About the Battery Backup Runtime
This calculator estimates how many hours a home battery storage system can power your essential circuits during a grid outage. It's meant for homeowners with a solar battery, standby battery, or whole-home backup system who want a quick runtime estimate based on the battery's rated capacity and the load they plan to keep running, such as a fridge, lighting, and WiFi.
How It Works
You enter the battery's total capacity in kilowatt-hours, its usable depth of discharge as a percentage, and the combined power draw of the loads you want to keep running in kilowatts. The calculator multiplies capacity by depth of discharge to find the usable energy in the battery, then divides that usable energy by your load to get an estimated runtime in hours.
Formula & Methodology
Depth of discharge represents the share of the battery's rated capacity that can actually be drawn down before the battery management system cuts off output to protect the battery's chemistry and lifespan; the remainder is reserved buffer. Multiply the rated capacity by this percentage to get usable kilowatt-hours. Then divide usable kilowatt-hours by your steady load in kilowatts (kWh divided by kW cancels to hours) to get the estimated number of hours the battery can sustain that load before depleting.
Examples
Standard home battery on essential circuits
A 13.5 kWh battery with 90% usable depth of discharge provides 12.15 kWh of usable energy. Powering a 1.5 kW essential load (fridge, lights, WiFi), that supports roughly 8.1 hours of runtime.
Smaller battery on a lighter load
A 10 kWh battery with 85% depth of discharge gives 8.5 kWh usable. Running a lighter 0.8 kW load stretches that to about 10.6 hours of estimated runtime.
Advantages
- Gives a fast runtime estimate from just three numbers, without requiring detailed appliance-by-appliance load modeling.
- Separates rated battery capacity from usable capacity, reflecting how depth of discharge limits actually work in real systems.
- Makes explicit that the result is a straightforward capacity-over-load estimate, so users know to treat inverter losses and load variability as additional real-world factors.
Common Mistakes
- Using a battery's full rated capacity instead of its manufacturer-specified usable depth of discharge, which overstates how long backup power will last.
- Underestimating total load by forgetting appliances that cycle on intermittently, like a well pump, sump pump, or furnace blower, which add to the average power draw even if they aren't running constantly.
- Assuming the calculated runtime is exact rather than an estimate, when inverter efficiency and real-world load spikes typically shorten actual backup time somewhat.
Edge Cases to Watch For
- If the load is entered as zero or a negative number, the calculator returns an error instead of dividing by zero or producing an infinite runtime.
- The estimate assumes a constant, steady load in kilowatts; real household loads spike when appliances like a refrigerator compressor or well pump start up, which can shorten actual runtime versus this average-based estimate.
- The calculation doesn't separately account for inverter efficiency losses, which typically shave a few percent off usable energy in a real system.
- Depth of discharge limits are set by the manufacturer or battery chemistry; entering a depth of discharge higher than what your specific battery actually allows will overstate usable capacity.
Common Use Cases
- Homeowners sizing a battery backup system and deciding which circuits to prioritize as 'essential loads' during an outage.
- People comparing battery capacity options from different manufacturers by translating rated kWh into an expected hours-of-backup figure.
- Solar-plus-storage owners planning for outage scenarios, like extended power loss during storm season, and checking whether their system covers the necessary duration.