Battery Runtime Calculator
Enter your battery's capacity and the power or current of the connected load to estimate how long the battery will last. The calculator accounts for efficiency losses and depth of discharge (DoD) to give you a realistic runtime estimate, complete with a projected end time based on your current local time.
Combine Depth of Discharge limit with inverter inefficiency. e.g. 85%.
Understanding Battery Runtime and Depth of Discharge (DoD)
Our battery runtime calculator estimates how long your battery will power a specific load before it needs recharging. Whether you're sizing a battery bank for a solar off-grid system, finding out how long a UPS will keep your computer running during an outage, or just calculating drone flight times, understanding the relationship between capacity, load, and efficiency is crucial.
One of the most important factors in this calculation is the Depth of Discharge (DoD). DoD refers to the percentage of the battery's total capacity that you actually use. While it might seem intuitive to use 100% of a battery's rated capacity, doing so regularly can severely reduce the lifespan of certain battery chemistries. By incorporating DoD and other efficiency losses (like inverter inefficiency) into the calculation, you get a much more realistic estimate of your safe, usable runtime.
How battery runtime is calculated
The basic principle of calculating battery runtime involves dividing the usable energy capacity of the battery by the power being drawn by the load. Here are the full formulas based on whether you're using Watt-hours (Wh) or Amp-hours (Ah):
Runtime (h) = (Capacity (Wh) × Efficiency % ÷ 100) ÷ Load (W)
Runtime (h) = (Capacity (Ah) × Efficiency % ÷ 100) ÷ Load Current (A)
Worked example: Let's calculate the runtime for a standard 100Ah 12V battery powering a 60W load. We'll assume this is a LiFePO4 battery, safely discharged to 80% DoD.
- Convert load to Amps: 60W ÷ 12V = 5 Amps.
- Calculate usable capacity: 100Ah × 80% = 80Ah.
- Calculate runtime: 80Ah ÷ 5A = 16 hours.
If this were a lead-acid battery restricted to 50% DoD to prevent damage, the usable capacity would only be 50Ah, resulting in a runtime of 10 hours for the same 60W load.
Depth of Discharge (DoD) and why it matters
Not all batteries are created equal when it comes to how deeply they can be discharged. The chemistry of the battery dictates its recommended Depth of Discharge.
- Lead-Acid (AGM, Gel, Flooded): These batteries are very sensitive to deep discharges. It is universally recommended not to discharge them below 50% DoD. Pushing them to 80% or 100% DoD will drastically shorten their cycle life from several hundred cycles to less than a hundred.
- Lithium-ion / LiFePO4 (Lithium Iron Phosphate): These modern batteries are highly resilient and can safely be discharged to 80%, 90%, or even 100% DoD without significant damage. A LiFePO4 battery discharged to 80% DoD can often last for 3,000 to 5,000 cycles, making them far more economical over the long term despite a higher upfront cost.
Battery Type Reference Guide
Use this reference table to understand the typical characteristics and safe runtimes of various common battery types when subjected to a 100W load.
| Battery Type | Nominal Voltage | Typical Capacity | Recommended DoD | Safe Runtime at 100W load |
|---|---|---|---|---|
| 18650 Li-ion Cell | 3.7V | 3.5Ah (13Wh) | 80% | ~6 minutes |
| Smartphone Battery | 3.8V | 3.0Ah (11.4Wh) | 80% | ~5.5 minutes |
| Typical Power Bank | 3.7V (Internal) | 10,000mAh (37Wh) | 85% | ~19 minutes |
| Lead-Acid Car Battery | 12V | 100Ah (1200Wh) | 50% | ~6 hours |
| LiFePO4 Solar Battery | 12V | 100Ah (1200Wh) | 80-100% | ~9.6 - 12 hours |
Frequently Asked Questions
How long will a 100Ah 12V battery last?
It depends on the load and depth of discharge. If you draw 5 amps (60W at 12V) and use 80% depth of discharge (typical for LiFePO4), you have 80Ah of usable capacity. At a 5-amp draw, the calculation is 80Ah / 5A = 16 hours of runtime.
What is depth of discharge?
Depth of Discharge (DoD) is the percentage of the battery's total capacity that has been used. For example, a 100Ah battery with a 50% DoD recommendation should only be discharged until 50Ah remain. Limiting DoD is critical to prolonging the lifespan of certain chemistries, especially lead-acid batteries.
How does temperature affect battery runtime?
Temperature has a profound effect on battery performance. Cold temperatures increase internal resistance and slow down chemical reactions, which can significantly reduce the usable capacity of a battery (often dropping by 20-50% in freezing conditions). Hot temperatures can temporarily increase capacity slightly but will drastically accelerate degradation and reduce the overall lifespan of the battery.
Li-ion vs lead acid runtime comparison
A 100Ah Lithium-ion (or LiFePO4) battery can safely be discharged to 80-100% DoD, providing 80-100Ah of usable capacity. In contrast, a 100Ah Lead-Acid battery is typically recommended to only be discharged to 50% DoD to prevent damage, yielding only 50Ah of usable capacity. Additionally, Lead-Acid capacity drops under heavy loads due to Peukert's Law, whereas Lithium maintains its capacity much better under high discharge rates.
Why is my battery not lasting as long as calculated?
Several real-world factors reduce actual runtime compared to theoretical calculations. Common reasons include inverter inefficiency (typically losing 10-15% of energy when converting DC to AC), battery aging and natural degradation, extreme temperatures, high discharge rates (which lower effective capacity in lead-acid batteries), and hidden parasitic loads from battery management systems or connected devices that draw small amounts of current continuously.
Methodology
The calculations provided by this tool use standard electrical power formulas combined with practical efficiency coefficients. We allow users to factor in the combined effects of Depth of Discharge (DoD) limits and system inefficiencies (such as inverter losses) via a single percentage input. It's important to note that this calculator provides a linear estimate. It does not dynamically model Peukert's Law for lead-acid batteries under varying high loads, nor does it adjust for ambient temperature fluctuations. For mission-critical applications, always consult with a battery engineer and refer to the specific discharge curves provided by the battery manufacturer.