A home battery should be sized from the loads you actually need during an outage, not from the total list of appliances in the house. The first estimate is an energy calculation; the final design must also check inverter power, surge demand, wiring, transfer equipment, safety, and local installation requirements.
Define the essential load
List the devices that must remain on: refrigeration, lighting, communications, medical equipment, fans, pumps, or selected outlets. Record their approximate watts and expected hours of use. Separating essential loads from optional loads usually produces a smaller, more useful backup plan than trying to run the whole house indefinitely.
Loads with thermostats or motors do not draw their nameplate power every minute. Use a realistic average for runtime estimates and keep a separate peak or startup value for inverter sizing. If the consequence of an undersized battery is serious, measure the load or ask a qualified professional to validate it.
Convert load and hours into battery energy
Essential energy is approximately total average watts divided by 1,000, multiplied by backup hours. Then account for the reserve you do not want to use, battery depth of discharge, and inverter or conversion losses. A battery rated at 10 kWh does not necessarily provide 10 kWh to the loads.
The home-battery calculator reports the assumptions separately so you can compare a short critical-load backup with a longer whole-home scenario. Test low, likely, and high load cases instead of relying on one number when outage duration is uncertain.
Energy capacity and power capacity are different
Battery capacity in kWh describes how long the battery can supply energy. Inverter power in kW describes how much load can run at once. A refrigerator, pump, compressor, or power tool may have a startup surge that exceeds its running wattage. Size the inverter and transfer equipment for the simultaneous running and surge requirements, not just the battery’s energy total.
A battery can have enough stored energy but still fail to start a motor or support several large loads at the same time. Put high-load appliances on a priority list, stagger them where possible, and confirm the system’s continuous and peak ratings from the manufacturer documentation.
Validate the design locally
Product availability, chemistry, warranty, temperature limits, fire protection, installation rules, and utility interconnection requirements vary by location. NumberTrove’s result is a planning estimate and does not replace a one-line electrical design, permit review, or qualified installer assessment.
Before buying, compare the proposed battery and inverter with the actual datasheets, warranty conditions, replacement plan, and expected cycle life. Recalculate when the tariff, solar system, outage pattern, or essential-load list changes.
Sources
The figures and rules in this guide are checked against:
- Solar Integration: Solar Energy and Storage Basics — U.S. Department of Energy
- Electricity Explained: Energy Storage for Electricity Generation — U.S. Energy Information Administration
- NFPA 855: Standard for the Installation of Stationary Energy Storage Systems — National Fire Protection Association
- How Powerwall Works — Tesla Support