Battery runtime and essential-load sizing
How This Calculation Works
The runtime estimate converts usable battery capacity into watt-hours, applies the selected depth-of-discharge reserve, and divides by the average connected load. It also considers inverter losses, so a 13.5 kWh nameplate does not mean 13.5 kWh reaches 120-volt loads. Runtime changes as refrigerators cycle, pumps start, chargers run, and temperatures affect the battery. Use the Whole-Home Energy Load Calculator to build a realistic list, then test both average watts and starting surge.
Applicable Code Rules & Standards
CEC/OESC requirements for energy storage address equipment listing, disconnecting means, wiring, overcurrent protection, location, ventilation or separation, labelling, and emergency access. Ontario installations may require ESA notification and utility coordination. NEC projects commonly reference Article 706 and related PV and transfer-equipment rules, but jurisdiction-specific requirements always control. A runtime result does not confirm that an inverter can legally or safely connect to a dwelling panel; transfer equipment must prevent unintended backfeed.
Common DIY/Installation Errors
- Using total battery nameplate capacity instead of usable capacity and reserve.
- Forgetting refrigerator, well-pump, furnace, or sump-pump starting surge.
- Connecting a portable generator or inverter to a receptacle without listed transfer protection.
- Placing batteries where temperature, water, combustible storage, or ventilation violates the listing.
- Assuming a battery can power a whole panel when only selected essential circuits were designed for backup.
Make an essential-load list with both running watts and startup behaviour. Decide whether refrigeration, heat, communications, medical equipment, sump protection, lighting, and water systems must remain available, then identify what can be switched off during an outage. The result should include a reserve for cold weather and battery ageing. Test the completed system under supervision and review its alarms, shutdown procedure, ventilation, clearances, and reset instructions with every household member.
Runtime should be treated as a range rather than a promise. Seasonal temperature, battery age, inverter idle draw, charging state, and a changing household load can move the result substantially. A written emergency plan is as important as the kWh number.
When to Hire a Licensed Professional
Hire a professional for a hardwired battery, transfer switch, generator inlet, essential-load panel, solar-plus-storage system, or any connection to fixed building wiring. The installer must size conductors and protection, verify neutral switching and grounding, provide labels, coordinate permits, and test islanding or transfer behaviour. Keep the system isolated and call for help if it smells hot, swells, leaks, alarms, or repeatedly shuts down.