HOME BATTERY SIZING
How Many kWh Do You Need to Power a House?
The answer depends on which appliances you want to run, how long the outage lasts, and how much power those appliances draw. A whole home and a small set of essential loads can have very different energy needs.
Quick answer: there is no single kWh number that fits every home. Estimate the energy used by the loads you want to back up during the outage, then account for system losses and check that the inverter can supply the required power.
What does kWh mean?
Kilowatt-hours (kWh) measure energy over time. A 1,000-watt appliance running for one hour uses about 1 kWh. Kilowatts (kW), by contrast, measure the rate of power at a moment in time. Battery capacity in kWh tells you how much energy is stored; inverter output in kW helps determine which appliances can run at once.
Start with the appliances you need
List the loads that matter during an outage, such as a refrigerator, lights, Wi-Fi, selected outlets, a well pump, or air conditioning. For each one, look for its nameplate or manufacturer documentation rather than relying only on a generic estimate. Motors and compressors may draw a brief startup surge that is higher than their normal running load.
Estimate energy for the outage duration
A simple first estimate is:
Energy (kWh) = average load (kW) × hours of backup
For example, if the appliances average 1.2 kW over the period and you want 10 hours of backup, they would use about 12 kWh before accounting for conversion losses, reserve capacity, or changes in usage. This is an example of the calculation, not a prediction for a particular home.
Remember duty cycle and real usage
Some appliances do not run continuously. A refrigerator cycles on and off, while an air conditioner’s demand changes with weather, thermostat settings, insulation, and home size. Average energy use can therefore be lower than multiplying every appliance’s full running wattage by every hour. However, assumptions about cycling can be wrong for your actual equipment, so treat generic estimates as preliminary.
Check power output as well as energy capacity
A battery may have enough stored energy but still be unable to operate all selected loads simultaneously. Check the system’s continuous output rating and its surge rating, including how long it can sustain surge output. Startup requirements can vary widely, and multiple appliances may start close together. A qualified installer should verify the equipment combination and electrical design.
How to get a useful preliminary estimate
- Choose only the appliances you want available during an outage.
- Set a realistic backup duration, such as 12, 24, or 48 hours.
- Use the Backup Clarity calculator to explore an illustrative estimate.
- Read the methodology and assumptions, then compare its generic assumptions with actual equipment specifications.
- Ask a qualified installer or electrician to verify usable capacity, continuous output, startup surge, transfer equipment, permits, and installation requirements.
Important limitations
Backup Clarity’s calculator uses simplified, generic appliance and battery assumptions. It does not inspect your home, measure actual loads, model every operating condition, or provide an electrical design. The result does not guarantee runtime or establish that a specific system is safe or compatible. Use manufacturer data and professional advice before purchasing or installing equipment.