LoadCairn / UPS runtime guide
UPS runtime calculator guide
Estimate battery backup time from a connected load and battery-bank details. The UPS runtime calculator makes its energy method visible so engineers, facility managers, technicians, MEP consultants, and students can review the assumptions behind a planning estimate.
Estimate method
Nominal energy / planning only
Nominal bank energy = V × Ah × series batteries × parallel strings
Runtime = bank energy × efficiency fraction × remaining-capacity fraction ÷ load (W)
Efficiency and remaining capacity are percentages converted to fractions. The displayed runtime rounds to the nearest minute.
01 / Inputs and formula
What the estimate uses
First, nominal bank energy in watt-hours is voltage per battery multiplied by rated amp-hours per battery, batteries in series per string, and parallel strings. The calculator then applies UPS efficiency and remaining battery capacity before dividing by connected load in watts.
Runtime (hours) = nominal bank energy × (efficiency / 100) × (remaining capacity / 100) ÷ connected load (W).
The calculator displays the result rounded to the nearest minute.
Supported inputs
- Connected real-power load
- The equipment load in watts (W).
- Nominal battery voltage
- The voltage rating for one battery (V).
- Rated battery capacity
- The amp-hour rating for one battery (Ah).
- Batteries in series per string
- The count of batteries connected in each series string.
- Parallel strings
- The number of matching battery strings connected in parallel.
- UPS efficiency
- The estimated efficiency as a percentage.
- Remaining battery capacity
- The estimated capacity remaining as a percentage.
02 / Assumptions and limits
A planning estimate, with boundaries
The result is a planning estimate, not a guaranteed or model-specific runtime. Actual backup time depends on the UPS, batteries, operating conditions, and load behavior.
Assumes
- The battery bank starts fully charged.
- The connected real-power load stays constant.
- Series and parallel battery arithmetic is ideal.
Does not model
- No chemistry-specific Peukert or discharge-rate curve is applied.
- No temperature correction is applied.
- Inverter cutoff and battery-management limits are not modeled.
- Power factor and inrush are not modeled.
- The estimate is not calibrated to a specific manufacturer or UPS model.
For equipment decisions, consult the UPS and battery manufacturer’s data for the specific system and operating conditions.
03 / Run an estimate
Put your battery-bank values into the calculator
Change the inputs to compare planning scenarios, then review the formula and limits beside the result.