Lead-acid batteries remain the most affordable option for stationary storage, backup power, and vehicle starting. But their runtime calculations require more nuance than lithium — the Peukert effect, sulfation risk, and low DoD limits mean the number on the label tells only part of the story. This calculator gives you a baseline estimate with lead-acid-appropriate defaults.
The Peukert Effect in Lead-Acid Batteries
A 100Ah lead-acid battery rated at the 20-hour rate (C20) delivers 5A for 20 hours. Simple math says it should also deliver 50A for 2 hours or 100A for 1 hour. It does not. At 50A, you might get only 1.5 hours (75Ah delivered). At 100A, maybe 40 minutes (67Ah delivered).
The Peukert equation quantifies this: t = C / I^k, where t is time, C is the rated capacity at C20, I is the discharge current, and k is the Peukert exponent. For lead-acid batteries, k ranges from 1.1 (premium AGM) to 1.4 (cheap flooded). Higher k means more capacity loss at high discharge rates.
What causes this? At high currents, lead sulfate crystals form faster than they can dissolve back into the electrolyte. These crystals block the plate surface, preventing further chemical reaction. The energy is not lost — it is trapped in the plates and becomes available when the battery rests or is recharged. But in practice, if you need 100A right now, the Peukert effect means your battery delivers less total energy than the label suggests.
The runtime calculator above uses the simplified linear formula. For loads below C/5 (20A for a 100Ah battery), it is reasonably accurate. For higher loads, reduce the calculated runtime by 10-25% to account for Peukert losses.
Flooded Lead-Acid vs Sealed Lead-Acid
| Feature | Flooded (FLA) | Sealed (SLA/VRLA) |
|---|---|---|
| Maintenance | Add distilled water every 1-3 months | Maintenance-free |
| Gassing | Produces hydrogen when charging — needs ventilation | Minimal gassing (recombination) |
| Mounting | Must be upright — acid can spill | Any orientation (no free liquid) |
| Cost per Ah | Lowest ($0.40-0.80/Ah) | Higher ($1.50-3.00/Ah for AGM) |
| Cycle Life at 50% DoD | 600-1,000 cycles | 400-800 cycles (AGM), 700-1,200 (Gel) |
| Self-Discharge | 5-15% per month | 3-5% per month |
| Charge Acceptance | Moderate | Good (AGM can accept higher charge rates) |
Flooded batteries win on cost and cycle life but require maintenance and ventilation. Sealed (AGM or Gel) batteries trade cycle life for convenience. For off-grid cabins where you can maintain the batteries, flooded is the value play. For RVs, boats, and enclosed spaces, sealed is safer and more practical.
Frequently Asked Questions
Planning solar charging for your lead-acid bank? The <a href="/solar/solar-battery-charge-time-calculator">solar charge time calculator</a> accounts for the slow absorption phase that lead-acid requires.
Lead-acid batteries are proven, affordable, and widely available. They are also heavy, maintenance-intensive, and limited to 50% usable capacity. Run the numbers above with 50% DoD and 85% efficiency — those are the honest defaults for lead-acid. If the runtime falls short, you need a bigger bank or a different chemistry.
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Written and maintained by Dan Dadovic, Developer & Off-Grid Energy Enthusiast. On the energy side, Dan has hands-on experience with residential solar panel installation, DIY battery bank construction, off-grid power systems, and wind power — all from building and maintaining his own systems..
Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.