Battery State of Charge (SoC): How the % Is Worked Out

Quick answer

Battery State of Charge (SoC) is how full a battery is right now, shown as a percent of the energy it can hold. On a home battery it is the percentage in your app, and it is an estimate worked out from current, voltage and temperature.

No sensor reads it directly. The battery’s electronics calculate it many times a second, and the guess can drift and then be corrected.

Quick facts

The key facts about battery state of charge, with sources:

Shown as: A percent, 0% to 100%
How it is found: Estimated from voltage, current, temperature and age ¹
Main method: Coulomb counting: adding up current in and out over time ²
Opposite of: Depth of discharge (DoD = 100% − SoC)
Enphase low limits: 5% total: a 3% shutdown level plus a 2% safety-critical limit ³
Tesla’s term: “State of Energy (SOE)”, e.g. 25% at storage on the Powerwall 3 sheet ⁴

Key takeaways

SoC is the fuel gauge of a battery: the percent of energy left.
It is calculated, not measured. Small errors build up and get corrected later.
LFP batteries are hard to read by voltage alone, so their percent can jump near full or empty.
Your backup reserve is a SoC number: the battery stops daily use when it reaches it.
A sudden jump is usually a recalibration, not lost energy.

SoC is a fuel gauge, not a meter

Sandia National Laboratories defines SoC as the ratio between the remaining energy and the maximum energy a storage system can hold ¹. In plain words: 60% means about six-tenths of the tank is left.

For some storage, SoC can be read straight off. A pumped-hydro plant just checks its water level. A lithium battery cannot. Its SoC has to be estimated from voltage and current ¹.

This page covers how that estimate is made and how to read it. How deep you should drain a battery is on the depth of discharge page. The device that does the maths is the battery management system.

Sources: [1]

How the percent is worked out, step by step

Most home batteries use a method called coulomb counting, checked against voltage. A coulomb is a unit of electric charge.

The battery starts from a known point, such as “full” after a long charge.
A sensor measures the current flowing in or out, many times a second.
The electronics add up that current over time. Charge in raises the count; charge out lowers it.
The count is divided by the battery’s capacity to give a percent.
When the battery rests or reaches full or empty, the voltage is used to check and correct the count.

Researchers at Stanford note coulomb counting is widely used because it needs little computing power. But it drifts over time from small current-sensor errors, a wrong starting point and an out-of-date capacity figure ².

Sources: [2] [1]

Ways to estimate SoC

Methods described in the Sandia chapter and the Stanford study. A real system blends them.
Method	How it works	Weak spot
Coulomb counting	Adds up current in and out over time ²	Small errors pile up; needs a correct starting point
Voltage lookup	Reads the resting voltage and matches it to a chart	Needs a long rest; weak on LFP’s flat middle range ²
Model-based filters	A software “copy” of the battery, such as a Kalman filter, compares predicted and measured voltage	Less effective on LFP for the same flat-voltage reason ²
Combined	Counting most of the time, reset by voltage at rest or at full	Corrections can make the percent jump

Why the percentage jumps

Most home batteries now use LFP (lithium iron phosphate) cells. Their resting voltage barely changes across a wide middle band. The Stanford team found it is flat over roughly 30% to 80% SoC, and in some cells 20% to 90% ². In that band, voltage cannot tell 40% from 70% well.

LFP also has hysteresis. That means the voltage at a given SoC depends on whether the battery was just charging or discharging. The study found a gap between measured and true resting voltage even after 48 hours of rest ².

So the battery leans on counting through the middle. Errors build quietly. Near full or empty, where voltage moves fast, the electronics see the truth and snap the number to it.

That is the jump you notice, from 92% to 100% or from 12% to 7%. It is a correction of the display, not energy that appeared or vanished. See LFP batteries for more on the chemistry.

Sources: [2]

Example: SoC, kWh left and the reserve

An example using the Powerwall 3’s 13.5 kWh figure from Tesla’s data sheet. The 20% reserve and 55% reading are chosen for illustration.

Item	Math	Result
Battery energy	Tesla data sheet ⁴	13.5 kWh
App shows 55%	13.5 × 0.55	About 7.4 kWh stored
Backup reserve set to 20%	13.5 × 0.20	2.7 kWh kept for outages
Energy left for daily use	7.4 − 2.7	About 4.7 kWh
If the grid fails now	All 7.4 kWh is available to backed-up loads	Down to the maker’s floor

The kWh figures are approximate because SoC itself is an estimate, and some energy is lost converting it. How long 7.4 kWh lasts depends on your load; see battery backup duration.

Where you see SoC

The app: the big percent, often with a daily chart of charge rising and falling.
The reserve setting: a SoC floor for daily use, chosen by you.
Storm or outage modes: features that charge the battery to a high SoC ahead of bad weather, where offered.
Data sheets: test notes such as Tesla’s “State of Energy (SOE): 25% initial” for storage ⁴.
Your utility program: if you join one, its rules may say how low the battery can go.

What SoC is good for, and where it falls short

Useful for

Knowing roughly how much backup you have right now.
Setting a reserve that protects outage power.
Spotting a battery that no longer fills or empties as it used to.
Timing heavy loads for when the battery is fuller.

Falls short on

It is an estimate and can be off by a few points.
It is a percent of the current capacity, which shrinks with age.
It does not say how much power the battery can deliver.
Different apps define 0% differently.

What 0% and 100% really mean

Zero on the app is rarely empty cells.

Enphase explains that its battery stops serving the home at 5% of its own scale: a 3% “battery shutdown level” keeps the electronics alive overnight, and a further 2% “safety-critical limit” protects the battery if it sits for a long time with no power coming in ³.

Other makers draw the line in their own place and may hide it from the display. That is why a battery can read 0% yet still wake up and charge at sunrise. The usable capacity page compares how makers state that floor.

Sources: [3]

How SoC habits touch lifespan, warranty and cost

Holding a lithium battery at high SoC for long stretches wears it. The US Department of Energy names this as one of the biggest drivers of capacity fade for backup use ⁶. A battery that sits at 100% all year, waiting for a storm, ages faster than one that cycles in the middle.

Warranties are written in capacity, not SoC. Tesla, for example, promises Powerwall keeps 70% of 13.5 kWh at 10 years ⁵. Your SoC habits help decide how close to that floor your battery ends up. We do not quote battery prices; see solar cost for what drives a quote.

Sources: [6] [5]

How SoC limits are set at install

The maker fixes the hidden floor and ceiling in the battery’s firmware.
The installer commissions the system and confirms the app reads correctly.
The installer sets the operating mode and a starting backup reserve.
You can change the reserve later in the app; it is a setting, not wiring.
Firmware updates may change how SoC is estimated or shown.

Any electrical work on the system is for a licensed electrician or installer.

Good SoC habits

Raise the reserve before a forecast storm and lower it afterwards.
If the battery sits unused for months, ask your installer whether a lower resting SoC suits your model.
Let the battery reach full now and then if your app allows; it helps the gauge recalibrate.
Glance at the daily SoC chart weekly. A normal day shows a smooth rise and fall.

Warning signs and when to call a pro

SoC stuck at one number for days while the house uses power.
Large jumps every day, not just near full or empty.
The battery shuts off at a high SoC, well above your reserve.
It never reaches 100% on sunny days with little load.
Any fault code, heat, swelling or smell. Keep away and call the installer.

Safety rule

Do not try to “reset” a battery by switching breakers or forcing a full drain. Ask the installer or maker to recalibrate it through the app or firmware.

Standards behind SoC

No US rule tells makers how to calculate or show SoC. It is a design choice inside the battery management system, which Sandia describes as measuring voltage, current and temperature and estimating states such as SoC and state of health ¹.

The safety standard for home storage, UL 9540 (third edition, published by ULSE), tests the whole system, including its controls ⁷. Local building codes then decide where it can go. Your installer and permit office handle both.

Sources: [1] [7]

SoC vs related battery terms

Term	What it tells you	Changes how often
State of charge	How full the battery is right now	Every second
Depth of discharge	How much was drained in a cycle	Every cycle
State of health	Capacity left compared with new	Slowly, over years
Usable capacity	Energy you can spend when full	Fixed rating, fades with age
Backup reserve	The SoC kept for outages	When you change it

Misconceptions

Myth The percent is measured exactly.: Reality It is estimated from current, voltage and temperature, and it drifts ¹ ².
Myth A jump from 90% to 100% means free energy.: Reality It means the gauge corrected itself near full, where voltage gives a clear signal.
Myth 0% means the cells are empty.: Reality Makers keep a hidden floor; Enphase stops at 5% of its own scale ³.
Myth Keeping it at 100% is best for backup.: Reality It gives the most outage energy, but long stays at high SoC speed up ageing ⁶.

SoC planning in SC, GA and VA

Storm season is when SoC matters most in the Southeast. South Carolina customers averaged nearly 53 hours without power in 2024, mostly from Hurricane Helene ⁸. A battery that starts an outage at a high SoC buys more hours.

That is a reason to raise the reserve when a storm is forecast and lower it after. Georgia and Virginia utilities run their own rate and program rules; check yours before joining any program that controls your SoC.

Reading SoC in your own decisions

Treat the percent as close, not exact. Leave a margin.
Pick a reserve that matches your typical outage length.
Before buying, ask how the app shows 0% and whether a storm mode exists.
If the gauge behaves oddly, ask for a recalibration before assuming a fault.

Next steps

Battery sizing toolSee how much stored energy your loads need
Battery storageCompare storage options with vetted installers
Battery backup vs generatorWhen each makes sense

Questions about battery state of charge

What does state of charge mean on a solar battery?

It is how full the battery is, as a percent. 100% is full and the bottom of the scale is the point where the battery stops serving the home. Most apps show it as a big number with a daily chart. It is an estimate made by the battery’s electronics.

Why does my battery percentage jump suddenly?

Usually the gauge is correcting itself. Most home batteries track charge by adding up current, and small errors build up. Near full or empty, the voltage gives a clear signal and the percent snaps to the right value. LFP batteries do this more because their voltage is flat across the middle range.

How is state of charge calculated?

Mostly by coulomb counting. The battery measures current in and out and adds it up over time, starting from a known point such as full. It then checks the result against voltage when the battery rests or reaches full or empty. Temperature and age are also used to adjust the estimate.

Is SoC the same as depth of discharge?

They are two sides of one number. SoC is how full the battery is; depth of discharge is how much has been taken out. At 70% SoC, the depth of discharge is 30%. SoC is the live reading in your app; DoD is how makers describe cycles and wear.

What is a good state of charge to keep a home battery at?

For daily use, letting it cycle between your reserve and full is normal. For long stretches of no use, the DOE notes high SoC is a main cause of capacity fade, so ask your installer whether a lower resting level suits your model. Raise the level before storms when backup matters most.

Why does my battery say 0% but still turn on in the morning?

Because 0% on the app is not truly empty. Makers keep a hidden reserve so the electronics can stay alive and restart charging from solar. Enphase, for example, stops serving loads at 5% of its scale and keeps that energy for overnight electronics and protection.

How accurate is the battery percentage?

Close, but not exact. Accuracy depends on the sensors, the method and how recently the gauge was corrected. LFP batteries are harder to read by voltage, so they lean on counting, which drifts between corrections. Treat a reading as a good guide and leave a margin in an outage.

What is state of energy?

State of energy is a close cousin of SoC. It describes remaining energy in watt-hours as a share of the total, rather than remaining charge. Tesla uses the term on its Powerwall 3 data sheet. For a homeowner, the two read almost the same on the app.

Should I fully charge my battery before a storm?

Yes, if an outage is likely. A higher state of charge at the start means more hours of backup. Many apps let you raise the backup reserve or turn on a storm mode. After the storm passes, lower the reserve again so the battery does not sit full for months.

Sources

Sandia National Laboratories, DOE Energy Storage Handbook, Ch. 15: Energy Storage Management Systems (Nguyen et al.), retrieved October 8, 2026.
Che, Xu, Teodorescu, Hu and Onori, Enhanced SOC Estimation for LFP Batteries, ACS Energy Letters 10 (2025) 741–749, retrieved October 8, 2026.
Enphase Energy, Technical note: IQ Battery usable capacity (TEN-00026-2.0, November 2024), retrieved October 8, 2026.
Tesla, Powerwall 3 Datasheet (2024), retrieved October 8, 2026.
Tesla, Powerwall Limited Warranty (USA), retrieved October 8, 2026.
US DOE, Energy Storage Grand Challenge Technology Strategy Assessment: Lithium-ion Batteries (2023), retrieved October 8, 2026.
ULSE, ULSE publishes third edition of UL 9540, Energy Storage Systems and Equipment, retrieved October 8, 2026.
US EIA, Today in Energy: Hurricanes in 2024 led to the most hours without power in 10 years, retrieved October 8, 2026.

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: SoC definition and estimation inputs from the Sandia ESMS chapter; coulomb counting drift and LFP flat-voltage and hysteresis findings from the 2025 ACS Energy Letters study; low-SoC limits from the Enphase technical note; high-SoC ageing from DOE; warranty from Tesla. The example uses a reserve and reading chosen for illustration.

Suggest a correction. We fix errors and say what changed.