Depth of Discharge (DoD) Explained
Depth of discharge is the share of a battery's nominal capacity you can safely use before it stops supplying power. It is the single most important number when comparing two batteries, because a 10kWh battery with 80% DoD delivers less usable energy than an 8.5kWh battery at 100% DoD. If you are sizing a system, read it alongside our solar battery storage guide and the LiFePO4 vs NMC comparison, since chemistry decides how much DoD you actually get.
Quick Answer
Depth of discharge (DoD) is the percentage of a battery's nominal capacity you can safely use. Usable capacity = nominal capacity multiplied by DoD. Modern LiFePO4 home batteries allow around 90 to 100% DoD, so a 10kWh nominal LFP battery gives roughly 9 to 10kWh usable. Older chemistries were limited to about 50 to 80% to protect lifespan. Always size a battery on usable kWh, not the headline nominal figure.
What is depth of discharge?
Depth of discharge (DoD) is the percentage of a battery's nominal (rated) capacity that you can draw out in normal use. A battery is rarely run all the way to empty: the battery management system (BMS) reserves a slice at the bottom to protect the cells. DoD describes how big the usable slice is.
The flip side of DoD is state of charge (SoC). If a battery is discharged to a 10% state of charge, it has been taken to 90% depth of discharge. A battery rated for 100% DoD can be drained fully without the BMS holding energy back, while a battery rated for 80% DoD keeps roughly a fifth in reserve at all times.
The core formula
Usable capacity = nominal capacity × depth of discharge
- 10kWh nominal × 100% DoD = 10kWh usable
- 10kWh nominal × 90% DoD = 9kWh usable
- 10kWh nominal × 80% DoD = 8kWh usable
- 10kWh nominal × 50% DoD = 5kWh usable
Two batteries with the same nominal kWh can deliver very different usable energy depending on rated DoD.
Usable vs nominal capacity: the number that matters
Manufacturers usually headline the nominal capacity because it is the bigger figure, but the number you actually live with is usable capacity. When you compare quotes, line them up on usable kWh. The table below shows how the same nominal rating produces different usable energy at different DoD levels.
| Nominal capacity | Rated DoD | Usable capacity | Energy held in reserve |
|---|---|---|---|
| 10 kWh | 100% | 10.0 kWh | 0 kWh |
| 10 kWh | 95% | 9.5 kWh | 0.5 kWh |
| 10 kWh | 90% | 9.0 kWh | 1.0 kWh |
| 10 kWh | 80% | 8.0 kWh | 2.0 kWh |
| 10 kWh | 50% | 5.0 kWh | 5.0 kWh |
Most modern LiFePO4 datasheets quote around 95 to 100% usable, so the gap between nominal and usable is small. With older or cheaper chemistries the gap can be large enough to change which battery is genuinely bigger. See the popular models in our solar battery brands roundup, which lists usable figures.
Indicative figures for illustration. Always confirm rated DoD and usable capacity on the manufacturer datasheet for the specific model quoted.
Depth of discharge by battery chemistry
How much DoD you get, and how it affects lifespan, depends on the cell chemistry. Deeper regular discharge can shorten cycle life on some chemistries, but lithium iron phosphate (LiFePO4, or LFP) tolerates near-full discharge with minimal degradation, which is why it dominates Irish home storage. For a full chemistry comparison, see our LiFePO4 vs NMC guide.
| Chemistry | Typical rated DoD | Effect of deep discharge | Common use |
|---|---|---|---|
| LiFePO4 (LFP) | ~90 to 100% | Tolerates near-full DoD with minimal degradation | Most current Irish home batteries |
| Lithium NMC | ~80 to 100% | Handles deep DoD well, slightly more sensitive than LFP | Some compact all-in-one units |
| Older lithium / hybrid | ~70 to 80% | Reserve kept to protect lifespan | Earlier home storage products |
| Lead-acid (legacy) | ~50% | Deep discharge sharply shortens life | Older off-grid and backup setups |
Cycle life is always quoted at a given DoD, so check the footnote on any cycle figure. A battery rated for thousands of cycles at 90% DoD is a stronger claim than the same cycle count measured at 50% DoD.
Typical ranges only. Exact rated DoD and cycle life vary by model and manufacturer.
Sizing a battery on usable capacity in Ireland
In Ireland the practical takeaway is simple: compare batteries on usable kWh, not the nominal headline, and most current LiFePO4 datasheets quote around 95 to 100% usable. DoD does not change the grants or the tax position, but it does change how much stored energy you have for evening use and during outages.
What DoD affects
- Evening self-use from solar stored during the day, the main reason Irish homes add a battery.
- Backup duration during an ESB Networks outage, if the system supports backup.
- How much you export under the Clean Export Guarantee (CEG) versus storing it for later.
What DoD does not change
- The SEAI grant, which applies to the solar PV portion and is capped at €1,800; batteries are not separately grant-funded.
- The 0% VAT rate on the supply and installation of domestic solar.
- Your CEG export tariff, which is set by your supplier, not by the battery.
Ireland's low-light, diffuse climate means batteries often charge from modest daytime generation, so squeezing full usable capacity out of each cycle matters more here than in sunnier regions.
Frequently Asked Questions About Depth of Discharge
What does depth of discharge mean for a solar battery?
Depth of discharge (DoD) is the percentage of a battery's nominal capacity you can safely use before the battery management system stops supplying power. Usable capacity equals nominal capacity multiplied by DoD, so a 10kWh nominal battery at 90% DoD gives around 9kWh usable.
What is the difference between usable and nominal capacity?
Nominal capacity is the total rated size of the battery. Usable capacity is the part you can actually draw out, which is nominal capacity multiplied by the rated depth of discharge. Most modern LiFePO4 batteries quote around 95 to 100% usable, so the gap is small, but with older chemistries it can be large.
What is a good depth of discharge for a home battery?
For modern lithium home batteries, a rated DoD of around 90 to 100% is typical and good. LiFePO4 batteries tolerate near-full discharge with minimal degradation. Older chemistries were limited to about 50 to 80% to protect lifespan, which left more energy unused in reserve.
Does deep discharging damage a LiFePO4 battery?
Lithium iron phosphate (LiFePO4) tolerates near-full depth of discharge with minimal degradation, which is why most LFP home batteries are rated for around 90 to 100% DoD and allow daily deep cycling. On some other chemistries, repeated deep discharge can shorten cycle life more noticeably.
How does depth of discharge affect battery sizing in Ireland?
When sizing a battery, compare usable kWh rather than the nominal headline figure, because two batteries with the same nominal rating can deliver different usable energy. Most current LiFePO4 datasheets sold in Ireland quote around 95 to 100% usable. DoD does not change the SEAI grant, the 0% VAT rate, or your CEG export tariff.
Related Guides
LiFePO4 vs NMC Batteries
Battery chemistry compared on safety, lifespan, and value.
Battery Storage
Solar battery costs, sizing, popular models, and payback analysis.
Solar Battery Brands
Best home battery brands in Ireland ranked by capacity, warranty, and compatibility.
AC vs DC Coupled Batteries
Retrofit vs new-install battery coupling and efficiency.
Sources
- US Department of Energy: Batteries explainer, energy.gov
- NREL: Battery storage performance and degradation, nrel.gov
- SEAI: Solar Electricity Grant, seai.ie
Last updated: June 2026
Fact-checked by John Rooney, Solar Energy Editor. Editorial policy
John Rooney is the founder of Solar Info and has been covering the Irish solar energy market since 2023. He fact-checks all content against official SEAI data and maintains relationships with SEAI-registered installers across Ireland.
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