Estimate Battery AC Runtime
An inverter runtime estimator helps answer a simple but important question: how long can your battery bank power an AC load through an inverter? This matters because most battery systems are rated in voltage and amp-hours, while household appliances and backup loads are usually rated in watts. A good runtime estimate bridges that gap and turns battery capacity into something practical and usable.
Unlike a basic battery runtime formula, an inverter runtime calculator also accounts for conversion losses. When a battery powers an inverter, not all of the battery energy becomes AC output. Some energy is lost as heat inside the inverter, which means the usable runtime is always somewhat lower than a perfect no-loss calculation.
This page expands the simple runtime formula into a more practical planning guide. It explains how to estimate runtime from battery voltage, amp-hours, depth of discharge, inverter efficiency, and AC load, while also covering real-world limits such as surge loads, battery aging, temperature, and standby losses.
Short answer: To estimate inverter runtime, convert battery capacity into watt-hours, adjust for usable depth of discharge, account for inverter efficiency, and divide by the AC load in watts. Runtime falls when load rises, efficiency drops, or usable battery energy is reduced.
Free Inverter Runtime Estimator
Estimate usable battery energy, AC runtime, HH:MM runtime, battery current draw, and planning margin
Battery Bank
Inverter and Load
How inverter runtime is calculated
The basic idea is simple. First, convert the battery bank into stored energy in watt-hours. You do that by multiplying voltage by amp-hours. A 24V battery bank with 200Ah stores about 4,800Wh of nominal energy. But that is only the starting point.
Next, you adjust for depth of discharge. If you only want to use 80% of the battery, then only 80% of that nominal energy counts as usable energy. After that, you adjust again for inverter efficiency because the inverter converts DC battery energy into AC power with some losses. Only then do you divide by the AC load in watts to estimate runtime.
In equation form, the logic is: battery watt-hours, multiplied by usable DoD, multiplied by inverter efficiency, divided by the AC load. This produces a runtime estimate in hours, which is often easier to understand when also shown as hours and minutes.
Core formulas
| Formula | Meaning | Why it matters |
|---|---|---|
| Battery energy (Wh) = Voltage × Capacity | Nominal stored battery energy | Converts battery specs into energy terms |
| Usable energy = Battery energy × DoD | Energy available before inverter losses | Reflects how much of the battery you allow yourself to use |
| Delivered AC energy = Usable energy × inverter efficiency | Energy available to AC loads | Accounts for inverter conversion losses |
| Runtime (hours) = Delivered AC energy ÷ load | Estimated duration of power delivery | Turns battery capacity into practical runtime |
Why inverter efficiency matters
Inverter efficiency has a direct impact on runtime because every percentage point lost inside the inverter becomes heat instead of useful AC power. If you ignore inverter efficiency, you will overestimate how long the battery bank can actually run your load. In smaller systems the error may look modest, but in long-runtime backup systems it can become significant.
For example, suppose a battery bank has 2,000Wh of usable DC energy. At 100% theoretical conversion, that could support a 500W load for 4 hours. But at 90% inverter efficiency, only about 1,800Wh reaches the load, reducing runtime to about 3.6 hours. The difference is meaningful when planning outage backup or off-grid autonomy.
Efficiency can also vary with load level. Many inverters perform best around moderate load and may be less efficient at very low or very high output. That means the real runtime can change depending not only on the battery bank and load size, but also on how closely the inverter is matched to the actual load profile.
Practical takeaway: A battery bank does not power AC loads directly. It powers the inverter first. Any runtime estimate that ignores inverter efficiency is usually too optimistic.
Why depth of discharge matters
Depth of discharge controls how much of the battery energy you are willing to use. A larger DoD increases runtime because more energy becomes available to the inverter. But that does not always mean it is the best operating choice, because deeper discharge can also increase battery wear and shorten cycle life over time.
For example, a lithium battery bank used at 90% DoD will generally deliver more runtime than the same bank limited to 70% DoD. However, some users intentionally keep a more conservative DoD to preserve battery lifespan, maintain a backup reserve, or protect performance in cold weather. This is why a runtime number should always be understood together with the chosen DoD assumption.
In real battery planning, DoD is one of the easiest ways to trade runtime against long-term battery health. Larger battery banks often allow users to operate at a gentler DoD while still achieving the required runtime.
Example calculation
Consider a 24V, 200Ah battery bank powering a 500W AC load through a 90% efficient inverter at 80% DoD. The nominal battery energy is 24 × 200 = 4,800Wh. At 80% DoD, usable DC energy becomes 3,840Wh. After 90% inverter efficiency, delivered AC energy becomes about 3,456Wh.
Now divide 3,456Wh by the 500W load and the runtime estimate becomes about 6.91 hours. In time format, that is roughly 6 hours and 55 minutes. This is the same kind of estimate the calculator performs automatically, but with extra outputs that make the result easier to interpret in real battery planning.
The same method also shows how sensitive runtime is to changes in load. If the load doubles from 500W to 1,000W, the runtime is cut roughly in half. This is why managing load is often just as important as adding more battery capacity.
What changes the real runtime
Battery temperature
Cold weather can reduce usable capacity, especially compared with room-temperature ratings, which shortens actual runtime.
Battery aging
Older batteries usually deliver less usable capacity than new ones, which means less runtime at the same load.
Voltage sag
Under high load, battery voltage can drop enough to trigger inverter shutdown before all theoretical energy is used.
Inverter self-consumption
Even with no major load, many inverters consume some power just by being on, which reduces net runtime.
Load variation
Real loads often cycle up and down rather than staying constant, so the true runtime may differ from a steady-load estimate.
Battery limits
BMS current limits, cutoff voltages, and manufacturer protection settings may end runtime earlier than a simple energy model suggests.
Continuous load versus surge load
This calculator is meant for continuous power planning, not surge analysis. Many appliances, especially those with motors or compressors, draw a much higher current for a short time during startup. That short surge may not consume much energy overall, but it can still determine whether the inverter can start the load at all.
A refrigerator, pump, air compressor, or power tool may have a modest running wattage yet still require a startup surge several times higher. If the inverter cannot handle that surge, the system may shut down even if the average runtime calculation looks excellent. That is why runtime and surge capability are related but separate planning steps.
For inverter selection, always check both continuous watt rating and surge watt rating. For runtime planning, use the continuous or average AC load you expect during actual operation.
Battery current draw
Another useful runtime-planning detail is battery-side current. Inverter loads that look modest on the AC side can require large current from the battery bank, especially in 12V systems. For example, a 1,200W AC load at high efficiency may still require well over 100A from a 12V battery bank.
This matters because high current affects cable size, fuse rating, connector choice, battery heating, and voltage sag. It can also influence how much usable runtime the battery actually delivers, since heavy current draw may reduce effective capacity compared with a lighter load. That is one reason higher-voltage battery systems such as 24V or 48V are often preferred for larger inverter setups.
The calculator therefore includes battery-side current as an extra output. It gives a more complete picture of how hard the battery bank is being worked, not just how long the load may run.
Planning tip: If your runtime estimate looks fine but the calculated battery current is extremely high, the system may still need a redesign. Runtime, cable sizing, BMS limits, and inverter input current all need to agree.
Where this tool helps
An inverter runtime estimator is useful in home backup, solar battery storage, cabins, RVs, vans, marine systems, and temporary power setups. It helps translate battery-bank size into practical expectations such as how long a fridge, router, lighting circuit, TV, computer, or small appliance can run during an outage or off-grid period.
It is also useful when comparing alternative designs. You can test whether it is better to reduce the load, increase battery voltage, add battery capacity, accept a deeper DoD, or improve inverter efficiency. In many cases, a simple load reduction provides the cheapest path to longer runtime.
For system design, the tool also helps reveal when a battery bank is probably too small for the intended inverter load. If runtime is very short or battery current is too high, that is often a sign the design needs more capacity, higher voltage, lower load, or a different inverter strategy.
Common mistakes
- Ignoring inverter efficiency and assuming all battery energy reaches the AC load.
- Using nominal battery capacity without considering depth of discharge.
- Forgetting that aging and cold temperature can reduce available capacity.
- Planning only for average watts while ignoring startup surge requirements.
- Overlooking battery-side current and focusing only on AC load watts.
- Using a perfect-theory runtime number as a guaranteed real-world result.
These mistakes usually lead to overly optimistic expectations. A realistic inverter runtime plan should include some margin, especially for emergency backup systems where reliability matters more than squeezing every last minute from the battery.
How to use the result
- Enter battery voltage and capacity to calculate nominal battery energy.
- Choose a realistic DoD based on battery type and how conservatively you want to operate.
- Enter the continuous AC load, not just occasional peaks.
- Enter a realistic inverter efficiency figure and an optional reserve margin.
- Check runtime, delivered energy, and battery current together before finalizing the design.
It is often helpful to test several load levels and reserve margins rather than relying on one single scenario. That quickly shows whether your battery system has comfortable backup margin or whether it is operating too close to the edge.
Important note: This estimator is a planning tool, not a substitute for equipment specifications. For critical backup systems, always verify inverter low-voltage cutoff, surge capability, standby draw, battery temperature derating, cable losses, and battery manufacturer limits.