For batteries, kW measures how much power the system can deliver at once, while kWh measures how much energy it can store or supply over time. The kW rating determines which loads can run together; usable kWh helps determine how long they can run. A suitable battery system needs enough continuous power, adequate short-duration starting capability, and sufficient available energy—not simply the largest capacity number.
Battery Ratings That Determine What Runs and for How Long
| Battery specification | What it tells you | What to compare it with | What it does not establish |
|---|---|---|---|
| Continuous output, kW | How much power the system can sustain under its rated conditions | Total running power of loads operating simultaneously | How many hours those loads can run |
| Peak or surge output, kW or starting-current rating | Short-duration load support, subject to a stated time limit or motor-start specification | Starting requirements plus other loads already running | Permission to operate continuously at the peak rating |
| Usable energy, kWh | Energy available within the manufacturer’s defined operating limits and measurement boundary | Required load energy over the intended backup period | Whether the inverter can supply the load’s instantaneous power |
| Nominal or total energy, kWh | A stated energy capacity whose definition must be checked | Manufacturer-defined usable capacity and applicable restrictions | Energy guaranteed to reach your AC loads |
| Charging power, kW | How quickly energy can enter the system under specified conditions | Available charging source and required recharge time | Discharge power or stored energy capacity |
EIA distinguishes storage power capacity from energy capacity, while manufacturer specifications separately identify continuous output, starting capability, charging limits, and energy-rating conditions. Read these as separate limits rather than interchangeable measures.
kW Is the Delivery Rate; kWh Is the Energy Quantity
Think of a battery as a water tank. The amount of water available resembles its energy capacity in kWh, while the rate at which water can flow through the outlet resembles its power capability in kW. A large tank with a restricted outlet can supply a small flow for a long time, but it cannot necessarily supply a large flow on demand.
For a constant load:
[
E_{\text{load}} = P_{\text{load}} \times t
]
where:
- (E_{\text{load}}) is energy consumed, in kWh.
- (P_{\text{load}}) is load power, in kW.
- (t) is operating time, in hours.
For a hypothetical 2 kW load running for 3 hours:
[
E_{\text{load}} = 2\text{ kW} \times 3\text{ h} = 6\text{ kWh}
]
This requires 6 kWh delivered to the load. It does not automatically mean a battery labeled “6 kWh” will provide that runtime: the label’s energy boundary, available charge, operating limits, and losses still matter. Storage systems consume more electricity during charging than they subsequently deliver.
The “h” in kWh means hours multiplied by power. It does not mean “kilowatts per hour,” and a 10 kWh battery does not necessarily have a 10 kW output rating.
Continuous Power and Surge Power Solve Different Problems
Continuous output is the rating to compare with loads that remain on together. Peak output is temporary support, so its duration matters as much as its magnitude.
For a real North American example, Enphase lists the IQ Battery 5P, SKU IQBATTERY-5P-1P-NA, with 5.0 kWh of usable energy and 3.84 kW of continuous output. Its published peak output is 7.68 kW for 3 seconds or 6.14 kW for 10 seconds; those are not continuous operating ratings. These specifications were checked on October 12, 2026.
A hypothetical 5 kW steady load exceeds that unit’s listed 3.84 kW continuous output even though it falls below the 3-second peak rating. Conversely, a brief starting event must fit the applicable surge limit and duration while accounting for other loads already running.
Common pitfall: A peak-power headline is not a continuous-power rating. Compare the load’s starting requirement with the manufacturer’s time-qualified surge specification or motor-start guidance—not just the largest number on the product page.
Manufacturers may express motor-start capability as current rather than kW. Tesla’s 2025 U.S. Powerwall 3 datasheet, for example, lists load-start capability in locked-rotor amps, or LRA. Do not treat an LRA rating as an interchangeable kW rating; use the manufacturer’s load-compatibility guidance for the relevant voltage and equipment.
Read the Battery Label Before Calculating Runtime
Separate nominal energy from usable energy
“Nominal,” “total,” and “usable” are not interchangeable labels. Check how the manufacturer defines the energy value, including whether it represents DC battery energy or AC-delivered energy.
Tesla’s 2025 U.S. Powerwall 3 datasheet labels its energy rating as 13.5 kWh AC and specifies test conditions of 25°C, beginning of life, and 3.3 kW charge/discharge power. Those conditions help explain what the number represents; it is not an unconditional promise for every load and environment.
If a published usable-energy value already excludes a protected reserve, do not subtract that same reserve again. If it already represents AC-delivered energy, do not automatically apply another DC-to-AC conversion loss.
Check output limits separately from capacity
Look for continuous discharge power, surge duration, charging power, output voltage, and the operating mode to which each rating applies.
Charging and discharging limits can differ. The same Tesla datasheet lists configurable AC output ratings up to 11.5 kW but a maximum continuous AC charging power of 5 kW for Powerwall 3 without Expansion units. Neither value replaces the energy rating.
Read the conditions attached to the numbers
Check temperature limits, rating footnotes, system configuration, and required backup equipment. Tesla identifies possible performance derating above 40°C, while Enphase states that IQ Battery 5P backup capability requires an IQ System Controller 3/3G. A battery’s kW and kWh figures alone therefore do not establish a complete backup system’s capabilities.
Size Both Power and Energy: A Hand-Checkable Example
The following example is hypothetical, not a product specification or a recommended battery size.
Assume a backup system must serve a constant 2.0 kW AC load for 4 hours, with no solar contribution or other charging during the outage.
| Input | Assumed value | Meaning |
|---|---|---|
| Simultaneous AC load | 2.0 kW | Constant running load |
| Required backup duration | 4 hours | Time without charging |
| Nominal DC battery energy | 10.0 kWh | Starting energy-capacity assumption |
| Starting state of charge | 100% | Battery begins fully charged |
| Minimum retained state of charge | 20% | Only 80% of nominal energy is available for this scenario |
| DC-to-AC discharge efficiency | 90% | Assumed one-way conversion efficiency |
| Separate auxiliary consumption | Excluded | Must be added if not covered by the efficiency assumption |
- Calculate the required load energy.
[
E_{\text{required,AC}}
= 2.0\text{ kW} \times 4\text{ h}
= 8.0\text{ kWh}
] - Calculate the available DC energy above the retained charge level.
[
E_{\text{available,DC}}
= 10.0\text{ kWh} \times (1.00 – 0.20)
= 8.0\text{ kWh}
] - Apply the assumed one-way discharge efficiency.
[
E_{\text{available,AC}}
= 8.0\text{ kWh} \times 0.90
= 7.2\text{ kWh}
] - Estimate runtime at the constant AC load.
[
t
= \frac{7.2\text{ kWh}}{2.0\text{ kW}}
= 3.6\text{ h}
]That is 3 hours and 36 minutes—short of the 4-hour target.
- Calculate the nominal DC energy needed under the same assumptions.
[
E_{\text{nominal,DC}}
= \frac{8.0\text{ kWh}}{0.80 \times 0.90}
= \frac{8.0}{0.72}
\approx 11.1\text{ kWh}
]This is the calculated energy requirement before additional allowances. The system must also sustain the 2.0 kW running load and satisfy any starting requirements.
The example demonstrates why you must size both power and energy. A system with enough kWh can still fail the continuous-output check; a system with enough kW can still run out of energy too soon.
Use average power for energy, but simultaneous power for output limits
For loads that cycle or operate for different periods, calculate energy separately:
[
E_{\text{total}} = \sum (P_i \times t_i)
]
For example, a hypothetical 1.0 kW load operating for 2 hours plus a 0.5 kW load operating for 4 hours consumes:
[
E_{\text{total}}
= (1.0 \times 2) + (0.5 \times 4)
= 4.0\text{ kWh}
]
If they overlap, their simultaneous running power is 1.5 kW. Dividing 4.0 kWh by the 4-hour period gives a 1.0 kW average, but that average would understate the output needed during overlap.
Do not substitute round-trip efficiency for discharge efficiency
Round-trip efficiency includes charging and discharging losses. A runtime calculation beginning with stored DC energy instead needs an appropriate discharge-side efficiency assumption.
Enphase lists different DC and AC round-trip efficiencies for the IQ Battery 5P. Those values describe specific energy paths; they should not automatically be used as one-way discharge multipliers in a runtime calculation.
For the corresponding calculations, use the Battery Capacity Calculator and Battery Runtime Calculator with the same energy boundary and assumptions.
Battery Specification and Runtime Checklist
- Record the continuous output rating separately from the peak rating.
- Record the peak-rating duration or manufacturer-defined motor-start capability.
- Add the running power of loads that may operate simultaneously.
- Check starting requirements while accounting for other operating loads.
- Identify whether the energy rating is nominal, total, or usable.
- Confirm whether the stated energy is measured on the DC or AC side.
- Use the expected starting charge and the applicable retained-charge limit.
- Apply only losses and reserves not already included in the energy rating.
- Calculate energy from actual operating hours or justified duty-cycle assumptions.
- Check temperature conditions, output voltage, operating mode, and required backup equipment.
These calculations support load planning and product comparison; they do not establish installation approval or guaranteed runtime. For a premises-connected system, final equipment selection and installation must follow manufacturer instructions, the locally adopted National Electrical Code, and the authority having jurisdiction’s requirements.