Quick answer
Depth of Discharge (DoD) is the share of a battery's capacity that has been drawn out, as a percentage: a full battery is at 0% DoD and an empty one at 100%. State of charge (SoC) is the mirror image, the share still left.
A battery at 30% SoC is at 70% DoD. Deeper average discharges generally mean fewer cycles before the battery wears out.
Quick facts
The key facts about depth of discharge, with sources:
- Formula
- DoD = 100% − SoC
- Full battery
- 0% DoD, 100% SoC 1
- Empty battery
- 100% DoD, 0% SoC 1
- Shown in
- Battery apps as SoC, the percentage on screen 2
- Effect on life
- Deeper average discharge, shorter cycle life 1
- On current lithium data sheets
- Usually built into "usable capacity" rather than listed as a DoD figure 3
- Lithium calendar ageing
- Sitting at high SoC for long periods is a main driver of capacity fade 6
Key takeaways
- Depth of discharge is how much of the battery you have used. State of charge is how much is left.
- They always add up to 100%. The percent in your battery app is state of charge.
- Deeper cycles wear batteries faster. The effect was huge in lead-acid and is milder in lithium.
- Lithium home batteries hide DoD inside "usable capacity" and state life in warranty cycles.
- Your real choice is the backup reserve: how much charge to keep for outages.
What depth of discharge means, in plain words
Picture a fuel gauge. State of charge is where the needle sits. Depth of discharge is how far it has dropped from full. If the gauge shows 30%, you have used 70%.
NREL defines state of charge as the battery's present level of charge, from completely discharged to fully charged. It notes that this level decides what the battery can do at any given time 2.
DoD is also used per cycle. A "50% DoD cycle" means the battery went from full to half and back. Battery makers and labs use this to describe how hard a battery is worked.
Sources: [2]
SoC and DoD side by side
| State of charge (app shows) | Depth of discharge | Energy used | Energy left |
|---|---|---|---|
| 100% | 0% | 0 kWh | 10 kWh |
| 80% | 20% | 2 kWh | 8 kWh |
| 50% | 50% | 5 kWh | 5 kWh |
| 20% | 80% | 8 kWh | 2 kWh |
| 0% | 100% | 10 kWh | 0 kWh |
Why depth of discharge matters
Each cycle wears a battery a little, and deep cycles wear it more. Sandia National Laboratories put it plainly in a 2004 report: "The deeper batteries are discharged on average, the shorter their so-called cycle life."
NREL describes state of charge as the battery's present level of charge, from completely discharged to full, and notes that it governs what the battery can do at any moment. A battery sitting at 15% SoC on a summer evening cannot cover an outage that starts at 9 p.m.
The top end matters too. The DOE says that for lithium-ion, keeping a high state of charge for long stretches is one of the biggest causes of capacity fade over time 6. So both very deep and very full can cost life, in different ways.
The battery management system estimates SoC by tracking energy in and out and cell voltage, and it stops discharge before the cells are truly empty.
Ways DoD shows up on paper
| Term | Where you see it | What it tells you |
|---|---|---|
| Rated DoD | Older or lead-acid data sheets | How far the maker lets you drain it |
| Usable capacity | Current lithium data sheets 3 | DoD already applied, given in kWh |
| Cycle count at a DoD | Lab reports and some warranties | Wear at a set depth per cycle |
| Backup reserve | Your battery app | The SoC floor you choose for daily use |
| Storage SoC | Shipping and storage specs | Powerwall 3 is stored at 25% state of energy 4 |
The data that made DoD famous
DoD became a headline spec in the lead-acid era, when it changed cycle life dramatically. Sandia's figures for sealed lead-acid at 25°C (77°F):
| Depth of discharge per cycle | Typical cycles (sealed lead-acid) |
|---|---|
| 100% (full discharge) | 150–200 1 |
| 50% (partial) | 400–500 1 |
| 30% (shallow) | 1,000 and more 1 |
These numbers are for lead-acid, not lithium. They show the principle, not what a modern home battery will do. Lithium-ion batteries tolerate deep cycling far better, which is why their makers state cycle counts in the warranty instead.
Example: what a 20% backup reserve leaves for daily use
A worked example on a published capacity. The 20% reserve is an assumed setting, not a maker default.
| Step | Math | Result |
|---|---|---|
| Usable capacity | Enphase IQ Battery 5P 3 | 5.0 kWh |
| Reserve kept for outages | 5.0 × 20% | 1.0 kWh |
| Daily use (DoD stops at 80%) | 5.0 − 1.0 | 4.0 kWh per day |
Raise the reserve to 50% and daily use falls to 2.5 kWh. That is the trade-off you set in the app.
Where DoD hides on a lithium data sheet
Current lithium data sheets rarely print a DoD percentage. The battery management system enforces the limits, and the maker publishes a usable capacity that already sits inside them.
The Enphase IQ Battery 5P is a clear case. It lists 5.0 kWh total and 5.0 kWh usable, and the footnote explains that the usable figure contains a 2% safety-critical limit kept for long outages plus 3% held overnight to run the battery electronics.
Its warranty is written in cycles: more than 60% capacity for up to 15 years or 6,000 cycles, whichever comes first.
If a quote lists "100% DoD", read it as "the usable capacity is the whole published figure", then check what reserve the installer will set for backup.
Sources: [3]
Deep vs shallow daily cycling
Using more of the battery each day
- More daily savings from stored solar.
- Less unused capacity sitting idle.
- Fewer kWh needed for the same daily job.
Limits of the DoD idea
The classic DoD-versus-cycles curve comes from lead-acid. Sandia's numbers show the trend clearly, but they are not a guide to lithium 1. HyreSolar found no primary-source lithium curve to publish for home batteries.
SoC is also an estimate. The app's percent is calculated, not measured directly, so it can drift. Heat and age change how much energy each percent holds. DOE notes that high SoC held over time adds to calendar ageing, which a DoD figure alone does not capture 6.
How DoD ties to lifespan, warranty and cost
Warranties turn DoD into terms you can hold the maker to. Enphase covers more than 60% capacity for 15 years or 6,000 cycles, whichever comes first 3. Tesla covers 70% at 10 years with unlimited cycles for solar self-use, time-of-use and backup. For other uses, Tesla caps total output at 37.8 MWh 5.
NLR models a home battery over a 15-year life at about one cycle a day, with replacement possible in that time 7. A battery cycled deeply every day may hit a cycle cap before the year limit.
On cost, compare price per usable kWh. A battery that lets you use more of its pack gives more per dollar. HyreSolar does not print prices without dated quotes.
The SoC settings you will actually choose
Your installer sets these at turn-on, and you can change most in the app.
- Backup reserve. The SoC the battery will not go below in daily use, kept for outages. Higher reserve, less daily saving.
- Storm or outage-alert mode. Some systems charge to full ahead of forecast severe weather.
- Grid-charge limits. Whether the battery may refill from the grid, which can affect export-credit rules.
- Operating mode. Self-consumption, time-of-use or backup-only, each of which cycles the battery differently.
Good habits for battery charge levels
Let the battery do its job. Normal daily cycling within the maker's limits is what it is built for. If the system is backup-only and sits at 100% for months, ask your installer if a lower ceiling or reserve is an option, since DOE ties long high-SoC periods to fade 6.
Keep it in a mild spot. Enphase lists a best range of 0°C to 30°C (32°F to 86°F) 3. Check the app now and then for a SoC that jumps oddly, which can mean the gauge needs a recalibration by the installer.
Warning signs and when to call a pro
- SoC jumps from a high figure to near empty in minutes.
- The battery stops at a lower SoC than your reserve setting.
- It never charges past a set level on sunny days.
- An outage drained it far faster than the percent suggested.
- Call your installer. These can point to a cell or sensor problem only they should check.
Rules and standards
No US building code sets a depth of discharge for home batteries. The battery management system and the product listing handle it. Home units must be listed to UL 9540, with placement set by IRC R328 and NFPA 855 8.
Utility rules can affect how you charge, though. On Dominion Energy South Carolina's Solar Choice rider, storage counts only if it charges solely from onsite renewables 9. That can shape grid-charge settings. Check your own utility's rider.
DoD vs related battery terms
| Term | What it measures | Unit |
|---|---|---|
| Depth of discharge | Share of capacity used | % |
| State of charge | Share of capacity left | % |
| Capacity | Total energy the battery holds | kWh |
| Round-trip efficiency | Share of energy you get back | % |
| Cycle life | Cycles before notable fade 2 | Cycles |
Common mistakes
- Myth A higher DoD on a data sheet means a better battery.
- Reality It means the maker lets you use more of the pack. Compare usable kWh and the cycle warranty, which capture the same trade-off.
- Myth State of charge and capacity are the same thing.
- Reality Capacity is the size of the tank; SoC is how full it is right now. A battery that has lost capacity can still show 100% SoC.
- Myth Lithium batteries follow the lead-acid cycle chart.
- Reality Sandia's chart is for sealed lead-acid. Lithium handles deep cycles far better 1.
Reserve settings in SC, GA and VA
Where long outages are common, a higher reserve makes more sense. South Carolina customers averaged nearly 53 hours without power in 2024, the most in the US, mostly from Hurricane Helene 10.
A storm mode that charges to full before severe weather fits hurricane season. In Georgia and Virginia, check your utility's storage and export rules before allowing grid charging.
Choosing your reserve
- Outages are rare and short: a low reserve gives more daily savings.
- Outages are long or someone needs medical power: set a high reserve.
- Hurricane or ice-storm season: use storm mode if your system offers it.
- Next step: ask your installer what reserve they plan to set and why.
Next steps
- Battery sizing calculatorAllows for the reserve you plan to keep
- Home battery storage
- Battery chemistry and warranty terms compared
Questions about depth of discharge
What depth of discharge is safe for a lithium battery?
For a packaged home battery, whatever the battery management system allows. The maker has already set the limits inside the usable capacity, so running it down to the reserve you choose is normal use. Enphase, for example, keeps a 2% safety limit and 3% for electronics within its usable figure.
What does 80% depth of discharge mean?
That 80% of the battery's capacity has been used and 20% remains. On a 10 kWh battery that is 8 kWh out and 2 kWh left. Your app would show it as 20% state of charge, since the two numbers always add up to 100%.
Is state of charge the same as battery percentage?
Yes. The percentage in a battery app is its state of charge. Subtract it from 100 to get depth of discharge. Keep in mind it is an estimate the battery management system calculates, so it can drift a little until the system recalibrates.
Does charging to 100% damage a home battery?
Packaged home batteries manage their own upper limits, and many charge to full every day by design. The DOE notes that sitting at a high charge for long periods speeds capacity fade in lithium-ion. If the maker offers a lower ceiling for longevity, the app will show it.
What is the difference between DoD and capacity?
Capacity is how much energy the battery can hold, in kWh. DoD is how much of that you have used, in percent. A 10 kWh battery at 40% DoD has used 4 kWh. Capacity shrinks slowly with age; DoD changes every hour.
Does depth of discharge affect a battery warranty?
Indirectly. Many warranties cap cycles or total throughput, and deeper daily use spends those faster. Enphase covers 15 years or 6,000 cycles, whichever comes first. Tesla gives unlimited cycles for solar self-use, time-of-use and backup on Powerwall, with a throughput cap for other uses.
What backup reserve should I set?
It depends on how often you lose power and what must stay on. A higher reserve keeps more for outages but saves less each day. In areas with long storm outages, many owners keep more in reserve or use a storm mode. Ask your installer to show both options in kWh.
Why does my battery show 0% but still power the house?
Many batteries keep a hidden buffer below what the app calls zero, to protect the cells and run their own electronics. Enphase keeps 5% for those jobs inside its usable figure. Once that is used, the system shuts down and waits for solar to recharge it.
Sources
- Sandia National Laboratories, Temperature Effects on Sealed Lead Acid Batteries and Charging Techniques to Prolong Cycle Life (SAND2004-3149, June 2004), retrieved .
- NREL, Grid-Scale Battery Storage: Frequently Asked Questions (NREL/TP-6A20-74426, 2019), retrieved .
- 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 .
- US DOE Office of Electricity, Lithium-ion Batteries Technology Strategy Assessment (DOE/OE-0031, July 2023), 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 .
- Dominion Energy South Carolina, Residential Solar Choice rider (Rider to Rate 5), 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: DoD definition and lead-acid cycle figures from Sandia SAND2004-3149, labelled as lead-acid; SoC definition from NREL; high-SoC calendar ageing from the DOE lithium-ion assessment; lithium examples from the Enphase and Tesla data sheets and Tesla US warranty.
No lithium DoD-versus-cycle curve is published because none was found in a primary source. The reserve example uses an assumed setting.
Suggest a correction. We fix errors and say what changed.