What the Battery Runtime Calculator Actually Estimates
The Battery Runtime Calculator converts battery capacity into nominal watt-hours and then reduces that energy by the planned depth of discharge. It also converts the stated device load into a battery-side power demand using the entered conversion efficiency and any fixed overhead. Dividing planned usable energy by battery-side power produces the estimated runtime in hours.
Watt-hours make different voltages comparable. A 100Ah battery at 12.8V contains about 1,280Wh nominally, while 100Ah at 25.6V contains about 2,560Wh. The same Ah rating does not mean the same stored energy.
Battery Runtime Calculator Formula, Units and Inputs
The Battery Runtime Calculator starts with the energy relationship watt-hours = amp-hours × volts. Watt-hours describe stored energy, which can be directly compared with a load measured in watts. The calculator then applies the fraction of rated energy you plan to use. A 90% depth of discharge means the estimate reserves 10% of nominal energy rather than assuming every rated watt-hour is available.
Nominal energy (Wh) = Ah × V. Planned usable energy (Wh) = nominal Wh × DoD fraction. Battery-side watts = device watts ÷ efficiency fraction + fixed overhead watts. Estimated runtime (h) = planned usable Wh ÷ battery-side watts. Approximate battery current (A) = battery-side watts ÷ nominal battery voltage.
The efficiency field represents conversion loss. A 500W AC appliance at 92% inverter efficiency requires about 543.5W from the battery before fixed overhead. For a simplified direct-DC load with negligible conversion loss, use 100%.
Fixed overhead matters most with small loads or long runtimes. Inverter idle draw and always-on electronics can consume meaningful energy. Prefer manufacturer data because idle power and efficiency vary by model and load.
Battery Runtime Calculator: 7 Smart, Essential Checks
Use these seven checks whenever the Battery Runtime Calculator returns a plausible number. They connect the simple energy equation with the electrical and operating limits that determine whether the system can actually deliver that runtime.
- Use the correct nominal voltage. Enter the nominal voltage for the battery bank as configured, not a fully charged open-circuit reading. A published 12V-class LiFePO4 product may use 12.8V nominal; for example, Renogy lists 12.8V and 100Ah as 1,280Wh for one current model.
- Use a realistic average load. Nameplate maximum power may overstate a cycling appliance’s average demand, while a low steady rating may miss startup surges. Measure average energy use over a representative period when possible.
- Separate depth of discharge from efficiency. DoD controls how much battery energy you plan to remove. Efficiency accounts for conversion losses. Combining them into one unexplained percentage makes the result harder to audit and update.
- Include inverter overhead. Inverters have both conversion losses and no-load consumption. Victron documentation shows that these values differ by model and system voltage, which is why the Battery Runtime Calculator leaves them editable instead of assuming one universal number.
- Check current and surge limits. Energy may be sufficient while current is not. Compare the result’s approximate battery current with the battery’s continuous discharge rating, BMS limit, inverter input requirement, cable ampacity and protective-device ratings.
- Allow for real-world derating. Battery age, temperature, voltage sag, wiring resistance, cell balance and low-voltage cutoffs can reduce delivered energy. A planning reserve is useful when shutdown would be costly or unsafe.
- Verify the complete system. The result cannot approve a battery, charger, inverter or installation. Manufacturer instructions and applicable electrical or fire-safety requirements take priority over a calculator output.
How to Use the Battery Runtime Calculator
Enter capacity in amp-hours and the nominal voltage of the complete bank. Series wiring raises voltage while matched-string Ah stays the same; parallel wiring raises Ah while voltage stays the same. Use the finished bank rating.
Enter average load power in watts. Add simultaneous loads together, and use measured average power for cycling equipment when possible. A refrigerator or pump may spend much of its time off, so its running wattage alone can overstate long-term demand.
Choose the planned depth of discharge based on the reserve you want and the battery manufacturer’s recommendations. The Battery Runtime Calculator does not prescribe a universal DoD because cycle-life guidance, BMS thresholds and warranty terms differ among products. Leaving some energy unused can also provide a buffer for temperature, aging or unexpected load increases.
Enter conversion efficiency from the inverter or converter documentation and add fixed overhead that remains on. Then compare the estimated battery current with BMS, wiring, fuse, connector and inverter requirements.
Battery Runtime Calculator Examples
Example 1: 12.8V 100Ah battery with a 500W AC load
For this Battery Runtime Calculator example, a 12.8V 100Ah battery stores 1,280Wh nominally. At 90% planned DoD, usable energy is 1,152Wh. With a 500W output load, 92% conversion efficiency and 5W of fixed overhead, battery-side demand is about 548.5W. Estimated runtime is therefore about 2.10 hours, or roughly 2 hours 6 minutes.
Example 2: 24V 200Ah bank with a 300W load
A 24V 200Ah battery bank contains 4,800Wh nominally. At 80% DoD, planned usable energy is 3,840Wh. If the 300W load is supplied through a 94% efficient converter with 8W of fixed overhead, battery-side demand is about 327.1W. The Battery Runtime Calculator estimate is about 11.74 hours before additional real-world derating.
Example 3: 51.2V 200Ah backup bank with a 1,200W load
A 51.2V 200Ah bank has 10,240Wh of nominal energy. At 85% DoD, 8,704Wh is planned for use. With a 1,200W AC load, 93% efficiency and 20W of inverter overhead, battery-side demand is about 1,310.3W. The Battery Runtime Calculator returns approximately 6.64 hours, assuming the load stays near 1,200W.
| Scenario | Nominal energy | Planned usable energy | Battery-side demand | Estimated runtime |
|---|---|---|---|---|
| 12.8V · 100Ah · 500W | 1,280Wh | 1,152Wh at 90% DoD | 548.5W | 2.10h |
| 24V · 200Ah · 300W | 4,800Wh | 3,840Wh at 80% DoD | 327.1W | 11.74h |
| 51.2V · 200Ah · 1,200W | 10,240Wh | 8,704Wh at 85% DoD | 1,310.3W | 6.64h |
Examples use steady average loads and user-entered planning assumptions. They are not product performance guarantees.
Battery Runtime Calculator for DC Loads, AC Loads and Inverters
If measured battery-side power is already known for a DC load, set efficiency to 100% and enter that wattage directly. For a DC-DC converter, include its efficiency or use measured battery input power.
AC loads need an inverter, so the battery must supply more than the appliance’s output watts. An inverter’s efficiency is not a fixed law; it changes with design, input voltage and operating point. One Victron Phoenix manual lists full-load efficiency values from 89% to 94% across different variants and also lists different no-load currents. That variation is why measured battery-side power is the strongest input when it is available.
The Battery Runtime Calculator also shows approximate battery current because a low-voltage system can require substantial current at high power. A 1,000W battery-side load at 12.8V is about 78A, while the same 1,000W at 51.2V is about 19.5A. Lower current can reduce voltage drop and conductor losses, but system voltage must match the battery, inverter, charger, protection and load architecture.
Why Real Runtime Can Differ From the Battery Runtime Calculator
The Battery Runtime Calculator assumes a steady average power demand and uses nominal battery energy. Real batteries do not deliver energy under perfectly constant conditions. Voltage changes during discharge, equipment may shut down at a preset voltage, and internal resistance causes voltage sag under load. A system can therefore stop before every planned watt-hour is delivered.
Temperature, battery age, high current and cycling loads can all change delivered energy. These effects matter most for cold conditions, older packs, long runtimes and systems operating near current or low-voltage limits.
A BMS or inverter can disconnect during over-current, over-discharge or transient events according to its design. For critical applications, compare estimates with logged monitor data and maintain additional reserve.
CPSC notes that high-energy batteries and chargers can present overheating, fire, shock and burn hazards and emphasizes the need for appropriate safeguards and use as a system. Use the Battery Runtime Calculator for planning energy, not as an instruction to modify battery packs, bypass protection or operate equipment beyond its ratings.
Using the Battery Runtime Calculator for Solar, RV, Marine and Backup Power
For solar systems, use runtime alongside daily energy demand. List each load, its average watts and the hours it operates, then calculate watt-hours per day. The Battery Runtime Calculator answers the shorter question of how long a known bank supports a known average load; solar sizing also needs recharge energy, solar-resource assumptions, controller limits and reserve for periods with little generation.
For RV and marine systems, separate continuous loads from brief high-power loads. Energy determines runtime, while surge current and cable voltage drop may determine whether pumps, compressors or cooking loads start successfully.
For backup power, identify critical loads first. A router, modem, lighting and small electronics may allow many hours of operation from a modest bank, while resistive heating or air conditioning can consume energy quickly. The Battery Runtime Calculator is useful for comparing “essential-only” and “whole-load” scenarios before deciding whether more battery capacity or load shedding is the better strategy.
When the result shows insufficient runtime, there are only a few fundamental options: increase usable battery energy, reduce average power, improve conversion efficiency, reduce fixed overhead or recharge during operation. Use the Battery Capacity Calculator to work backward from a target runtime and the Load Current Calculator to examine the corresponding current.
Battery Runtime Calculator Data Sources and Assumptions
The Battery Runtime Calculator uses user-entered electrical values rather than hidden chemistry presets. For a concrete example of product-specific data, Renogy’s 12.8V 100Ah LiFePO4 specification lists 1,280Wh energy and separate continuous-current limits. That illustrates why voltage, capacity and current ratings should come from the exact battery rather than a generic chemistry label.
For inverter planning, the Victron Phoenix Inverter 750 manual provides model-specific full-load efficiency and no-load current values. These values show that conversion loss and idle draw are equipment characteristics, not universal constants. Use the documentation for the inverter and operating point closest to your system.
For safety context, the U.S. Consumer Product Safety Commission battery resource discusses battery and charger hazards. For larger stationary systems, the U.S. Department of Energy lithium-ion ESS safety guidance identifies codes and standards that may apply. Manufacturer instructions and local requirements still take priority.