Quick answer
Battery Capacity is the amount of energy a battery can store, given for home batteries in kilowatt-hours (kWh). Usable capacity is the part you can actually draw out; nominal or total capacity can be larger, because the manager keeps some charge in reserve to protect the cells.
Usable capacity is the figure to size and compare on.
Quick facts
The key facts about battery capacity, with sources:
- Unit
- Kilowatt-hours (kWh); utility systems use MWh 5
- Nominal vs usable
- Total stored energy vs the share the controls let you use
- Enphase IQ Battery 5P
- 5.0 kWh total, 5.0 kWh usable 1
- Tesla Powerwall 3
- 13.5 kWh nominal battery energy, AC 2
- FranklinWH aPower 2
- 15 kWh AC usable system energy per unit 4
- Not the same as
- Power rating (kW), which limits how much can run at once
- Changes with age
- Yes. Warranties guarantee a retained share, such as >60% at 15 years or 6,000 cycles 1
Key takeaways
- Capacity is how much energy a battery holds, in kWh. It decides how long it can run your loads.
- Compare batteries on usable kWh, not nominal or total kWh.
- You never get the full label back: reserve, conversion losses, heat and age all take a share.
- Capacity fades with years and cycles. Warranties promise a floor, such as 60% or 70%.
- Codes cap kWh per unit and per room, so more capacity can mean a different install spot.
Capacity is the size of the tank
NREL defines energy capacity as the maximum amount of stored energy, in kWh or MWh. A 10 kWh battery could, in principle, supply 1 kW for ten hours or 5 kW for two.
Capacity says nothing about how fast the energy can come out. That is the power rating, in kW. DOE's storage primer makes the point with a grid example: the same stored energy can deliver a lot of power briefly or less power for longer, which is why storage is quoted with both numbers.
The EIA uses the same split. Energy capacity is the total a system can store or release. Power capacity is the most it can deliver at once 8. A kilowatt-hour is the same unit on your power bill, which makes capacity easy to compare with your use.
How a battery reports its capacity
Inside a lithium-ion battery, charging moves lithium ions to one side and discharging moves them back 7. The number of ions that can make the trip sets the capacity.
The battery management system counts energy in and out and shows it as a percent, called state of charge. It also stops discharge before the cells are empty. Enphase builds that limit into its usable figure: a 2% safety reserve for long outages and 3% to run the electronics at night 1.
Nominal, total and usable capacity
| Term on the data sheet | Meaning | What to do with it |
|---|---|---|
| Nominal or rated capacity | The capacity the cells are rated for under test conditions. | Treat it as the upper bound. |
| Total capacity | All the energy in the pack, including any reserve. | Compare it with usable to see how much is held back. |
| Usable capacity | Energy available to your loads in normal operation. | Use this for sizing and price-per-kWh comparisons. |
| Backup reserve | A share you set in the app to keep for outages. | Lowers the energy available for daily savings, by your choice. |
| AC vs DC capacity | AC figures are measured after the inverter; DC figures at the cells. | AC is closer to what reaches your outlets. |
Reading three real data sheets
The three products below publish capacity in different ways. Neither is wrong; they are answering different questions.
| Battery | Published figure | Fine print |
|---|---|---|
| Enphase IQ Battery 5P | 5.0 kWh total, 5.0 kWh usable 1 | The usable figure includes a 2% safety-critical limit kept for long outages and a further 3% held overnight to power the battery electronics 1. |
| Tesla Powerwall 3 | 13.5 kWh nominal battery energy, AC 2 | Stated at 25°C at beginning of life, at 3.3 kW charge and discharge. The data sheet does not list a separate usable figure. |
| FranklinWH aPower 2 | 15 kWh AC usable per unit 4 | At beginning of life, 3 kW charge and discharge, 25°C (77°F) 4. |
Before comparing quotes, put all products on the same basis: ask each installer for the usable kWh at commissioning and the backup reserve they plan to set.
Example: what 13.5 kWh runs overnight
A worked example using Tesla's published figure. The load is an assumption for illustration, not a typical home.
| Step | Math | Result |
|---|---|---|
| Stored energy | Powerwall 3 nominal energy 2 | 13.5 kWh |
| Keep a 20% reserve for outages | 13.5 × 0.8 | 10.8 kWh for daily use |
| Steady evening load of 0.9 kW | 10.8 ÷ 0.9 | About 12 hours |
The 13.5 kWh is measured AC, so inverter losses on the way out are mostly already counted. A big load such as an AC unit would shorten the run a lot.
Where you will see a capacity number
- On the data sheet, as "usable energy", "nominal energy" or "total capacity".
- On the quote, often as a price per kWh. Ask which kWh figure it uses.
- In the app, as state of charge in percent and, on some systems, kWh left.
- In the warranty, as the share of rated capacity promised after a set number of years.
- On the permit plans, because codes cap kWh per unit and per location 10.
More capacity: what it gets you, and what it does not
What it does well
- Longer backup in an outage.
- Room to store all of a sunny day's surplus.
- A buffer that lets you keep a reserve and still save on the bill.
Why you get less than the label
- Reserve and limits. The battery management system stops discharge before the cells are empty; see depth of discharge.
- Conversion losses. Energy out is less than energy in; the AC round-trip efficiency is 90% on the Enphase and FranklinWH sheets 1 4.
- Temperature. Tesla notes performance may be de-rated above 40°C (104°F) 2.
- Ageing. Capacity fades with cycles and years. Warranties promise a floor, not the original number.
How capacity drives cost, lifespan and warranty
Capacity is one of the biggest cost levers on a quote. NLR's home battery cost model treats the battery pack as its own cost line, separate from the inverter, labor, permits and overhead 9.
More kWh means a bigger pack. HyreSolar does not print prices without dated quotes; ask for a price per usable kWh.
Warranties are written in capacity terms. Tesla promises Powerwall 3 keeps 70% of 13.5 kWh at 10 years 3.
Enphase promises more than 60% for 15 years or 6,000 cycles 1. FranklinWH lists 15 years or 60 MWh of total throughput 4.
Throughput is the total energy the battery has delivered over its life.
NLR models a home battery over a 15-year life at about one cycle a day, with fade that may need a replacement in that time 9.
How capacity is sized for a home
- List the loads the battery must carry and for how long: overnight self-use, a set outage length, or both.
- Multiply each load's watts by its hours and add them up to get kWh.
- Add headroom for the round-trip loss and any backup reserve you will keep.
- Check the power rating can start and run those loads together; a large tank with a small tap still trips.
- Have the installer confirm the total kWh fits the code limit for the chosen location.
The battery sizing calculator does this arithmetic with your loads.
Keeping capacity from fading early
The DOE says holding lithium-ion cells at a high charge for long periods is one of the main causes of capacity fade 7. If your system sits full for months waiting for an outage, ask your installer about a lower reserve.
Heat matters too. Enphase lists a best operating range of 0°C to 30°C (32°F to 86°F) 1. A shaded wall or a garage that stays mild helps. Check the usable kWh in the app once or twice a year and compare it with the warranty floor.
Warning signs of lost capacity
- The battery runs out hours earlier than it did last year, with the same loads.
- It never reaches 100% on clear days.
- The app shows a cell-balance or capacity fault.
- Call your installer and ask for a capacity test against the warranty. Do not open the unit.
Capacity limits in the building code
Codes regulate capacity directly. Under the 2021 IRC R328 and NFPA 855 Chapter 15, as UL summarises them, each unit may be up to 20 kWh.
The total may be up to 40 kWh in utility closets, basements and storage rooms. It may be up to 80 kWh in garages, accessory buildings, on outside walls or outdoors. Larger figures need UL 9540A test data 10.
That is why a maker's limit and the code limit can differ. FranklinWH allows up to 15 aPower 2 units, or 225 kWh, on one aGate 4. A home still has to fit that capacity into code-allowed spots. Your local adopted code edition decides; South Carolina enforces the 2021 codes 12.
Capacity vs related battery specs
| Spec | Unit | Question it answers |
|---|---|---|
| Capacity | kWh | How much energy is stored? |
| Power rating | kW | How much can run at once? |
| Round-trip efficiency | % | How much of what goes in comes back? |
| Depth of discharge | % | How far may it be drained? |
| Capacity retention | % of original | How much is left after years of use? |
| Throughput | MWh | How much total energy the warranty covers? |
Misconceptions about capacity
- Myth A bigger battery can run more things at once.
- Reality Capacity sets how long. Power rating sets how much at once.
- Myth Two 13.5 kWh batteries from different makers hold the same.
- Reality One may quote nominal and the other usable. Ask for usable kWh on both.
Capacity choices in SC, GA and VA
Outage length is the main local driver of how much capacity backup needs. South Carolina customers averaged nearly 53 hours without power in 2024, the most of any state, mostly from Hurricane Helene 11.
For daily savings, what your utility pays for exports matters most; SC rules differ by utility. Georgia Power and Virginia utilities set their own terms, so check with yours.
How much capacity is right for you?
- For daily savings only: aim to store a typical sunny day's surplus, no more.
- For backup: size to the loads you must keep on, times the outage length you plan for.
- For both: add the two and keep a reserve setting you are comfortable with.
- On every quote, ask for usable kWh, the reserve setting, and the warranty floor.
Next steps
- Battery sizing toolTurn your loads into kWh and kW
- Compare home battery storagePrices are on quotes; ask for $ per usable kWh
- What home battery data sheets do and do not say
- What drives solar and storage cost
Questions about battery capacity
What is the difference between kWh and kW on a battery?
kWh is energy, the size of the tank. kW is power, how fast it can be emptied. A battery needs enough of both: kWh for how long, kW for how much at once.
A 13.5 kWh battery rated 11.5 kW could, for example, run 1 kW for over 13 hours or its full 11.5 kW for a little over one hour.
How much battery capacity does a house need?
It depends on what you want to run and for how long. Add up the watt-hours of the loads you want covered, allow for round-trip losses and any reserve, and check the power rating. A calculator with your own loads beats any rule of thumb, because a fridge-and-lights plan and a whole-house plan differ hugely.
Does battery capacity decrease over time?
Yes. Lithium-ion batteries lose capacity with cycles and calendar age. Warranties state the floor: Enphase warrants more than 60% of capacity for up to 15 years or 6,000 cycles on the IQ Battery 5P, whichever comes first. Tesla warrants 70% of 13.5 kWh at 10 years on Powerwall 3.
Can I add capacity later?
Many systems allow it. Tesla supports up to four Powerwall 3 units together, and FranklinWH up to 15 aPower 2 units on one aGate. Adding units can change the permit, because residential codes cap total kWh by location. Older and newer units may also age differently, so ask your installer how they will be matched.
What does usable capacity mean?
Usable capacity is the energy you can actually draw for your home in normal use. It leaves out charge the battery keeps back to protect its cells or power its own electronics. Enphase, for instance, keeps 5% inside its 5.0 kWh usable figure for safety and overnight electronics. Use usable kWh to compare quotes.
Is nominal capacity the same as usable capacity?
Not always. Nominal capacity is the rated figure under test conditions. Usable capacity is what the controls let you use. Some makers publish both and they match; others publish only nominal. Ask the installer for the usable kWh at commissioning so different brands are compared fairly.
How is battery capacity measured?
In kilowatt-hours, the same unit as your power bill. Makers test it at set conditions, often 25°C at beginning of life and a set charge rate, which the data sheet footnotes. Real capacity in your garage can be a bit lower in heat or cold, and it shrinks slowly with age.
What is the most battery capacity allowed in a home?
Under the 2021 IRC R328 and NFPA 855 rules, each unit may be up to 20 kWh. Totals are capped at 40 kWh in utility rooms and basements, and 80 kWh in garages, on outside walls or outdoors. More is allowed only with UL 9540A test data. Your local code edition decides.
Sources
- Enphase Energy, IQ Battery 5P data sheet (DSH-00010-11.0-EN-2025-05-06), retrieved .
- Tesla, Powerwall 3 Datasheet (2024), retrieved .
- Tesla, Powerwall Limited Warranty (USA), effective 21 August 2023, retrieved .
- FranklinWH, aPower 2 data sheet (2025), retrieved .
- NREL, Grid-Scale Battery Storage: Frequently Asked Questions (NREL/TP-6A20-74426, 2019), retrieved .
- US DOE Solar Energy Technologies Office, Solar-Plus-Storage 101, retrieved .
- US DOE Office of Electricity, Lithium-ion Batteries Technology Strategy Assessment (DOE/OE-0031, July 2023), retrieved .
- US EIA, Energy storage for electricity generation, retrieved .
- National Laboratory of the Rockies (NLR, formerly NREL), Annual Technology Baseline 2024: Residential Battery Storage, retrieved .
- UL, 2021 Residential ESS Requirements (IRC R328 and NFPA 855 Chapter 15 summary, 2021), retrieved .
- US EIA, Today in Energy: Hurricanes in 2024 led to the most hours without power in 10 years, retrieved .
- SC Building Codes Council, code adoption, retrieved .
Expert review
Written by the HyreSolar Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Capacity figures and footnotes copied from the Enphase, Tesla and FranklinWH data sheets and the Tesla US warranty; definitions from NREL, EIA and DOE; code limits from UL's summary of 2021 IRC R328 / NFPA 855; lifetime and cost structure from the NLR ATB.
No capacity was inferred where a data sheet is silent. The runtime example uses an assumed load, labelled as such.
Suggest a correction. We fix errors and say what changed.