A battery’s advertised capacity can describe more energy than its system makes available to your devices. To compare batteries fairly, identify what each capacity rating means, check the limits that affect usable energy, and match the result to the loads you actually need to run.
What nominal and usable capacity mean
Nominal capacity is the battery’s stated or rated storage capacity under defined conditions. It may be shown in kilowatt-hours (kWh) or watt-hours (Wh). One kWh equals 1,000 Wh. A label such as “5 kWh battery” often refers to nominal capacity, but the product specification should confirm what the number represents.
Usable capacity is the energy the system allows you to draw for normal operation. It is usually lower than nominal capacity because the battery management system (BMS) may reserve energy at the top or bottom of its charge range. Those reserves help protect the battery, support reliable operation, or keep the system within its designed limits.
For example, if a battery has 10 kWh nominal capacity and its manufacturer specifies 9 kWh usable, the system makes 9 kWh available under the stated conditions. The remaining 1 kWh should not be treated as energy you can routinely use. Do not infer usable capacity from nominal capacity alone unless the manufacturer provides a clear, applicable rating.
Capacity is not the same as power. Capacity describes an amount of energy; power, usually measured in watts (W) or kilowatts (kW), describes how quickly energy can be delivered. A battery can have enough energy for a long-running load but still be unable to start or operate that load if its continuous or surge-power limits are too low.
Find comparable specifications
Before comparing products, collect the relevant figures from the product datasheet or manual. A product page may round values or omit test conditions, so use the detailed manufacturer documentation when possible.
Record:
- Nominal capacity, with units.
- Usable capacity, with units and the conditions attached to it.
- Maximum depth of discharge (DoD), if specified.
- Continuous output power and short-duration surge power.
- Whether the figure refers to the battery cells, the complete battery system, or energy delivered at the AC outlets.
- Any temperature, charge-rate, inverter, or operating-mode conditions that qualify the rating.
- Whether the battery is modular and whether adding units changes the stated usable capacity.
Use the same units before doing arithmetic. If one product is listed in kWh and another in Wh, convert them first: 2,400 Wh is 2.4 kWh. Avoid comparing a cell-level or DC-side figure with an AC-output figure as if they were measured at the same point in the system.
| Specification | What it describes | How to use it |
|---|---|---|
| Nominal capacity | Rated stored energy under defined conditions | Useful for understanding the battery’s overall size |
| Usable capacity | Energy the system permits you to draw under stated conditions | Best starting point for estimating runtime |
| DoD | Portion of capacity allowed to be discharged | A limit or operating parameter; check how the manufacturer defines it |
| Continuous power | Output the system can sustain | Compare with the combined watts of loads running at once |
| Surge power | Higher output allowed briefly | Check against startup demands and the specified surge duration |
If a listing only gives nominal capacity, look for the usable figure in the datasheet or ask the manufacturer or seller where it is documented. If you cannot confirm it, mark the comparison as incomplete rather than assuming the entire nominal rating is available.
Understand depth of discharge and reserves
Depth of discharge is commonly written as a percentage. It describes how much of a battery’s capacity has been discharged, according to the product’s stated reference. A system that allows a 90% DoD may permit more of its nominal energy to be used than one that allows 80%, but DoD by itself does not settle the comparison.
A rough estimate sometimes used for planning is:
estimated usable capacity = nominal capacity × allowed DoD
For a 10 kWh battery with an allowed DoD of 90%, that calculation gives 9 kWh. Treat it as an estimate only. The manufacturer’s usable-capacity rating takes precedence because the product may define DoD differently, include reserves, or account for other system limits.
Do not assume that every battery should be discharged to the maximum permitted level every day. The manufacturer may specify operating ranges, reserve settings, or care instructions that affect normal use and warranty conditions. Follow the manual for settings and battery care, particularly when configuring a home storage system or connecting compatible equipment.
Also distinguish a fixed reserve from a user-configurable backup reserve. A home battery may keep some energy aside for an outage or other chosen purpose. That reserve can reduce the amount available for routine self-consumption without changing the battery’s underlying capacity. When comparing systems, note whether published usable capacity is measured before or after a configurable reserve is applied.
Estimate the energy your loads need
Start with the appliances and devices you want to power, not the battery size you hope to buy. List each load, its wattage, and how long it runs during the period you want to cover. The basic estimate is:
energy needed (Wh) = load power (W) × runtime (hours)
For instance, a 60 W device used for 5 hours needs about 300 Wh at that device’s stated power. For multiple devices, calculate each one and add the results. If a device’s label gives only volts and amps, watts can often be estimated as volts × amps for a simple DC rating. For AC equipment, use its rated input power where available, and account for startup requirements separately.
A practical planning list might include:
- Essential loads during an outage, such as selected lights, communications equipment, or a refrigerator.
- Portable devices for a trip or work session.
- The hours each item must operate, including whether it runs continuously or cycles on and off.
- Loads that will run at the same time, which matter for output-power limits.
- A margin for uncertainty in actual consumption and operating conditions.
Nameplate power is a starting point, not always a precise prediction of energy use. Some devices cycle, change power with their operating mode, or draw more during startup. For a household load, use the appliance manual or a reliable energy measurement when available, then allow for variation. Do not count on a battery’s full usable rating reaching every AC appliance: inverter and conversion losses can reduce delivered energy, and the amount depends on system design and operating conditions.
Compare batteries on an equal basis
Suppose two products advertise the following ratings: Battery A has 5 kWh nominal and 4.5 kWh usable; Battery B has 5 kWh nominal and 4.0 kWh usable. Battery A offers more stated usable energy, but that fact alone does not establish which is the better fit. Check output power, surge capability, system compatibility, operating conditions, and what each rating includes.
A useful comparison process is:
- Compare usable capacity first. Use the manufacturer’s stated figure for the complete system at the same measurement point, if possible.
- Check the basis of the rating. Confirm whether it is DC energy from the battery or energy delivered through the inverter’s AC outlets.
- Check power limits. Add the watts of loads you expect to run together and compare that total with continuous output. Check motor or compressor startup against surge ratings and duration.
- Check configuration. Verify that the listed capacity applies to the number of battery modules and accessories included in the configuration you are considering.
- Check conditions. Note temperature range, charge or discharge rate, and any other conditions that could make the published capacity differ from your use.
- Compare the usable fraction only as context. Divide usable capacity by nominal capacity to see how much of the nominal rating is available under the stated definition. Do not use that fraction as a substitute for the usable-energy figure.
When two manufacturers state capacity differently, write down the unresolved difference. A clearly qualified rating is more useful than a larger headline number whose meaning is unclear.
Account for AC and DC conversion losses
Battery cells store energy as direct current (DC). Many household appliances use alternating current (AC), so a system with an inverter changes the energy’s form before it reaches those appliances. The conversion uses some energy, and the system may also consume power for controls, cooling, or standby operation.
That means a stated usable capacity may not equal the energy delivered to AC loads. Check how the manufacturer measures it: some specifications describe energy available from the battery, while others may state output energy after conversion under particular conditions. Read the definition and test conditions before applying the figure to an appliance runtime estimate.
If the manufacturer provides an efficiency figure, it may help with an estimate, but use it only when it applies to the same operating mode and measurement path. A simple calculation is:
estimated AC energy = usable DC energy × applicable conversion efficiency
For example, if 4 kWh of usable DC energy is paired with an applicable 90% conversion efficiency, the estimate is 3.6 kWh delivered as AC energy. This is illustrative arithmetic, not a guarantee of a product’s output. Real delivered energy can vary with load, temperature, inverter behavior, and system settings. If no relevant efficiency information is published, keep the estimate conservative and avoid presenting an exact runtime as certain.
Apply the comparison to different uses
Solar planning
For a solar-plus-storage system, estimate the energy you want to shift from daytime generation to later use, or the essential loads you want covered during a grid outage. Compare that need with usable capacity at the relevant output point. Then confirm that the battery, inverter, solar equipment, and controls are designed to work together. Compatibility includes voltage and communication requirements, approved system configurations, and the inverter’s charging and discharge limits; check the equipment documentation rather than relying on a capacity match alone.
A larger battery does not create more solar generation. It can store more energy only when there is enough available charging energy and the system can accept it. Consider when your solar system generates, when household demand occurs, and whether the battery’s charge-power limits suit that pattern.
Portable power
For a portable power station, add the energy needs of the devices you plan to use and compare the total with the station’s usable capacity and outlet-specific limits. Confirm whether the advertised capacity refers to the internal battery or expected AC output. Check each outlet’s maximum power, the number of outlets available at once, and any restrictions that apply when charging and discharging simultaneously.
Household energy use
For routine household backup, separate essential circuits from loads that are convenient but energy-intensive. A few low-power devices can run for a long time, while heating, cooling, cooking, and other high-demand equipment can use substantial energy or exceed an inverter’s power limit. Estimate both total energy over the backup period and the highest combined power at any one time.
Troubleshoot a runtime that seems too short
If a battery reaches its reserve sooner than expected, first check the system’s displayed state of charge and reserve setting. A backup reserve or minimum-charge setting may intentionally keep energy unavailable for routine loads. Then compare actual device consumption with your original estimate; a cycling appliance or startup surge can make a simple wattage-times-hours calculation inaccurate.
Check the manual and system display for temperature limits, power limits, error messages, and operating modes. Cold or hot conditions, high discharge rates, aging, and conversion losses can affect available energy, but the size and behavior of those effects depend on the equipment. Avoid opening the battery or changing protective settings to chase a capacity number. Follow manufacturer instructions and use qualified service support where the product documentation calls for it.
If published figures still appear inconsistent, compare the exact model numbers, configuration, units, and measurement definitions. A nominal figure, a usable figure, and an AC-output figure can all be correct while describing different stages of the system. Keep those distinctions visible in your comparison so your runtime estimate rests on the energy your equipment can actually deliver.