Calculate Battery Runtime
A battery runtime calculator helps answer one of the most common battery questions: how long will this battery last? Whether you are building a small backup system, planning a solar battery bank, running marine electronics, powering an RV, or testing a portable lithium setup, runtime is usually the first number you need.
At the simplest level, battery runtime depends on stored energy and the size of the electrical load. But real battery systems are not perfectly efficient, and most users should not discharge a battery to absolute zero. That is why this calculator includes depth of discharge and inverter efficiency, so the result is more realistic than a basic Ah-only estimate.
This page gives you both a working calculator and a long-form explanation of the runtime formula, the variables that affect actual battery life, and the practical reasons your real-world result may be different from a perfect laboratory estimate. That makes it useful not only as a tool, but also as a planning reference.
Short answer: Battery runtime is estimated by converting battery capacity into usable watt-hours, then dividing by load power in watts. For better accuracy, you should account for depth of discharge and inverter efficiency instead of assuming that 100% of battery energy is available.
Free Battery Runtime Calculator
Estimate runtime, usable battery energy, DC current draw, and readable HH:MM runtime
How battery runtime is calculated
The core idea behind runtime is simple. A battery stores energy, and your load consumes energy. If you know how much usable energy is in the battery and how fast the load uses it, you can estimate how long the battery will last. In battery calculators, energy is usually expressed in watt-hours, while load is expressed in watts.
To calculate stored energy, you multiply battery voltage by battery capacity in amp-hours. That gives total watt-hours. However, not all of that energy should always be used. In many real systems, users intentionally avoid fully discharging the battery because deep discharge can reduce lifespan or because the battery management system may impose limits. That is where depth of discharge becomes important.
Then inverter efficiency is applied. If your battery is powering AC equipment through an inverter, some of the battery energy is lost during conversion from DC to AC. Even a high-quality inverter consumes a portion of the stored energy, which means runtime is shorter than the ideal DC-only result. For direct DC loads, users may sometimes set efficiency very high, but for AC loads the efficiency factor makes the estimate more realistic.
Core formulas
| Formula | Meaning | Practical use |
|---|---|---|
| Battery energy = V × Ah | Converts battery voltage and capacity into watt-hours | Shows the total theoretical stored energy |
| Usable energy = V × Ah × DoD | Adjusts total battery energy by the allowed depth of discharge | Reflects energy you actually plan to use |
| Delivered energy = usable energy × efficiency | Adjusts for inverter or system losses | Gives a more realistic AC-side energy figure |
| Runtime = delivered energy ÷ load | Divides available watt-hours by watts consumed | Estimates runtime in hours |
| Current draw = load ÷ voltage | Estimates battery-side current in amps | Useful for wiring, fuse planning, and discharge awareness |
Why DoD matters
Depth of discharge, often shortened to DoD, tells you what percentage of the battery's total capacity you plan to use. A battery used to 80% DoD means you are using 80% of its stored energy before recharge. This is extremely important because theoretical full-capacity runtime can give a misleading impression, especially when you want longer battery life or when the battery chemistry has recommended operating limits.
For many lithium systems, a relatively high DoD is practical, which is one reason lithium batteries are popular. But even then, users often do not want to plan around 100% discharge every cycle. A more conservative DoD can add margin, reduce stress on the system, and align the calculator result more closely with how the battery is used in real life.
DoD is also helpful when comparing chemistries. A battery with the same amp-hour rating can deliver very different practical runtime depending on how much of its capacity is normally usable. That is why runtime calculations become much more helpful once DoD is added to the formula.
Why inverter efficiency matters
If a battery powers an AC appliance through an inverter, the battery is not delivering energy directly to the appliance. Instead, the inverter converts DC battery power into AC output, and some of the energy is lost along the way. Those losses shorten runtime.
For example, suppose a battery has 960 Wh of usable energy after applying DoD. If the inverter is 90% efficient, only about 864 Wh reaches the load. That difference becomes meaningful as the load size grows. Ignoring inverter efficiency often makes runtime results look more generous than they really are.
Some users power DC devices directly from the battery without an inverter. In those cases, efficiency losses may be smaller, but there are still possible system losses in wiring, converters, or other electronics. The efficiency input is therefore a useful planning tool even beyond classic inverter-based systems.
Practical takeaway: A battery does not power your load with all of its nameplate energy. The usable result depends on both how deeply you allow the battery to discharge and how efficiently the system delivers that power to the load.
Example runtime calculation
Assume you have a 12V 100Ah battery, you plan to use 80% depth of discharge, your inverter efficiency is 90%, and your load is 200W. First calculate total battery energy: 12 × 100 = 1200 Wh. Then apply DoD: 1200 × 0.80 = 960 Wh usable. Then apply inverter efficiency: 960 × 0.90 = 864 Wh delivered to the load.
Finally, divide by the load: 864 ÷ 200 = 4.32 hours. In readable time, that is approximately 4 hours and 19 minutes. This is a practical example of why runtime is lower than many beginners expect when they only look at voltage and amp-hours without considering system losses.
If the same system used a 100W load instead, runtime would roughly double to 8.64 hours. If it used a 400W load, runtime would fall to about 2.16 hours. This shows how strongly load size affects runtime, even when the battery itself does not change.
What changes real runtime
Load variation
Many devices do not draw constant power. Compressors, pumps, refrigerators, and electronics can cycle on and off, so real average load may differ from the label.
Battery age
Older batteries usually hold less capacity than new ones, which reduces actual runtime compared with a fresh battery of the same rating.
Temperature
Cold weather can noticeably reduce available battery capacity and therefore shorten runtime, especially in exposed outdoor systems.
Discharge current
High current draw can reduce effective usable capacity and make the battery perform worse than a simple ideal formula suggests.
Battery chemistry
Different chemistries behave differently under load, across temperatures, and across different states of charge.
System losses
Inverters, converters, cabling, connectors, and battery management systems all introduce real losses that reduce net runtime.
Where this calculator is useful
This calculator is useful for a wide range of battery applications. In solar energy systems, it helps estimate how long batteries can support loads overnight or during an outage. In RV and van setups, it helps determine whether a battery bank can power lights, fans, refrigerators, and electronics through the night. In marine systems, it helps estimate run time for house loads, fish finders, pumps, and auxiliary equipment.
It is also useful for portable power stations, DIY battery packs, workshop backup systems, emergency battery banks, telecom backup, and small off-grid systems. Even when the final design requires more detailed engineering review, a runtime calculator provides a strong first estimate that helps narrow the correct battery size.
For commercial or industrial applications, the same logic still applies, although more variables usually need to be considered. The page is therefore a good starting point for planning, comparison, and rough system design before moving into more detailed specification work.
How to use the result properly
- Estimate your real average load rather than relying only on peak wattage.
- Choose a sensible depth of discharge based on battery chemistry and lifespan goals.
- Account for inverter efficiency if the load is AC.
- Add a safety margin so you are not designing to the absolute limit.
- Compare several scenarios, such as low-load, average-load, and heavy-load use.
For example, if your device runs at 150W most of the time but occasionally peaks at 300W, a runtime estimate based on 150W may be helpful for average planning while a 300W estimate gives a worst-case view. Using both numbers is often smarter than using either one alone.
It also helps to think in watt-hours, not only amp-hours. Watt-hours make comparisons easier when two batteries use different voltages. A 24V battery bank and a 12V battery bank may look very different in Ah, but runtime becomes much easier to compare once both are converted to stored energy.
Common mistakes
- Using battery nameplate amp-hours without converting them into watt-hours.
- Ignoring depth of discharge and assuming 100% of capacity is always usable.
- Ignoring inverter efficiency when powering AC appliances.
- Using rated appliance wattage instead of actual average power draw.
- Forgetting that temperature, battery age, and high current draw reduce real runtime.
These mistakes usually cause users to overestimate runtime. In other words, the battery appears to last longer on paper than it does in real use. A runtime calculator is most helpful when it includes enough realism to avoid that trap.
Important note: Runtime calculators are planning tools, not guarantees. In critical backup systems, medical applications, safety systems, or professional installations, always verify the design with manufacturer specifications and qualified engineering review.
Battery sizing connection
Runtime calculation and battery sizing are really two sides of the same question. If you already own a battery, runtime tells you how long it can support a load. If you know the runtime you need, you can reverse the formula to estimate how much battery capacity is required.
That is why runtime calculators are often used alongside capacity and charge-time calculators. A runtime result may tell you that your existing battery is too small, while a capacity calculator can help estimate the Ah or Wh needed to hit your target. Together, those tools make battery planning more complete.
For example, if your calculated runtime is only 4 hours but you need 8 hours, you may need about double the usable energy, assuming the load remains the same. That can mean a larger single battery, a larger battery bank, or a change in voltage and system configuration.