Enter your battery capacity, starting charge, and expected outage load to estimate how many hours your home battery can provide power.
Advanced options
This is an energy-based planning estimate. It does not prove that the battery can start or continuously power every device.
This comparison uses the entered continuous output limit. It is not a manufacturer approval or startup guarantee.
This is the energy used in the runtime estimate, not a new battery rating.
A negative value means the average load exceeds the entered output limit.
A negative value means the startup load exceeds the entered output limit. A separate surge rating is not modeled.
Table of contents
How to use our Home Battery Runtime Calculator
- Choose "Already usable capacity" if the listed kWh already excludes protected reserve or unavailable battery energy. Choose "Rated capacity" only when you have a separate usable-capacity percentage.
- Enter the battery capacity and its charge level at the beginning of the outage.
- Choose "Watts (W)" or "Kilowatts (kW)" before entering a new average outage load. Use the average power expected over the outage, not necessarily the sum of every appliance label.
- Open "Advanced options" if you need to include an extra documented conversion loss. Leave "Extra conversion efficiency (percent)" at 100 when your usable kWh value already includes the losses you want counted.
- For an optional power check, enter both "Battery system output limit (W)" and "Highest total startup load (W)." Include loads likely to be running while an appliance starts.
- Select "Calculate." Read "Estimated battery runtime" first. Then check the hours-and-minutes reading and, if shown, the power margins. A negative margin means the entered load is above the entered limit.
Definitions
kWh (kilowatt-hour): A unit of energy. More available kWh can run the same average load for more time.
Rated capacity: The stated battery energy before a separate usable-capacity percentage is applied.
Usable capacity: The share of rated battery energy the system makes available for use.
Starting charge: The percentage of available battery energy stored when the outage begins.
Average outage load: The average power used during the outage, entered in watts or kilowatts.
Startup load: The highest total power expected at one moment when equipment starts, including other loads already running.
Power margin: The entered battery-system output limit minus an entered load. A negative margin means that load exceeds the limit.
Common mistakes and quick fixes
Mistake: Selecting "Rated capacity" for a kWh figure already described as usable.
Fix: Select "Already usable capacity" so the calculator does not reduce the available energy twice.
Mistake: Using the total nameplate watts of every appliance as the average outage load.
Fix: Enter the average power expected over time. Appliances that cycle on and off can use less average power than their labels suggest.
Mistake: Entering watts while "Kilowatts (kW)" is selected, or entering kilowatts while "Watts (W)" is selected.
Fix: Choose the source unit before a new entry. A typed W or kW suffix can also identify the number's unit.
Mistake: Assuming a positive runtime proves every appliance will start.
Fix: Enter both optional power-check values when available. A passing check still does not account for every equipment-specific surge or voltage condition.
Mistake: Replacing a value after switching between watts and kilowatts.
Fix: Keep the converted number when switching units for an already-correct entry. It describes the same physical load.
Limitations & Key Assumptions / Boundary Conditions
- This is an energy-based planning estimate. Actual runtime changes when household power use changes during an outage.
- The estimate uses the capacity basis, usable-capacity percentage when needed, starting charge, extra conversion efficiency, and average load that you enter.
- Leave extra conversion efficiency at 100 percent when the usable kWh value already includes the losses you want counted. Applying losses twice makes the estimate too low.
- The optional power check compares entered loads with one entered continuous output limit. It does not model time-limited surge ratings, voltage, wiring, battery temperature, battery condition, or appliance-specific startup behavior.
- A nonnegative power margin is not manufacturer approval and does not guarantee an appliance will start or operate correctly.
Methodology
Runtime calculation
The calculator converts the average outage load to watts. A W or kW suffix typed with the load takes priority; otherwise, the selected average-load unit is used.
average_load_w = entered load in W, or entered load in kW * 1000
For "Rated capacity," the usable-capacity percentage reduces the entered battery capacity. For "Already usable capacity," the capacity factor is 1.
capacity_basis_factor = usable_capacity_percent / 100 for rated capacity; otherwise 1
Available energy is adjusted for starting charge and extra conversion efficiency. A blank efficiency is treated as 100 percent, so it adds no extra loss.
available_energy_kwh = battery_capacity_kwh * capacity_basis_factor * starting_charge_percent / 100 * conversion_efficiency_percent / 100
Estimated runtime equals available energy divided by average load in kilowatts.
estimated_runtime_hours = available_energy_kwh / (average_load_w / 1000)
For example, a 20 kWh rated battery with 80 percent usable capacity, 75 percent starting charge, and 90 percent extra conversion efficiency has 10.8 kWh available. At a 2 kW average load, estimated runtime is 10.8 / 2 = 5.4 hours, or 5 hours 24 minutes.
Optional power check
When both optional power values are entered, the calculator subtracts the average running load and the highest startup load from the entered battery-system output limit. Negative margins remain negative because they show how far the entered load exceeds that limit.
running_power_margin_w = system_output_limit_w - average_load_w
startup_power_margin_w = system_output_limit_w - highest_startup_load_w
The status reports a running-load exceedance first, then a startup-load exceedance, or "Within entered power limit" when both margins are zero or positive.