Battery Capacity Calculator

Undersizing a battery bank is a costly mistake — your system fails during the critical moment it was designed for. Oversizing by 50% wastes money and weight on capacity you will never use. The calculator for battery capacity finds the engineering-correct size by working backwards from your actual load, accounting for the depth of discharge limit that determines how much of the nominal capacity you can safely use, and the efficiency losses that mean you need to store more energy than you will actually consume.

The Battery Sizing Formula

Required battery capacity (in Ah at the system voltage V):

Required Ah = (Load power W × Runtime hours) / (Voltage V × DoD × Efficiency)

Or equivalently in Wh: Required Wh = (Load W × Runtime h) / (DoD × Efficiency)

For a 500W load running 4 hours from a 48V system with 80% DoD and 90% round-trip efficiency: Required Wh = (500 × 4) / (0.80 × 0.90) = 2,000 / 0.72 = 2,778 Wh; Required Ah = 2,778 / 48 = 57.9 Ah. The reserve factor adds further margin — a 20% reserve means ordering at least 57.9 × 1.20 = 69.5 Ah of nameplate capacity. Use this online calculator for any load and runtime combination. The battery life calculator solves the inverse: given a fixed battery, how long will it run?

Depth of Discharge: The Most Misunderstood Battery Parameter

Depth of discharge (DoD) is the fraction of nominal capacity that can be safely discharged before recharging is required — exceeding it degrades cycle life dramatically:

• Lead-acid (flooded): DoD 50% recommended maximum; 100% DoD reduces cycle life from 500+ cycles to under 200; designed for shallow cycling
• AGM/VRLA lead-acid: DoD 50–60%; slightly better than flooded but still depth-sensitive
• LiFePO4 (lithium iron phosphate): DoD 80–90% standard; 100% DoD acceptable for short periods; 2,000–6,000 cycles at 80% DoD — the dominant choice for solar and backup applications where cycle life justifies higher upfront cost
• NMC/NCA lithium (EV cells): typically used at 80–90% of nameplate (manufacturers reserve top and bottom 10% in BMS)

The energy consumption calculator and power and energy calculators provide complementary energy system design tools.

Round-Trip Efficiency: The Hidden Energy Loss

No battery stores and returns 100% of the energy put into it. Round-trip efficiency (RTE) is the ratio of energy out to energy in for one charge-discharge cycle:

• LiFePO4: 95–99% RTE — negligible losses; the primary reason lithium dominates applications where efficiency matters
• Lead-acid: 75–85% RTE; 15–25% of energy is lost as heat during charging
• Flow batteries (vanadium redox): 65–80% RTE; compensated by very long cycle life and independent power/energy scaling

In a solar storage system with lead-acid batteries, this efficiency loss means your solar panels must generate 15–25% more energy than your loads consume — a significant factor in panel sizing calculations.

Battery Aging: Planning for Capacity Fade

All batteries lose capacity over time. Lead-acid batteries reach end of life (defined as 80% of original capacity) in 3–7 years under typical solar cycling; LiFePO4 in 10–15+ years. For critical backup applications, size the battery to meet load requirements at end-of-life capacity, not nameplate — a system sized at 100% of nameplate capacity at installation will be undersized by 20% at end of life if no aging margin was included. Adding a 25% capacity margin over the calculated requirement effectively accounts for both aging and manufacturing tolerance variation.