Battery Depth of Discharge Calculator

About the Battery Depth of Discharge Calculator

Depth of discharge (DoD) is the percentage of a battery's total capacity that can be utilized before recharging. It directly determines the usable energy you get from a battery. A 20 kWh battery with 50% DoD gives only 10 kWh of usable energy, while the same battery with 80% DoD provides 16 kWh.

Different battery technologies have very different recommended DoD limits. Lead-acid batteries should not exceed 50% DoD to maintain reasonable cycle life. Lithium iron phosphate (LiFePO4) batteries can safely reach 80–90% DoD with minimal impact on longevity. Going beyond the recommended DoD dramatically shortens battery life.

This calculator helps you understand the real usable energy for any battery by applying the appropriate DoD. This is essential for accurate solar storage sizing — the rated capacity on the label is not what you actually get to use.

By calculating this metric accurately, energy analysts gain actionable insights that inform equipment selection, system design, and operational strategies for maximum efficiency and savings.

When This Page Helps

Battery manufacturers advertise total capacity, but usable capacity is always less. Knowing the difference prevents undersizing your storage system and running out of power when you need it most.

How to Use the Inputs

1. Enter the total rated capacity of your battery in kWh.
2. Enter the recommended depth of discharge percentage.
3. Review the usable energy and reserved energy.
4. Use the usable energy figure for your system sizing calculations.

Example Calculation

Tips & Best Practices

• Lead-acid: use 50% DoD maximum for reasonable cycle life.
• Lithium (LiFePO4): 80% DoD is standard; some support 90%.
• Lithium (NMC): 80–90% DoD is typical for home batteries like Powerwall.
• Discharging regularly beyond recommended DoD can halve battery life.
• When comparing batteries, compare usable kWh, not just rated capacity.
• Factor in round-trip efficiency (85–95%) for a complete picture of energy delivered.

DoD by Battery Chemistry

Lead-acid (flooded): 50% DoD, 500–1,200 cycles. Lead-acid (AGM/gel): 50% DoD, 500–1,000 cycles. Lithium NMC: 80–90% DoD, 3,000–5,000 cycles. Lithium LFP: 80–90% DoD, 5,000–8,000 cycles. Saltwater: 100% DoD, 3,000+ cycles.

Calculating True Delivered Energy

For the most accurate storage estimate, multiply rated capacity by DoD and then by round-trip efficiency. A 10 kWh battery at 80% DoD and 90% efficiency delivers 10 × 0.80 × 0.90 = 7.2 kWh to your loads per cycle.

Backup Reserve Settings

Many inverters let you set a reserve percentage for outage backup. Setting a 20% reserve on a 13.5 kWh Powerwall means only 10.8 kWh is available for daily cycling, with 2.7 kWh held for emergencies. Adjust this setting based on your outage risk.

Frequently Asked Questions

• Why can't I use 100% of my battery?

  Fully discharging any battery causes chemical stress that degrades the electrodes. For lead-acid, deep discharges cause sulfation. For lithium, it increases thermal stress. Limiting discharge depth extends cycle life by 2–5 times.

• What DoD does the Tesla Powerwall use?

  The Tesla Powerwall 2 has 13.5 kWh usable out of approximately 14 kWh total, representing about 96% DoD. Tesla manages the depth of discharge internally, so the 13.5 kWh specification is the actual usable amount.

• How does DoD affect cycle life?

  Higher DoD shortens cycle life. A lead-acid battery at 50% DoD may last 1,200 cycles, but at 80% DoD only 400 cycles. Lithium batteries are more resilient: 80% DoD may yield 4,000+ cycles while 100% DoD yields 2,000–3,000 cycles.

• Is usable capacity the same as what I get?

  Almost. Usable capacity is limited by DoD, but round-trip efficiency further reduces delivered energy. A 16 kWh usable battery with 90% round-trip efficiency delivers about 14.4 kWh to your loads. The rest is lost as heat during charging and discharging.

• What's the best DoD for daily solar cycling?

  For daily cycling, 50–70% DoD maximizes battery longevity. Many home battery systems automatically limit DoD to 80–90% and manage the remaining capacity for backup reserve. If outage protection isn't critical, limiting to 70% DoD extends battery life.

• Does temperature affect DoD?

  Yes. Cold temperatures reduce available capacity — a battery in freezing conditions may only deliver 60–70% of its rated capacity. Heat accelerates degradation. Most home batteries have built-in thermal management and should be installed in temperature-controlled spaces.