Home battery backup calculator

Quick answer

A home battery has to pass two separate tests. Its inverter and electrical architecture must be able to run and start the loads you care about, and its stored energy must be large enough to keep those loads operating for your target outage window.

This calculator therefore reports kW and kWh separately. It deliberately does not assume one Canadian refrigerator wattage, one motor-start multiplier, one inverter-efficiency factor, one battery reserve, or one winter solar derating percentage.

What the calculator is actually sizing

The power side and the energy side use different inputs because they answer different questions.

1. Managed continuous power

The calculator adds the running watts of loads that may operate automatically, then adds the largest load you said you would run separately. That produces a transparent managed-load envelope without pretending a hidden “diversity factor” knows which appliances will be on in your house.

2. Known starting-power envelope

For pumps, compressors and other loads that can have meaningful inrush, the calculator uses a starting value only when you enter one. If the value is unknown, the result says more data are required instead of applying a generic 2×, 3× or 5× multiplier.

3. Delivered energy target

Each load gets an energy-use model: continuous, duty-cycled, intermittent minutes per day, or a measured/planned average wattage. The calculator converts those inputs into average watts, multiplies by your outage duration, and adds only the system overhead and planning headroom you explicitly enter.

The result is load-side planning energy. It is not a battery nameplate recommendation. A system with a matching advertised kWh number may still deliver less usable AC energy because usable-capacity limits, conversion losses, standby draw, temperature, operating mode and battery condition vary by design.

Why whole-home backup still needs load management

“Whole-home” can describe how broadly a system is connected to the home's electrical distribution. It does not mean every range element, dryer, water heater, heat pump, EV charger and pump can run at full power simultaneously. A well-designed system may intentionally shed or defer large loads to stay inside the battery inverter's output limits.

That is why the calculator shows both the managed continuous target and all selected loads running at once. The difference is the value of deliberate load management.

What requires an electrician or exact equipment data

• 240 V loads: battery capacity does not solve a voltage or split-phase compatibility problem.
• Hardwired loads: furnaces, pumps, HVAC and household circuits turn this into an electrical-system integration project.
• Motor/compressor startup: use exact manufacturer or measured inrush/start data when it can determine whether the inverter will trip.
• Heat pumps: compressor behaviour and auxiliary resistance heat can change the answer dramatically.
• Whole-home integration: transfer/islanding equipment, service configuration, load control, permits and product approval belong in the installation design.

How to get better inputs

1. Use the exact equipment nameplate, manual or manufacturer data for voltage and running current/power.
2. For ordinary plug-in loads, use a suitable energy meter over a representative period when practical.
3. For cycling loads, measure the duty cycle rather than borrowing a generic percentage from the internet.
4. For pumps and compressors, find actual starting/inrush information instead of applying a universal multiplier.
5. Keep short, manually controlled loads separate from loads that can start on their own.
6. Ask the battery installer to map the load-side energy target to the system's actual usable capacity, discharge path, standby draw and temperature behaviour.

When this calculator is the wrong tool

• Use the portable power station runtime calculator for one plug-in battery station and a measured device load.
• Use the generator size calculator for a fuel generator's running and starting-watt requirements.
• Use the furnace outage guide for furnace-specific connection and equipment issues.
• Use the well-pump outage guide for voltage, controller and pump-start requirements.
• Use the sump-pump battery backup guide for pump cycling, backup architecture and water-risk planning.

Methodology

The calculation model was designed after September 2026 Canadian keyword research and a technical deep-research dossier, then independently checked against current Canadian electrical-safety and utility material. The dossier's fixed efficiency, standby-draw, motor multiplier and winter-solar defaults were rejected because they were not defensible across residential battery systems.

The calculator's central rule is intentionally simple: use instantaneous running/start data for power, and measured or user-defined operating profiles for energy. Missing consequential inputs stay visibly missing.