What Duty Cycle Means In Battery Planning
Duty cycle describes how much of a time interval a load spends in its active state. A 100% duty cycle means the equipment is continuously on. A 50% duty cycle means that, over the period being modeled, active time occupies half of the elapsed time. The switching pattern does not have to be exactly 30 minutes on and 30 minutes off; ten repeated cycles of three minutes on and three minutes off have the same 50% time fraction.
For runtime estimates, duty cycle is useful because battery depletion depends on energy over time. If a 140 W load is active only 35% of the time and consumes almost nothing while off, its average load is 140 × 0.35 = 49 W. A battery therefore sees a much smaller average energy demand than it would from a continuous 140 W load, even though the instantaneous active-state demand is still 140 W.
The important phrase is over the period being modeled. A cycling device may have a 20% duty cycle during mild conditions and 70% during a demanding period. Compressors, pumps, heaters and fans often respond to temperature, pressure, occupancy or control settings. A single percentage is most useful when it is based on measurements or a clearly stated scenario.
The Formula Used By This Calculator
The calculator first converts the duty-cycle percentage to a fraction. For a 35% duty cycle, the fraction is 0.35. It then multiplies active-state power by that fraction to obtain average duty-cycled power. Finally, it reduces available battery energy by the entered delivery efficiency and divides that delivered energy by average power.
Average power: Pavg = Pactive × D
Delivered energy: Edelivered = Eavailable × η
Elapsed runtime: t = Edelivered ÷ Pavg
In these expressions, D is duty cycle as a decimal and η is efficiency as a decimal. The units remain transparent: watt-hours divided by watts produce hours. This calculator expects available battery energy directly in watt-hours, which avoids mixing voltage and amp-hour assumptions inside the runtime formula.
What “available battery energy” should mean
Use the energy you expect to make available to this load during the modeled discharge, not automatically the battery's full nominal watt-hour rating. A reserve target, battery-management cutoff, usable-capacity limit, age-related capacity loss or other operating constraint may reduce the energy you intend to use. If another calculation has already reduced the battery to a usable watt-hour figure, do not apply the same reduction again through the efficiency field.
Worked Example: 1,100 Wh Battery, 140 W Active Load
Consider the default values in the calculator: 1,100 Wh of available battery energy, a device that draws 140 W while active, a 35% duty cycle and 91% delivery efficiency. The duty-cycled average load is 49 W. After the efficiency adjustment, the modeled delivered energy is 1,001 Wh. Dividing 1,001 Wh by 49 W gives an estimated elapsed runtime of about 20.43 hours.
During those 20.43 hours, the device would accumulate roughly 7.15 hours of active operation if the 35% duty cycle remained stable. That is different from saying the load can run continuously for 20.43 hours. The estimate includes both on-time and off-time because the battery is being modeled across elapsed clock time.
| Duty cycle | Average load at 140 W active | Estimated runtime* |
|---|---|---|
| 20% | 28 W | 35.75 h |
| 35% | 49 W | 20.43 h |
| 50% | 70 W | 14.30 h |
| 75% | 105 W | 9.53 h |
| 100% | 140 W | 7.15 h |
*Using 1,100 Wh available energy and 91% delivery efficiency. These values illustrate the mathematics; actual runtime depends on the battery and load.
The table shows why estimating duty cycle carefully matters. Moving from 35% to 50% does not look like a large percentage-point change, but it increases average load from 49 W to 70 W and shortens the modeled runtime by more than six hours.
How To Measure Duty Cycle In The Real World
For a device that has clear on and off states, observe it long enough to capture several representative cycles. Add the duration of every active interval, divide by the total observation time, and multiply by 100. If a compressor runs for 24 minutes during a 60-minute observation, its observed duty cycle is 40%. If it runs for 66 minutes during a three-hour observation, the duty cycle is about 36.7%.
A longer observation is usually more informative when cycles are irregular. One short cycle may be distorted by a door opening, a thermostat change, a pressure recovery event or a recently started system. Measuring across the same environmental conditions you expect during battery operation helps make the percentage more defensible.
Some equipment does not have a clean two-state pattern. Variable-speed compressors, modulating heaters, multi-speed fans and electronically controlled motors may draw several different power levels. In that case, a simple active-power × duty-cycle model can be too crude. A load-profile approach that records watts and duration for each state is more appropriate.
Use energy measurements when available
If a plug-in energy meter or DC shunt reports watt-hours consumed over several hours, you can calculate average power directly by dividing measured energy by elapsed time. That measurement automatically captures cycling and may be more reliable than estimating a duty cycle from visual observation. You can still use the duty-cycle calculator as a cross-check against the observed behavior.
Standby Power, Inverter Idle Draw And Other “Off” Loads
The simple formula on this page assumes the device consumes essentially zero power during the off part of its cycle. That can be reasonable for some switched loads, but many real systems continue to draw energy. Control boards, displays, relays, fans, network modules and inverters may remain powered while the main load is inactive.
When off-state consumption is material, a better average-power equation is:
Pavg = (Pon × D) + (Poff × (1 − D))
For example, suppose the active state is 140 W, the off state still draws 8 W, and the duty cycle is 35%. The average is not 49 W; it is 54.2 W. That extra 5.2 W of continuous background demand lowers runtime. If an inverter remains on solely to support the cycling appliance, its idle consumption may need to be included as part of the off-state load or incorporated into a measured system-level average.
Do not hide known standby consumption inside an arbitrary efficiency percentage. Efficiency is best used for conversion losses that scale with energy transfer, while a persistent standby draw is a power load in its own right.
Startup Surge Does Not Disappear At A Low Duty Cycle
A low duty cycle can make average energy consumption modest while leaving startup requirements unchanged. A compressor that runs only 25% of the time may still demand a brief inrush current every time it starts. The battery, BMS, inverter, wiring and connectors must tolerate that event even though the long-term average power is much lower.
This distinction is important because runtime and power capability answer different questions. Runtime asks how long the stored energy may last. Surge capability asks whether the system can deliver a short, high-power event without excessive voltage sag, protective shutdown or component stress. A battery can have enough watt-hours for the day and still be a poor match for the load's starting demand.
Frequent cycling can also increase the number of start events. Two scenarios with the same 35% duty cycle could behave differently if one uses long on/off periods and the other starts every few minutes. The energy-only model treats them alike, but equipment wear, startup losses and control behavior may not be alike.
Why Duty Cycle Changes With Conditions
Thermostatically controlled equipment is especially sensitive to its environment. A refrigerator or air conditioner generally runs longer when the temperature difference between the controlled space and its surroundings increases. Insulation, door openings, ventilation and internal heat sources can all change the on-time fraction. A pump may cycle more often with higher demand or a smaller pressure reservoir. A heater's duty cycle changes with heat loss and thermostat settings.
Battery conditions can influence the system too. Voltage sag, low-temperature performance, inverter behavior and control cutoffs may change how equipment operates near the end of discharge. The calculator intentionally does not try to predict those interactions. Instead, use separate duty-cycle scenarios—such as mild, typical and demanding—to see how sensitive runtime is to the percentage you choose.
For planning, the highest plausible duty cycle is often more useful than a comfortable best case. If your system still meets the required runtime at that higher percentage, you have more margin for changes in weather, usage or load behavior.
Common Mistakes With Duty-Cycled Runtime
Using nameplate maximum power as the normal active draw. A label can represent a maximum or rated condition rather than typical running power. Measure actual active watts when practical.
Confusing duty cycle with efficiency. A 40% duty cycle does not mean the device is 40% efficient. Duty cycle describes time spent active; efficiency describes energy retained through conversion.
Applying duty cycle twice. If you already measured average power over time, the cycling behavior is already included. Do not multiply that measured average by duty cycle again.
Ignoring off-state and inverter consumption. Background loads can dominate when the main device has a low duty cycle.
Assuming one percentage covers every season or operating mode. Cycling loads can change significantly with ambient conditions, settings and usage.
Treating the runtime result as a surge check. Average energy demand does not prove that the battery or inverter can start the load.
Using nominal battery energy without a usable-energy decision. Reserve targets, cutoff settings and real capacity affect the watt-hours you can actually allocate.
When A Duty-Cycle Model Is The Right Tool
This calculator is most useful when one dominant load alternates between a reasonably stable active power and a near-zero off state. Examples can include a pump controlled by a float switch, a fan controlled by temperature, a heater controlled by a thermostat, or a compressor that cycles between running and off.
Use a more detailed model when the system contains several overlapping loads, variable-speed equipment, substantial standby consumption, time-of-day schedules or state-dependent power levels. In those cases, calculate energy for each operating state or use measured watt-hours over a representative interval. The goal is not to force every system into a duty-cycle percentage; it is to choose the simplest model that still represents the load honestly.
Before acting on the estimate, verify electrical limits separately. Check battery discharge ratings, BMS continuous and peak current, inverter output and surge capability, wiring, fusing, connectors, temperature limits and the equipment manufacturer's operating requirements.