Battery Runtime Calculator

How is Battery Runtime Calculated?

Battery runtime depends on the stored energy (Wh) and the rate of energy consumption (W). The basic formula Runtime = Energy / Power gives a theoretical maximum, but real-world runtime is always lower due to efficiency losses, voltage sag, and the Peukert effect.

Peukert's law describes how battery capacity decreases at higher discharge rates. At low current draws, a battery delivers close to its rated Ah capacity. At high current draws, internal resistance causes voltage to drop faster and the battery reaches cutoff voltage sooner, effectively reducing usable capacity.

Discharge efficiency varies by chemistry: Li-ion achieves 90-95% efficiency at moderate C-rates, lead-acid 80-85%, and alkaline primary cells can drop below 70% at high loads.

For systems with variable loads (e.g., an inverter powering intermittent appliances), calculate runtime using the average power draw, not the peak. However, ensure the battery can deliver the peak current without excessive voltage sag — a 12V lead-acid battery may sag to 10.5V under high load, causing sensitive electronics to shut down even though energy remains.

Formula: Energy (Wh) = Capacity (Ah) × Voltage (V) Runtime (h) = Energy (Wh) × Efficiency / Load Power (W) Current Draw (A) = Load Power (W) / Voltage (V)

Example Calculation

A 12V 100Ah lead-acid battery powers a 200W load at 85% efficiency. Energy = 100 × 12 = 1200 Wh. Runtime = 1200 × 0.85 / 200 = 5.1 hours. Current draw = 200 / 12 = 16.7A (C/6 rate).

When to Use This Calculator

• Sizing a battery bank for off-grid solar, RV, or marine applications to meet a specific runtime target for known loads
• Estimating UPS backup duration for critical equipment during power outages
• Determining whether an existing battery can support a new accessory load (e.g., adding a winch, refrigerator, or communications equipment)
• Comparing battery options (lead-acid vs. Li-ion) by calculating runtime at the same load with each chemistry's typical efficiency

Common Mistakes to Avoid

• Using the nameplate Ah capacity without applying depth-of-discharge limits — lead-acid batteries should only be discharged to 50% to preserve cycle life, so a 100Ah battery provides only 50Ah of usable capacity
• Ignoring inverter efficiency — when powering AC loads from a DC battery, the inverter consumes 10-15% of energy; this must be factored into the total load
• Assuming battery capacity is constant across temperatures — at 0°C, lead-acid capacity drops ~30% and Li-ion drops ~10-20%; size batteries for the worst-case operating temperature
• Forgetting the Peukert effect for lead-acid — a 100Ah battery rated at C/20 (5A) may deliver only 70-80Ah at C/3 (33A); Li-ion is much less affected

How to Interpret Results

• Runtime shown assumes constant load at the specified efficiency; apply a depth-of-discharge limit before using as a sizing guide — 50% DoD for lead-acid, 80–90% for Li-ion
• Current Draw is the steady-state current at the given load — verify this is below the battery's maximum continuous discharge current rating to avoid voltage sag and overheating
• Energy (Wh) is theoretical stored energy; actual usable energy equals Energy × depth-of-discharge limit × efficiency factor combined
• At high discharge rates (>C/5 for lead-acid), real runtime will be shorter than calculated due to the Peukert effect — apply a 0.8–0.9 correction factor for lead-acid at C/3 or higher

Related Standards & References

• IEC 61960-3 — Secondary lithium cells and batteries for portable applications; defines capacity test methods and rating conventions
• IEEE 485 — Recommended Practice for Sizing Lead-Acid Batteries for Stationary Applications; provides Peukert correction methodology
• SAE J537 — Storage Batteries for automotive applications; defines testing, rating, and service life procedures
• IEC 62619 — Safety requirements for secondary lithium cells and batteries used in industrial applications, including discharge rate limits

Frequently Asked Questions

Why does my battery last shorter than the calculated runtime?

Several factors reduce actual runtime: aging (reduced capacity), temperature effects (capacity drops in cold), inverter losses if using AC devices, Peukert effect at high discharge rates, and the cutoff voltage limit which prevents using the last 10-20% of charge for battery health.

Does voltage affect runtime?

Higher voltage batteries can deliver the same power at lower current (P=V×I), which reduces I²R losses and the Peukert effect. This is why 48V systems are more efficient than 12V systems for the same load — less current means less wasted energy as heat.

How do I account for multiple loads on the same battery?

Sum all load powers to get total wattage. If loads are intermittent, calculate the duty-cycle-weighted average power. For example, a 100W light running continuously and a 500W pump running 20% of the time gives an average load of 100 + (500 × 0.2) = 200W.