Depth of discharge (DoD) is how much of the battery’s stored energy you use before recharging. It directly affects usable capacity and cycle life. This guide explains DoD and how to use it when sizing your solar battery bank.
What Is DoD?
DoD = percentage of the battery’s capacity that you discharge.
- 100% DoD = fully empty (avoid for most batteries).
- 80% DoD = you use 80% of the capacity, leave 20% in the battery.
- 50% DoD = you use half; common recommendation for lead-acid.
Usable capacity = Rated capacity × DoD.
So a 10 kWh battery at 80% DoD gives you 8 kWh usable. When sizing, you need enough usable energy for your days of autonomy; then Battery capacity = Usable ÷ DoD.
DoD by Chemistry
- LiFePO4: Often 80–90% DoD in normal use. Long cycle life even at high DoD. You need less total capacity for the same usable energy.
- Lead-acid (flooded, AGM, gel): Typically 50% DoD recommended to extend life. Deeper cycles shorten life. So you need roughly twice the lead-acid capacity for the same usable energy as LiFePO4.
- Sodium-ion: Check the datasheet; often similar to LiFePO4 (high usable DoD).
Why It Matters for Sizing
Battery capacity (Wh) = Daily use (Wh) × Days of autonomy ÷ DoD
Lower DoD (e.g. 50%) → larger battery for the same backup. Higher DoD (e.g. 85%) → smaller battery. That’s why LiFePO4 vs lead-acid comparison shows LiFePO4 needing fewer batteries for the same runtime.
Summary
Use the manufacturer’s recommended DoD for your chemistry when sizing. LiFePO4: 80–90%; lead-acid: ~50%. Plug that into the formula above or the WattSizing calculator to get the right battery capacity.
