Battery Runtime Calculator (Peukert's Law)
Use this tool to estimate real-world battery runtime at a given discharge current.
What this Peukert calculator does
This calculator estimates how long a battery can run under a specific load by applying Peukert's Law. The key idea is simple: battery capacity is not perfectly fixed. As current draw increases, effective capacity usually drops. That means runtime can be much shorter than a plain Ah ÷ A estimate suggests.
For lead-acid systems in off-grid power, marine applications, RV electrical systems, and backup setups, this gives a more realistic runtime expectation.
Peukert's Law formula
t = H × (Ir / I)k
Where:
- t = estimated discharge time (hours)
- H = rated hour period (such as 20 hours)
- Ir = rated current = C / H
- I = actual load current (A)
- k = Peukert exponent (battery dependent)
When k = 1, the battery behaves ideally and runtime scales linearly. In real life, most batteries have k > 1, which causes faster capacity loss at higher current.
How to use the calculator
- Enter rated Ah: from your battery label/spec sheet.
- Enter rating hours: usually 20 for lead-acid capacity ratings (C20).
- Enter k value: use manufacturer data when possible.
- Enter expected current draw: average current from your load.
- Set usable depth of discharge: 50% to 80% is common for long lead-acid life.
The tool returns full-discharge runtime plus usable runtime based on your depth-of-discharge setting.
Typical Peukert exponent values
| Battery Type | Typical k Range | Notes |
|---|---|---|
| Flooded lead-acid | 1.20 to 1.35 | Higher current has noticeable impact on usable capacity. |
| AGM lead-acid | 1.10 to 1.20 | Often performs better than flooded under high draw. |
| Gel lead-acid | 1.10 to 1.25 | Check datasheet; charging/discharge limits matter. |
| Lithium (LiFePO4) | ~1.03 to 1.08 | Peukert effect is smaller but still present in some models. |
Example calculation
Scenario
You have a 100 Ah battery rated at 20 hours, Peukert exponent of 1.20, and a 25 A load.
- Rated current: 100/20 = 5 A
- Runtime: 20 × (5/25)1.20 ≈ 2.9 hours
- At 80% usable DoD: 2.9 × 0.8 ≈ 2.3 hours
If you used a simple linear method (100 Ah ÷ 25 A = 4 hours), you would overestimate runtime significantly.
Practical tips for better battery planning
- Use average current draw: pulsing loads can distort runtime expectations.
- Account for temperature: cold batteries usually deliver less usable capacity.
- Avoid deep discharge when possible: especially for lead-acid longevity.
- Verify with real tests: calculators are models, not exact guarantees.
- Size with margin: design for reserve runtime to avoid unexpected cutoffs.
Limitations of the model
Peukert's Law is highly useful, but it is still a simplification. Real battery behavior is influenced by temperature, age, internal resistance, resting state, charge acceptance, and cutoff voltage in your inverter or load controller.
For mission-critical systems, combine this estimate with manufacturer discharge curves and real-world testing at expected operating conditions.
FAQ
Does this only work for lead-acid batteries?
It is most commonly used for lead-acid chemistry, where the effect is strongest. Lithium batteries can also be modeled, but the exponent is closer to 1 and the result is usually less dramatic.
Where do I find the Peukert exponent?
Check your battery datasheet first. If unavailable, use a typical value for your chemistry and then refine after measuring real runtime.
Why include depth of discharge?
Because many systems should not use 100% of capacity. Setting usable DoD gives a safer, more practical runtime estimate for daily operation.
Bottom line
A Peukert calculator helps you move from optimistic label numbers to realistic runtime planning. If you rely on battery power for backup, travel, marine, or off-grid use, this is one of the most important corrections you can make.