion losses, battery conditions and the actual start-of-outage charge. The resulting installed quantity should be calculated using the selected product's documented ratings, not a generic number of batteries per house.

Do not assume solar recharge is always available

Solar contribution depends on sunlight, array condition and whether the proposed configuration supports solar operation while islanded. Cloud cover, shade and seasonal production may reduce energy available during an outage. Tesla's overview describes storing energy and working with solar; it does not guarantee a daily recharge amount for a specific house.

For a design involving Sol-Ark 15K-2P-LV, the manual's backup-circuit and integration requirements must be reviewed. Product-specific operating modes and connection constraints are not interchangeable across inverter revisions.

Match the outage target to the design

An overnight target, a daytime interruption and a multi-day outage have different energy needs. Ask the designer to state the assumed outage duration, protected loads, initial charge, estimated solar contribution and the operating actions required to stay within limits. Generator integration, when proposed, must be supported by the applicable equipment and engineered connection.

Ask how a system responds to low charge and whether loads are shed automatically. Understand how homeowners receive alerts and what happens when equipment shuts down. Do not rely on an unqualified promise that whole-home backup means no circuit management.

Request a documented sizing recommendation

The final recommendation should include an electrical assessment, actual model ratings, a load schedule, starting-demand checks and a range of runtime scenarios. It should distinguish calculations from manufacturer specifications and from guaranteed outcomes. Current RS can evaluate the site and planned backup scope; this article does not assert a reviewed installation design for any particular customer.

Use documented product ratings without turning them into a runtime promise

The Powerwall 3 US datasheet lists 13.5 kWh nominal AC battery energy under stated test conditions and selectable nominal AC output up to 11.5 kW. The IQ Battery 5P North American datasheet lists 5.0 kWh usable capacity and 3.84 kVA continuous apparent power at 240 V, with reserve-related footnotes. Energy and output describe different limits; kW and kVA are also not interchangeable at every power factor.

Sol-Ark's September 2026 15K-2P-LV specification table separates 15 kW grid nominal real power from 12 kW continuous off-grid power at 240 V. It lists 13 kW for 30 minutes and 24 kW for 10 seconds as peak surge ratings. A backup calculation should use the relevant continuous rating, battery capability and temperature conditions, not the largest headline number.

The Limitless 18K-2P-LV Revision 8 datasheet instead lists 18 kW continuous off-grid output at 240 V and 36 kW surge for 10 seconds; at 208 V the corresponding figures are 15.6 kW and 31.2 kW. Its manual says battery-only output varies with state of charge and voltage, and limits combined battery charge/discharge to 350 A using both inputs, or 190 A for a single input. A bank must support the intended operating demand. Both the 15K and 18K list 200 A grid passthrough, which is not inverter-generated backup. Choosing the 18K does not by itself increase stored kWh or outage duration.

Have the proposal document at least a low-demand scenario and a high-demand weather scenario. Each should show initial charge, available energy after reserves, estimated average load and the assumptions for solar contribution. Turning a rated capacity into guaranteed hours without those assumptions is not a complete sizing recommendation.