Control Panels
UPS and Battery Runtime
How long a battery will carry a control load, from capacity, voltage, depth of discharge, and inverter efficiency.
Results
- Estimated runtime
- 2.21 h
- Nominal battery energy
- 172.8Wh
- Usable energy after all factors
- 99.5Wh
- Discharge current
- 2.08A
- Discharge rate
- C/3.5
133 minutes
A fast discharge. Real capacity will be noticeably below the rating.
The arithmetic
- Nominal energy = 7.20 Ah x 24 V = 172.8 Wh
- Usable = 172.8 x 0.80 depth x 0.90 efficiency x 0.80 derate = 99.5 Wh
- Runtime = 99.5 Wh / 45.0 W = 2.21 h
What this does not account for
- Battery capacity ratings are stated at a slow discharge rate, usually 20 hours. At a faster rate the usable capacity is lower, an effect described by Peukert law that this simple calculation does not model.
- Cold reduces capacity substantially. A battery in an unheated outdoor enclosure in winter delivers far less than its rating.
- Capacity falls throughout the battery life. Size for end of life, not for a new battery, and test the actual runtime rather than assuming it.
- A runtime calculation is not a substitute for a load test. Test the real system before you rely on it.
How this is calculated
Runtime is battery capacity in amp-hours multiplied by nominal voltage, multiplied by the usable depth of discharge and the conversion efficiency, divided by the load in watts. Real runtime is shorter than this figure because capacity falls at high discharge rates, an effect described by Peukert law.
Formulas
Runtime = (Ah x V x DoD x efficiency x (1 - derate)) / load watts
- DoD — usable depth of discharge as a fraction
Assumptions built into this calculator
- Constant load throughout the discharge.
- Does not model Peukert capacity loss at high discharge rates.
- Does not model temperature effects beyond the flat derate you enter.
Frequently asked questions
- Why is my real runtime shorter than this?
- Three reasons usually: the capacity rating assumes a slow discharge and yours is faster, the battery has aged, or it is cold. Test the actual runtime under real conditions rather than trusting any calculation.
- What should I back up at a remote site?
- Often just the controller and the communication path, long enough to report the outage and ride through a brief interruption. Backing the whole panel including drives is a much larger and usually unnecessary battery.
Direct contact
Have a controls question?
Reach Eric Sullivan directly about anything on this site, a controls or automation topic, or one of his personal projects.