Electrical calculator
Battery Runtime Calculator
Turn a battery’s voltage and amp hour label into a practical watt hour estimate, then allow for losses and depth of discharge.
Sage’s guided lesson
Let’s understand the result before using it
Estimate battery runtime from voltage, amp hour capacity, load power, efficiency, and usable capacity, with real-world limitations. I’ll show you what the numbers mean, how to check the math yourself, and where real-world conditions can change the answer.
Start with the system
What this calculator is actually doing
Nominal battery energy is voltage multiplied by amp hours. Real usable energy is lower because batteries have discharge limits, conversion losses, voltage sag, temperature effects, aging, and capacity that depends on discharge rate.
Convert the battery rating to watt hours, then apply realistic usable capacity and conversion efficiency before dividing by load.
Now for the arithmetic
How to do the math yourself
Nominal watt hours = volts × amp hours. Usable watt hours = nominal watt hours × efficiency × usable capacity. Runtime hours = usable watt hours ÷ load watts.
Worked example
A 12 V, 100 Ah battery powers a 300 W load.Nominal energy = 12 × 100 = 1,200 WhAt 90% conversion efficiency and 80% usable capacity: 1,200 × 0.90 × 0.80 = 864 WhRuntime ≈ 864 ÷ 300 = 2.88 hoursTreat the result as an estimate and avoid false minute level precision when load and battery behavior vary.
I know this is off topic, but this reminds me of a joke I heard when teaching this to one of my interns, What do you call a spider who just won an award? A web-inner. Yeah, I don’t see the connection either, but I think it is cute!
Before you build, wire, mix, or run it
Tips, shortcuts, and mistakes to avoid
- Use load wattage measured under normal operation and consider startup surges separately.
- Choose usable capacity based on chemistry, required battery life, and the manufacturer’s discharge guidance.
- Cold temperature, age, high discharge rate, inverter idle draw, and cable loss can reduce runtime substantially.
Battery chemistry, age, temperature, discharge rate, inverter cutoff, startup surges, and load cycling can shorten runtime.
How we got here
A little history and background
Rechargeable lead acid batteries enabled early portable and backup electrical systems. Nickel and lithium chemistries later improved energy density, but battery labels still require interpretation because rated capacity is measured under specified conditions.
What the result does not tell you
This is an energy estimate. It does not model discharge curves, Peukert effect in detail, cell balancing, cutoff voltage, startup surge, thermal limits, battery management systems, or charger compatibility.
Frequently asked questions
Why is runtime not simply amp hours divided by amps?
That shortcut can work for a direct DC load, but watt hours handle differing voltages and conversion losses more clearly.
What does usable capacity mean?
It is the fraction of nominal capacity you plan to remove before stopping, based on chemistry and life goals.
Why did my real battery run for less time?
Age, temperature, discharge rate, voltage cutoff, inverter loss, cable loss, and an inaccurate load estimate can all reduce runtime.
That wraps up the lesson
Class is over. It is officially cheese fries time.
You now know more about how the Battery Runtime Calculator actually works, not just which buttons to press. That earns cheese fries, Sage’s favorite way to celebrate a finished lesson.
Use the result within its limits
Batteries can deliver destructive fault current and may vent, burn, or explode. Use correct fusing, wiring, charging, ventilation, polarity, and manufacturer approved protection.