Battery Backup

How Battery Backup Runtime Is Calculated

A battery backup’s runtime is its usable energy, divided by the load it’s powering — and not every kWh of rated capacity is actually usable, since depth-of-discharge and inverter conversion both take a share off the top. Choose whether you want to find how long an existing battery lasts, or how big a battery you need for a target runtime, enter your load and the battery’s real- world derating factors, and this calculator returns the answer either direction.

This is distinct from the Generator Sizing Calculator calculator, which sizes a fuel- powered generator’s continuous wattage capacity (running load plus the largest motor-starting surge) rather than a battery’s stored energy and how long it lasts.

Key Factors to Consider

• A battery’s inverter has its own surge rating, separate from continuous output. Like a generator, powering a motor-driven appliance (a refrigerator compressor, a well pump) briefly draws far more than its steady running wattage to start up. This calculator’s runtime math assumes a steady average load — check that your battery system’s inverter surge rating actually covers your equipment’s startup draw, not just its continuous load.
• Solar recharging during a multi-day outage extends real-world runtime well beyond a single discharge cycle. If the battery is paired with solar panels, it can partially recharge each daylight hour during an extended outage — this calculator’s formula models one continuous discharge, not a system that’s topping back up during the day, so real backup duration across a multi-day outage with sun exposure can be meaningfully longer than the raw runtime figure alone suggests.
• Battery capacity gradually declines over the system’s lifetime, similar to any lithium battery. A system’s real usable capacity several years in is typically somewhat lower than its original rated spec — worth using a reduced capacity figure if you know your system has aged meaningfully.
• Extreme temperatures can reduce a battery’s real-world performance. A battery installed in an unconditioned garage or outdoor enclosure may deliver somewhat less usable capacity in very hot or cold conditions than its rated spec assumes.

Interpreting Your Results

The runtime figure this calculator returns is the battery’s own theoretical discharge time under a steady, constant load — a useful planning number, but real-world outages rarely draw power that evenly. A refrigerator compressor cycling on and off, a furnace blower kicking in, or someone running a microwave for a few minutes all create short spikes well above the average wattage you’ve entered, so the actual runtime you experience will typically land somewhat below the calculated figure rather than matching it exactly. Treat this number as a reasonable planning estimate, not a guaranteed countdown clock — and lean toward a smaller “critical loads only” average wattage figure rather than your home’s full potential draw if you want the estimate to hold up closer to reality.

When solving in reverse for a target runtime, the capacity figure this calculator returns is the minimum battery you’d need under ideal, steady-load conditions. Sizing up somewhat from that number — the same way an electrician oversizes a circuit rather than wiring to the exact minimum — gives you margin against real-world load spikes, a battery that’s aged past its first year or two, or an outage that runs longer than you initially planned for.

Common Mistakes

• Using the battery’s full rated capacity instead of its usable capacity. A battery labeled “13.5 kWh” doesn’t actually deliver 13.5 kWh to your appliances — depth-of-discharge limits and inverter conversion losses both take a real share off the top before any of that energy reaches a plug. Skipping those two derating factors is the single most common way a battery backup estimate ends up overly optimistic.
• Sizing to the home’s total potential draw instead of the actual critical-loads circuit. Most residential battery backup systems power a dedicated subpanel of essential circuits, not the entire house — using your home’s full electrical capacity (or a whole panel’s worth of breakers) as the “load” wildly overstates what the battery actually needs to support during an outage.
• Ignoring appliance startup surge and assuming average wattage is the whole story. A refrigerator or well pump’s running wattage might be modest, but its momentary startup draw when the compressor or motor kicks on can be several times higher — a battery’s inverter needs enough surge capacity for that spike even if the calculated average-load runtime looks comfortable.
• Treating one manufacturer’s depth-of-discharge or efficiency spec as universal. These figures genuinely vary by battery chemistry and inverter model — a lead-acid battery is typically only safely discharged to around 50% of its rated capacity to protect its lifespan, while many lithium systems can be discharged much deeper. Use your specific system’s own published numbers rather than assuming a one-size-fits-all default.

The Formula

$$\vC{\text{Usable Energy (Wh)}} = \vA{\text{Battery Capacity (kWh)}} \times 1000 \times \vD{\text{Depth of Discharge \%}} \times \vE{\text{Inverter Efficiency \%}}$$ $$\vF{\text{Runtime (hours)}} = \frac{\vC{\text{Usable Energy (Wh)}}}{\vB{\text{Load (W)}}}$$

The same equation solved in reverse gives the battery capacity needed for a target runtime.

Worked Example

A 13.5 kWh battery powering a 3,000 W load, at 90% depth of discharge and 90% inverter efficiency:

1. Usable energy: $\vA{13.5} \times 1000 \times \vD{0.90} \times \vE{0.90} = \vC{10,935} \text{ Wh}$.
2. Runtime: $\vC{10,935} \div \vB{3,000} \approx \vF{3.65} \text{ hours}$.

Reversing the question — powering a 1,500 W load for 8 hours at the same 90%/90% derating — needs about 14.8 kWh of rated battery capacity.

Useful to Know

Lithium battery systems are subject to a real, documented safety risk called thermal runaway — a chain reaction inside a damaged, overcharged, or manufacturing-defective cell that generates heat faster than it can dissipate, potentially leading to fire. This is why UL-listed home battery systems ship with dedicated battery-management electronics to prevent it, and why many jurisdictions require the system be installed to NFPA 855 (the U.S. standard for stationary energy storage systems) and local fire-code requirements — including proper ventilation, clearance from living spaces, and, in some areas, fire-department notification. None of this changes the runtime math above, but it’s worth knowing before choosing where a battery backup gets installed in your home: always use a qualified, licensed installer, and never disable or bypass a battery’s own protection electronics to try to get more of its rated capacity.