Quick answer
Usable Battery Capacity is the amount of stored energy, in kilowatt-hours (kWh), that a home battery will actually deliver to your house in normal use, after the maker’s built-in protective limits.
It is the number to compare quotes on. Each maker draws the line in a different place, and the backup reserve you set in the app takes a further share for outages.
Quick facts
The key facts about usable battery capacity, with sources:
Key takeaways
- Usable capacity is the share of a battery you can really spend on your home.
- Makers word it differently: Enphase says “usable”, Tesla says “nominal”, FranklinWH says “usable system energy”.
- Your backup reserve setting is a second, personal cut on top of the maker’s limits.
- Warranties measure capacity loss and total energy delivered, so read them next to the usable figure.
- Ask every installer for usable kWh at the AC output and the reserve they plan to set.
Usable capacity in plain words
A battery holds more energy than it lets you use. The cells need a small cushion at the bottom so they never go fully flat. The electronics also need a little power to stay awake. Usable capacity is what is left for your lights, fridge and outlets.
This page is about that line: where each maker draws it and what you can change. For the wider idea of how much a battery stores, and nominal vs total figures, see battery capacity. For how deep you choose to drain it each day, see depth of discharge.
The layers between the label and your outlets
- All energy in the cells → the maker keeps a protective floor (not shown to you)
- What is left → the published usable figure, often measured at the AC output
- Usable → minus the backup reserve you set → energy for daily bill savings
- Reserve → released to your home only when the grid goes down
- Over the years → usable kWh shrinks slowly as the cells age
How the line is drawn inside the battery
The battery management system tracks charge as a percent. The maker picks a low percent where it stops serving loads. Usable capacity is the energy between that point and full.
Enphase explains its own line in a technical note. Its usable figure includes a 2% “safety-critical limit”. That share protects the battery if a breaker trips while you are away and no grid power comes back. The battery waits at that low level instead of draining to a state it cannot recover from 2.
Enphase keeps a further 3% as a “battery shutdown level”. At that point the battery stops running the house. It uses the 3% to keep its electronics and status reports alive overnight, so it can start charging from solar at sunrise 2. In daily use the battery cycles between 100% and 5% 2.
How three makers state the figure
| Product | Data sheet wording | What the footnote adds |
|---|---|---|
| Enphase IQ Battery 5P | “Usable capacity 5.0 kWh” 1 | Includes a 2% safety-critical limit and 3% kept for the electronics at night 1 |
| Tesla Powerwall 3 | “Nominal Battery Energy 13.5 kWh AC” 3 | Values at 25°C (77°F), beginning of life, 3.3 kW charge and discharge 3 |
| FranklinWH aPower 2 | “Usable System Energy 15 kWh AC per unit” 5 | At beginning of life, 3 kW charge and discharge, 25°C 5 |
What the different words tell you
“Usable” means the maker has already taken its protective share out. “Nominal” is a rated figure; Tesla does not print a separate usable line for Powerwall 3, so the 13.5 kWh is the number its warranty measures against 4.
“AC” matters too. A figure measured at the AC output already counts the loss in the battery’s own inverter. A DC figure does not. Enphase lists a 90% AC round-trip efficiency and 96% DC 1, which shows how much the measuring point can move a number.
Test conditions matter last. Every sheet above states a temperature and a charge rate. A battery in a hot garage, or one pushed at full power, will not match the lab figure exactly.
Example: from usable kWh to spendable kWh
An example using the Enphase data sheet. The 20% backup reserve is a setting chosen for illustration, not a maker default.
| Step | Math | Result |
|---|---|---|
| Usable capacity per IQ Battery 5P | From the data sheet 1 | 5.0 kWh |
| Three units installed | 3 × 5.0 kWh | 15.0 kWh usable |
| Backup reserve set to 20% | 15.0 × 0.20 | 3.0 kWh held for outages |
| Left for daily savings | 15.0 − 3.0 | 12.0 kWh |
| Available in an outage | The full usable figure | Up to 15.0 kWh |
The maker’s 5% sits inside the 5.0 kWh already, so you do not subtract it again. The reserve is the only cut you control.
Where the usable figure shows up
- The data sheet: the spec table and its numbered footnotes.
- Your proposal: often as “storage size” or “kWh”, with no word on whether it is usable or nominal. Ask.
- The app: as percent charge and sometimes as kWh remaining.
- The warranty: as a capacity the battery must keep, and sometimes a throughput cap.
- The permit set: codes count kWh per unit and per room, as covered on the capacity page.
Why comparing on usable capacity helps
What it gets right
- It reflects energy you can actually spend.
- It stops a bigger “total” label from looking like more storage.
- It links straight to backup time and savings math.
- It is the basis many warranties use.
What it still misses
- Makers do not all publish it under the same name.
- It says nothing about how much power can flow at once.
- It is a beginning-of-life figure that drops with age.
- It ignores your own backup reserve setting.
Limits of the usable number
Usable kWh is a size, not a speed. A battery with lots of usable energy can still trip if you start too many big loads at once. That is set by the power rating.
It also does not tell you what happens in cold or heat. Data sheets state their test temperature for a reason. If a unit sits in a hot space, ask the installer what the maker says about output there.
Usable capacity, cost and warranty throughput
Price per usable kWh is a fairer yardstick than price per battery. We do not publish installed prices; see solar cost for what drives a quote.
Warranties hang off a capacity figure. Tesla warrants Powerwall 3 to keep 70% of 13.5 kWh at 10 years.
Daily solar use, time-of-use shifting and backup are unlimited cycles; any other use is capped at 37.8 MWh of throughput, measured at the battery’s AC output 4.
Throughput is the total energy a battery has delivered over its life.
FranklinWH lists a 15-year warranty or 60 MWh of aggregate throughput for aPower 2 5. Enphase warrants more than 60% capacity for up to 15 years or 6,000 cycles 1. Enphase counts one cycle as a full charge and discharge of the whole battery, reserves included 2.
To compare, divide warranted throughput by usable kWh. For FranklinWH that is 60,000 kWh ÷ 15 kWh = 4,000 full cycles’ worth. The NLR’s cost model assumes a 15-year life for home batteries 6.
How usable capacity is set during install
- The installer sizes storage from your loads and goals, using usable kWh.
- The design names each unit and the total usable kWh on the permit set.
- A licensed installer mounts and wires the system and passes the inspection.
- At commissioning, the installer sets the operating mode and the backup reserve in the app.
- You get the login and can see charge and reserve from then on.
All wiring is for a licensed electrician or installer. Changing the reserve is an app setting, not electrical work.
Backup reserve: the part you control
The backup reserve is a percent you set in the app. The battery will not dip below it for bill savings. It holds that energy for an outage.
A high reserve means more energy waiting for a blackout but less for daily savings. A low reserve flips that. Many owners raise it before a named storm and lower it after.
Check the reserve once or twice a year and after any software update. Also note the usable kWh your app reports when full; a slow fall over years is normal ageing.
Warning signs and when to call a pro
- A full battery shows far fewer kWh than the usable figure on the data sheet.
- The battery stops serving the home well above your reserve setting.
- The app shows a reserve you did not choose.
- Capacity drops quickly from one season to the next.
- Fault codes, swelling, hot spots or odd smells. Stay clear and call the installer.
Safety rule
Never open a battery cabinet or try to “unlock” hidden capacity. The protective floor is there to keep the cells safe. Only a licensed installer should service the system.
Rules that touch usable capacity
No US law sets how makers must word usable capacity. The figure comes from the maker’s own testing at the conditions in its footnotes.
Building codes count energy, not just units. UL’s summary of the 2021 IRC R328 and NFPA 855 sets limits of 20 kWh per unit, and 40 kWh or 80 kWh in total depending on where units sit 7. Your local adopted code edition decides, and your permit office checks it.
Sources: [7]
Usable capacity vs related terms
| Term | What it measures | Question it answers |
|---|---|---|
| Usable capacity | kWh the battery will deliver in normal use | How much can I spend? |
| Battery capacity | Stored energy, nominal or total | How big is the tank? |
| Depth of discharge | Share of capacity drained in a cycle | How deep do I go each day? |
| Power rating | kW the battery can supply at once | What can run at the same time? |
| Round-trip efficiency | Energy out ÷ energy in | How much is lost storing it? |
Misconceptions
- Myth You should subtract the maker’s reserve from the usable figure.
- Reality Not on Enphase: its 5.0 kWh already includes the 2% and 3% limits 1.
- Myth A larger nominal number always means more usable energy.
- Reality Only if both makers measure the same way. Ask for usable kWh at AC on both.
- Myth The backup reserve is wasted energy.
- Reality It is held, not lost. It runs your home when the grid fails.
- Myth Usable capacity is fixed for life.
- Reality It fades with age; warranties promise a floor such as 70% at 10 years 4.
Usable capacity in SC, GA and VA
Outage length is the local driver for how much usable energy to hold back. South Carolina customers averaged nearly 53 hours without power in 2024, the most of any state, largely from Hurricane Helene 8.
That is a reason to keep a higher reserve in storm season. Georgia and Virginia utilities set their own export and rate terms; check with yours before choosing how much usable energy to spend on daily savings.
Using the number on your quote
- Ask each installer: “What is the usable kWh at the AC output, per unit and in total?”
- Ask what backup reserve they plan to set, and why.
- Subtract the reserve to see what is left for daily savings.
- Read the warranty: retained capacity, years, and any throughput cap.
- Check the power rating covers the loads you want running together.
Next steps
- Battery sizing toolTurn your loads into a usable-kWh target
- Battery storageHow we match homes with storage installers
- Home battery chemistryHow LFP and NMC cells differ
Questions about usable battery capacity
What is usable capacity on a solar battery?
It is the energy, in kWh, the battery will actually deliver to your home in normal use. The maker holds back a small share to protect the cells and power the electronics. Enphase, for example, counts a 2% safety limit and a 3% electronics reserve inside its 5.0 kWh usable figure.
Is the Powerwall 3’s 13.5 kWh usable?
Tesla lists 13.5 kWh as “nominal battery energy, AC” and does not print a separate usable line on the Powerwall 3 data sheet. Its warranty measures retained capacity against that same 13.5 kWh. Ask your installer what the app shows as available when full and with your reserve set.
How much usable capacity does a FranklinWH aPower 2 have?
FranklinWH lists 15 kWh AC of usable system energy per aPower 2 unit, measured at beginning of life, at 3 kW charge and discharge and 25°C. Up to 15 units can sit on one aGate, though building codes limit how much energy can go in each part of a home.
Does the backup reserve reduce usable capacity?
It reduces what is available for daily savings, not what exists. The reserve is a percent you set in the app. The battery keeps that share for an outage and releases it when the grid goes down. A higher reserve means more backup and less bill savings.
What backup reserve should I set?
There is no single right number. It depends on how long your outages run, what you want to back up, and how your utility pays for stored energy. Many owners keep a modest reserve most of the year and raise it before a forecast storm. Your installer can set a starting point.
Why does my app show less than the usable capacity?
A few things cause it: the battery is not full, the temperature is outside the test range, the cells have aged, or the app shows energy above your reserve only. If a full battery reads far below the data sheet, and it is not cold or hot, ask your installer to check it.
What is warranty throughput?
Throughput is the total energy a battery delivers over its life, usually in megawatt-hours. Some warranties end at a throughput cap even if the years are not up. FranklinWH lists 60 MWh for aPower 2. Tesla caps some uses of Powerwall at 37.8 MWh, while daily solar, time-of-use and backup use are unlimited.
Should I compare batteries on usable or nominal capacity?
Usable, at the AC output, whenever you can. Nominal and total figures can include energy you never get to use. If one maker gives only a nominal figure, ask the installer for the energy the app reports when full, then compare like with like.
Can usable capacity be increased with a software update?
Sometimes a maker changes limits by firmware, but that is the maker’s call, made within its safety design. Never try to bypass the protective floor yourself. If you need more usable energy, the safe route is adding a unit where the system and the code allow it.
Sources
- Enphase Energy, IQ Battery 5P data sheet (DSH-00010-11.0-EN-2025-05-06), retrieved .
- Enphase Energy, Technical note: IQ Battery usable capacity (TEN-00026-2.0, November 2024), retrieved .
- Tesla, Powerwall 3 Datasheet (2024), retrieved .
- Tesla, Powerwall Limited Warranty (USA), retrieved .
- FranklinWH, aPower 2 data sheet (2025), retrieved .
- National Laboratory of the Rockies (NLR), Annual Technology Baseline 2024: Residential Battery Storage, retrieved .
- UL Solutions, ESS separation and maximum quantities: residential requirements (2021 IRC R328 / NFPA 855), retrieved .
- US EIA, Today in Energy: Hurricanes in 2024 led to the most hours without power in 10 years, 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: Usable-capacity wording and footnotes copied from the Enphase, Tesla and FranklinWH data sheets, the Enphase usable-capacity technical note and the Tesla US warranty; lifetime assumption from the NLR ATB; code limits from UL’s summary; outage data from EIA. The example uses a reserve chosen for illustration and labelled as such.
Suggest a correction. We fix errors and say what changed.