Estimated runtime--
Power Outage Runtime Calculator
Estimate outage runtime from stored watt-hours and average load watts for batteries, power stations, and backup planning.
Updated May 2026No signup requiredBuilt for mobile
Practical intro
Use this power outage runtime calculator when you know how much stored energy is available and want to estimate how long it may support an average load. It is useful for battery banks, portable power stations, UPS units, and simple backup plans where runtime depends on watt-hours divided by watts.
When to use this calculator
- Estimating how long a battery or power station can run selected essential loads.
- Comparing runtime after reducing the load list.
- Planning overnight battery use between generator charging windows.
- Checking whether stored energy is enough for communications, lights, fans, refrigeration cycles, or small devices.
Input explanation
- Stored energy is usable watt-hours, not always the full advertised battery rating.
- Average load is the average watts drawn over the runtime period.
- Loads that cycle, such as refrigerators, need an average over time rather than compressor running watts only.
- Inverter idle draw and conversion losses reduce real runtime if not already included.
- For critical loads, keep reserve instead of planning to drain the battery completely.
Formula or estimation method
Estimated runtime hours = stored watt-hours / average load watts.
Runtime with efficiency = stored watt-hours x efficiency fraction / load watts.
Average load = total watt-hours used during a period / hours in that period.
Required energy = desired runtime hours x average load watts.
Step-by-step worked example
If a battery system has 5,000 usable watt-hours and the average essential load is 700 watts, runtime is 5,000 / 700 = about 7.1 hours. Reducing the average load to 350 watts doubles the estimate to about 14.3 hours, before considering inverter idle draw, temperature, reserve, and battery aging.
Common mistakes
- Using rated battery capacity when only part of it is usable.
- Forgetting inverter losses and idle consumption.
- Using peak watts instead of average watts for cycling loads.
- Assuming runtime stays the same in cold weather or with aging batteries.
- Planning zero reserve for critical devices.
Planning notes and limitations
- Runtime improves fastest by reducing load watts, especially heat-producing appliances.
- A watt meter can help measure real average load instead of relying only on labels.
- Generator-plus-battery plans often use batteries overnight and generator runs during the day to recharge.
- For outage planning, make separate daytime and overnight load lists because the average load often changes when lights, cooking, internet, fans, or refrigeration cycles change.
- Keep a written reserve target for critical communications, refrigeration, or medical-support planning so the battery is not intentionally run to zero.
- This calculator estimates runtime only; it does not verify inverter size, wiring, fusing, transfer equipment, or device compatibility.
Backup power safety checklist
Before relying on the estimate, confirm the equipment manual, fuel or battery limits, ventilation needs, weather protection, cords, transfer method, and local requirements. Fuel-burning generators belong outdoors away from openings because carbon monoxide can be deadly. Electrical connections, transfer switches, high-current batteries, and permanent wiring should be handled under applicable code by qualified people.
Safety disclaimer
This calculator is for planning only. Follow manufacturer instructions, battery safety guidance, device manuals, local electrical code, and professional advice for critical, connected, or high-current backup systems.
FAQ
Power Outage Runtime Calculator questions
Why is real runtime shorter than calculated?
Losses, inverter idle draw, battery reserve, cold weather, aging, and higher-than-expected loads can reduce runtime.
Should I use rated or usable watt-hours?
Use usable watt-hours when possible. Some batteries should not be fully discharged.
How do I estimate average load?
Measure it with a watt meter or estimate each device's watts times hours, then divide by the total period.
Can this estimate refrigerator runtime?
Yes for rough planning if you use average duty-cycle watts and confirm starting surge separately.
How do I increase runtime?
Reduce load, add battery capacity, improve efficiency, recharge during the outage, or run only essential devices.
Before you buy equipment
Use this result as a screening estimate, then compare it with rated running watts, surge watts, usable watt-hours, fuel availability, recharge time, maintenance, and installation requirements. A backup plan is safer when it starts with fewer essential loads and leaves reserve instead of assuming every rating can be used at the same time.