Runtime Audit Suite
Forecast exactly how many hours your battery bank will sustain your loads before hitting critical discharge levels.
The Runtime Formula
Battery runtime is determined by how much energy is stored, how quickly it's consumed, and how deep you're allowed to discharge.
Runtime Calculation
Hours = (Ah × V × DoD) ÷ Watts Multiply battery capacity (Ah) by voltage to get Wh, apply DoD to get usable energy, then divide by load in watts to get runtime.
Usable Energy
Usable Wh = Ah × V × DoD Not all stored energy is accessible — DoD limits how deeply you can discharge without damaging the battery or shortening its lifespan.
How to Calculate Battery Runtime
Battery runtime tells you how long a battery bank can power a given load before needing a recharge. This calculation is critical for sizing backup power systems, off-grid solar storage, portable power stations, and UPS systems. The key variables are battery capacity (Ah), system voltage, Depth of Discharge (DoD), and the power draw of connected loads.
Why Peukert's Law Matters for Large Loads
Peukert's Law states that lead-acid batteries deliver less total energy when discharged at higher rates. A 100Ah battery discharged in 1 hour may actually deliver only 55–60Ah. This effect is minimal in lithium batteries but significant in lead-acid. For critical applications, use the C-rate adjusted capacity from your battery datasheet rather than the nominal Ah rating.
DoD Limits by Battery Chemistry
- Flooded Lead-Acid: Maximum safe DoD is 50%. Going deeper dramatically accelerates sulfation and plate degradation — expect 300–500 cycles at 50% DoD.
- AGM / Sealed Lead-Acid: Slightly better at 60–80% DoD depending on the manufacturer's rating. Maintenance-free but less tolerant of overcharging.
- Lithium Iron Phosphate (LiFePO4): Safe to 80–90% DoD with 2,000–5,000+ cycles at that depth. Best long-term value despite higher upfront cost.
- NMC / NCA Lithium: Used in EVs and some portable packs. Higher energy density than LiFePO4 but less thermally stable and typically limited to 80% DoD for longevity.
Sizing for Backup Power
- Define the Load: List all devices that must run during a power outage. Include their wattage and expected hours of operation.
- Calculate Total Wh: Sum all load × hours to get total watt-hours needed per outage event.
- Apply DoD Factor: Divide required Wh by your battery's DoD to get the required gross capacity.
- Convert to Ah: Divide gross Wh by system voltage to get required Ah.
- Add a Safety Buffer: Design for 80% of nominal capacity to account for aging — a battery that starts at 100% capacity will degrade to ~80% over its useful life.