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There is no universal runtime. Estimate it by dividing the battery’s usable energy in kilowatt-hours (kWh) by the average combined load in kilowatts (kW): hours = usable kWh ÷ average kW. A battery powering a refrigerator, communications equipment, lights, phone charging and other modest essentials may last much of a day or longer. Add electric heating, air conditioning, water heating, cooking, pumps or vehicle charging, and the same battery may last only a few hours.
The calculation that gives you a starting answer
Use the energy that the battery can actually deliver, not only the capacity printed on its case:
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Estimated runtime (hours) = usable battery energy (kWh) ÷ average total load (kW)
If your appliance estimate is in watts, convert it first. A 500-watt load is 0.5 kW. A 10 kWh battery divided by 0.5 kW produces a theoretical 20-hour result.
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That 20-hour figure is arithmetic, not a guaranteed field result. It does not yet subtract the battery’s reserve, inverter and wiring losses, standby consumption, temperature effects or changes in appliance demand. Anker SOLIX uses this type of 10 kWh-at-500 W example and cautions that practical runtime will be lower.
A manufacturer-published 2026 example gives roughly 8–14 hours for a 10–13.5 kWh battery under moderate essential-load assumptions. That range is an illustration for those assumptions, not an industry average or promise for every installation.
Why two homes get different runtimes
Usable energy is less than nominal capacity
Product labels commonly state a nominal storage capacity. The amount available to your circuits is governed by the system’s depth-of-discharge limit, reserve setting, battery chemistry, temperature and control software. A system may retain a reserve so it can protect the battery, restart safely or respond to grid conditions. Ask for the model’s usable energy at the intended operating conditions.
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When comparing systems, record usable kWh and the reserve assumption together. Comparing one product’s usable figure with another product’s nominal figure makes the larger-looking battery appear better than it is.
Average load matters more than a single appliance rating
The load is the combined demand of every backed-up circuit operating at the same time. A refrigerator, internet equipment, lighting, phone chargers and medical equipment may be modest together; adding an electric range, heat pump, resistance heater or water heater can multiply the demand.
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Loads also change over time. A refrigerator compressor runs, stops and restarts. A furnace or heat pump cycles with outdoor temperature and thermostat settings. Runtime therefore depends on the average energy used over the outage, not simply the maximum wattage printed on one appliance.
Power output can stop an appliance even when energy remains
Battery capacity answers “how long?” Output ratings answer “can it run this combination at all?” Check both:
- Continuous output (kW): the running demand the inverter can supply indefinitely within its rating.
- Peak or surge output: the short-term power available when a motor, compressor or pump starts.
- Starting behavior: a well pump, refrigerator, boiler pump or air-conditioner compressor may briefly require substantially more power than its running figure.
A battery with plenty of kWh can still trip or disconnect if simultaneous running loads exceed its continuous rating or a motor-start surge exceeds its peak rating.
Illustrative runtime calculations
The table shows the formula with a 10 kWh usable-energy assumption. These are planning examples, not measured performance from a particular home.
| Average combined load | Calculation | Theoretical runtime before losses and reserve |
|---|---|---|
| 0.25 kW (250 W) | 10 ÷ 0.25 | 40 hours |
| 0.5 kW (500 W) | 10 ÷ 0.5 | 20 hours |
| 1.0 kW | 10 ÷ 1.0 | 10 hours |
| 2.0 kW | 10 ÷ 2.0 | 5 hours |
| 4.0 kW | 10 ÷ 4.0 | 2.5 hours |
Real runtime will be shorter or longer than a simple scenario suggests because the load is not perfectly steady and the usable-energy figure may already include, or exclude, different reserves and conversion assumptions. Keep the assumptions beside every estimate.
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How to estimate runtime for your own house
- Define “essential.” Decide which circuits must remain on during an outage. Priorities differ: one household may need a refrigerator and router; another may need a well pump, medical equipment, a boiler control or a sump pump.
- List the backed-up appliances. Include anything connected to the backup panel or transfer equipment, not just plug-in devices in the room where the battery is located.
- Measure or obtain representative energy use. For plug-connected appliances, a plug-in electricity monitor or watt-hour meter can record consumption over an appropriate interval. For hardwired, high-voltage or permanently installed equipment, use the equipment documentation or have a qualified electrician perform the measurement.
- Handle cycling equipment correctly. Record a refrigerator or freezer over many hours, ideally through normal compressor cycles. A brief running-watt reading can overstate or understate its energy use across a full outage.
- Estimate the concurrent average load. Add the average demand of appliances likely to operate during the same period. Separate occasional high loads, such as a microwave or pump, from continuous background loads, but include their expected operating time.
- Use usable kWh. Apply the manufacturer’s usable-energy value for the exact model and configuration. Account for the reserve level you intend to keep.
- Allow for losses and uncertainty. Inverter conversion, battery management, standby electronics, temperature and changing demand all reduce the result from ideal arithmetic. Leave operating margin rather than planning to exhaust the battery.
- Check output ratings. Confirm continuous kW for simultaneous running loads and peak kW for compressor and motor startup. An installer should verify the actual circuits and starting characteristics.
- Test the plan safely. Follow the manufacturer’s outage-test procedure or have the installer perform a controlled test. Do not disconnect utility wiring or defeat required interlocks.
Which appliances usually change the answer most?
| Load type | Why it affects planning | What to verify |
|---|---|---|
| Refrigerator or freezer | Cycles rather than running continuously; startup can create a surge. | Energy over a representative interval and compressor-start capability. |
| Router, modem, alarms and controls | Often operate continuously, so small demand accumulates over a long outage. | Actual wattage and whether they share the backed-up circuit. |
| Lighting and phone charging | Usually intermittent, but usage varies with household behavior. | Which rooms and devices are essential, plus expected hours of use. |
| Medical equipment | May be safety-critical and may require uninterrupted or carefully conditioned power. | Manufacturer requirements, battery runtime and a professional backup plan. |
| Well, sump and sewage pumps | Motor startup can exceed running demand; repeated cycles can consume significant energy. | Continuous and surge ratings, cycle frequency and automatic controls. |
| Heating and air conditioning | Can be a dominant energy load and may exceed the inverter’s output. | Whether the system backs up the equipment, its starting demand and outdoor-condition assumptions. |
| Electric water heating and cooking | High-power elements can shorten runtime rapidly. | Whether these circuits are intentionally included and how they are scheduled. |
| Electric-vehicle charging | Consumes power for long periods and can overwhelm a residential backup inverter. | Charging limits, load-management controls and whether charging is disabled during an outage. |
Can a home battery run a refrigerator?
Often, yes, provided the battery system is connected to the refrigerator’s circuit and its inverter can handle both the refrigerator’s running demand and compressor-start surge. The remaining question is duration.
Use measured refrigerator energy over a representative period whenever possible. A short test that captures only the compressor running can produce a misleading estimate because the compressor cycles. If measurement is unavailable, consult the refrigerator manual, nameplate and energy information, then include uncertainty in the calculation.
Also check what else starts at the same time. A refrigerator may run successfully by itself but fail to start when a pump or another motor is already using the inverter’s surge capacity.
What “one day” means in battery planning
Solar United Neighbors’ 2024 Battery Storage Guide states: “For most battery backup systems the standard length of run time is one day.” In context, this is a broad planning convention for selected backup loads, not a guarantee that every battery will power an entire home for 24 hours.
A one-day plan normally depends on choosing priority circuits, limiting large discretionary loads and sizing storage for the household’s expected demand. A home that includes electric heating, cooling, pumping or cooking on the backup panel may need substantially more storage, a way to shed loads, or a different outage strategy.
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The available guidance does not establish a robust, independently measured population statistic for the universal average runtime of home batteries. Vendor examples and general planning statements should not be presented as such a statistic.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Comparing two battery options
Use the exact model, firmware, battery count, inverter and backup configuration under consideration. Compare these attributes rather than capacity alone:
| Attribute | Question to ask | Why it matters |
|---|---|---|
| Usable energy (kWh) | How much energy can the loads receive after reserve and product limits? | Sets the potential runtime. |
| Continuous output (kW) | Can the system carry all intended loads at once? | Prevents overload during normal operation. |
| Peak output and duration | Can it start compressors and pumps, and for how long? | Addresses motor-start surges. |
| Backup configuration | Which circuits are actually isolated and powered during a grid outage? | A battery cannot power circuits that were not wired into the backup system. |
| Recharge capability | Can compatible solar or another source recharge it, and at what rate? | Determines whether a multi-day outage can be extended. |
| Published runtime assumptions | What load, state of charge, reserve, efficiency and environmental conditions produced the advertised hours? | Makes vendor estimates comparable and prevents overgeneralization. |
| Installation and expansion | Is it a fixed system or portable station, and are added batteries or panels approved for that configuration? | Compatibility and wiring determine what can safely be expanded. |
Can solar keep the battery running?
Compatible solar charging can extend backup, especially during a long outage, but it is conditional. Actual generation depends on sunlight, weather, array orientation, shading, charge-controller and inverter compatibility, and the system’s charging limits. Solar production may be lowest when household demand is highest or may stop overnight.
Do not subtract a full day’s expected solar production from your load estimate unless the system documentation and local conditions support that assumption. Treat solar as variable energy, retain a reserve, and plan for periods when generation is insufficient.
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- Using nominal kWh as usable energy: obtain the manufacturer’s usable figure and reserve settings.
- Dividing by appliance nameplate watts alone: use average energy over time, especially for cycling appliances.
- Ignoring startup surge: check peak output for every compressor, pump and motor.
- Assuming every household circuit is backed up: identify the actual backup panel, transfer equipment and priority circuits.
- Planning to run high-power heating, cooling or cooking continuously: calculate those loads separately and consider load-shedding controls or alternate fuel sources.
- Treating a vendor’s hours as a guarantee: carry the stated load and operating assumptions with the number.
- Assuming solar always restores what was consumed: model cloudy periods, nighttime and charging limits.
- Using a plug-in meter on hardwired equipment: obtain a professional measurement instead; ordinary plug-in meters are not suitable for high-voltage or permanently wired loads.
Installation and safety checks
A fixed home battery must be installed so it isolates from the utility grid during an outage. Backup wiring, transfer equipment, grounding, overcurrent protection and local permitting are site-specific. Have an appropriately qualified installer confirm that the selected circuits, inverter output and motor-start requirements match the battery system.
Keep the manufacturer’s operating limits, temperature requirements, clearances, inspection instructions and emergency shutdown procedure accessible. Never back-feed a house through an improvised cord or bypass a transfer device. For medical equipment, water systems or other safety-critical loads, develop a separate continuity plan rather than relying on an untested runtime estimate.
A practical outage checklist
- Write down the exact circuits and appliances the battery supplies.
- Record usable kWh, continuous kW, peak kW and reserve settings for the installed configuration.
- Measure representative energy use for refrigerators and other cycling loads.
- Identify loads that must be disabled first, such as vehicle charging or electric water heating.
- Confirm motor-start capability for pumps, compressors and similar equipment.
- Estimate runtime for normal, high-demand and worst-weather conditions.
- Verify how solar charging works when the grid is down and how much output is realistically available.
- Review the plan with the installer and test it using the manufacturer’s procedure.
Bottom line
A home battery’s outage runtime is a load-and-configuration calculation, not a fixed product characteristic. Start with usable kWh ÷ average kW, then account for reserve, conversion losses, cycling behavior, motor-start surges and the circuits actually connected. Modest essential loads can fit a roughly day-long plan in some systems; heavy heating, cooling, pumping or cooking loads can reduce the same battery to hours. Use the exact model’s documentation and a qualified load assessment before treating any estimate as a promise.
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