UPS runtime, with the math in view.
Default inputs: 1,000 W load · 4 × 12 V, 100 Ah batteries · 90% efficiency · 80% remaining capacity.
Usable energy (Wh) = battery-bank energy × (UPS efficiency % ÷ 100) × (remaining battery capacity % ÷ 100)
Current calculation
12 V × 100 Ah × 4 batteries × 1 strings = 4,800 Wh4,800 Wh × (90% ÷ 100) × (80% ÷ 100) = 3,456 Wh3,456 Wh ÷ 1,000 W = 3.46 h; rounded to the nearest minute = 3 h 27 min
- Nominal bank energy
- 4,800 Wh
- Usable energy after factors
- 3,456 Wh
- Runtime in seconds
- 12,441.6 seconds
Assumptions and limits
- The bank starts fully charged.
- The load is constant real power measured in watts.
- Series and parallel capacity arithmetic is ideal.
- Nameplate Ah is treated as available energy before the explicit efficiency and remaining-capacity factors.
- No chemistry-specific Peukert or discharge-rate curve is applied.
- No temperature correction is applied.
- No inverter cutoff, battery-management limit, power-factor or inrush behavior, or manufacturer calibration is modeled.
Every result is an estimate for planning only—not a substitute for a licensed engineer’s design or manufacturer data.
The toolkit / 29 calculators
Twenty-nine planning calculators. One clear method.
Built for engineers, facility managers, technicians, MEP consultants, and students who want to inspect the reasoning—not just copy a number.
Not for: stamped construction documents, final equipment selection, or model-specific runtime guarantees. Use a licensed engineer and manufacturer data for those.
Browse all calculatorsPower & UPS
- 01
UPS runtime
Estimate runtime from load, battery-bank configuration, UPS efficiency, and remaining capacity.
Read the UPS runtime calculator guideLOAD · BANK CONFIGURATION · EFFICIENCY · REMAINING CAPACITY
- 03
Rack power
Plan rack power density and evaluate PDU capacity around the intended load.
RACK DENSITY · POWER · PDU CAPACITY
- 05
Generator sizing
Estimate standby generator kW and kVA requirements with running-load, power-factor, margin, and motor-start inputs.
RUNNING LOAD · POWER FACTOR · MARGIN · MOTOR START
- 08
kW / kVA / amps
Convert between kW, kVA and amps for single- and three-phase circuits, with power factor shown.
kW · kVA · AMPS · POWER FACTOR
- 09
Generator fuel burn & runtime
Estimate diesel burn rate, runtime on a tank, and the tank size you need for a target runtime.
LOAD · FUEL RATE · TANK · TARGET RUNTIME
- 10
UPS battery sizing
Work back from a target runtime to the watts each battery must deliver.
LOAD · RUNTIME · FACTORS · BATTERY DATASHEET
- 17
Energy cost
Estimate per-rack and facility energy and demand costs, including a 0.1 PUE savings comparison.
IT kW · RACKS · RATE · PUE · DEMAND CHARGE
- 19
Transformer sizing
Estimate demand load, required transformer capacity, standard size, and loading from transparent planning factors.
CONNECTED LOAD · DEMAND FACTOR · POWER FACTOR · GROWTH
- 23
Power factor correction
Estimate three-phase capacitor correction, kVA freed, and line-current change from current and target power factor.
LOAD · CURRENT PF · TARGET PF · LINE VOLTAGE
- 26
Motor starting & generator step load
Estimate motor starting kVA, three-phase current, and optional generator step load from motor HP, NEMA code, and starter planning factor.
MOTOR HP · NEMA CODE · STARTER · VOLTAGE · GENERATOR kVA
- 28
Available fault current
Estimate three-phase transformer available fault current and fault MVA from kVA, secondary voltage, and impedance.
TRANSFORMER kVA · SECONDARY V · %Z
Cooling & Airflow
- 02
Cooling load
Build a cooling-load estimate from IT demand, lighting, people, and the building envelope.
Read the cooling load formulas and assumptionsIT LOAD · LIGHTING · PEOPLE · ENVELOPE
- 06
Cooling airflow
Estimate sensible airflow in CFM from heat load and temperature rise.
SENSIBLE LOAD · TEMPERATURE RISE · AIRFLOW
- 07
Dew point & humidity
Find dew point, relative humidity and grains of moisture, and check condensation margin against the ASHRAE recommended envelope.
TEMPERATURE · RELATIVE HUMIDITY · DEW POINT
- 29
Humidification Load
Estimate ventilation humidification demand from outdoor and indoor design conditions, with a visible water-rate trace.
OUTSIDE-AIR FLOW · OUTDOOR DESIGN · INDOOR TARGET
- 11
Chilled water flow (GPM)
Convert cooling load and ΔT into chilled-water flow.
COOLING LOAD · ΔT · FLOW · PIPE VELOCITY
- 18
Cooling Unit Count & Redundancy
Estimate required cooling units and compare N+1, N+2, and 2N using effective capacity after derating.
COOLING LOAD · RATED CAPACITY · DERATE · REDUNDANCY
- 27
BTU to Tons Cooling Converter
Convert BTU/hr to refrigeration tons or tons to BTU/hr, with the conversion arithmetic and thermal kW equivalent shown.
BTU/HR · TONS · THERMAL kW
Cabling
- 14
Voltage drop
Estimate single- or three-phase conductor voltage drop with copper or aluminum circular-mil data.
PHASE · VOLTAGE · CURRENT · MATERIAL · LENGTH
- 15
Cable tray fill
Compare cable cross-sectional area with estimated ladder and ventilated tray capacity.
TRAY WIDTH · CABLE DIAMETERS · QUANTITIES
- 16
Conduit fill
Estimate raceway fill from NEC Table 4 dimensions and Table 5 conductor areas.
RACEWAY · TRADE SIZE · CONDUCTOR SIZE · COUNT
- 25
MPO Fiber Breakout Planner
Plan duplex LC links, MPO trunk strand capacity, cassette count, spare links, and enclosure rack units.
LC LINKS · MPO TRUNK FIBERS · SPARE CAPACITY · CASSETTES · RU
Reliability & Network
- 04
PUE
Estimate power usage effectiveness, data center infrastructure efficiency, and non-IT overhead from facility and IT power.
FACILITY POWER · IT POWER · PUE · DCiE
- 12
Availability & downtime
Turn nines into minutes of downtime and compare N, N+1 and 2N.
AVAILABILITY · DOWNTIME · REDUNDANCY
- 13
Fiber loss budget
Add up fiber, connector and splice loss and check it against 10G–100G limits.
Read the fiber loss formulas and assumptionsFIBER · CONNECTORS · SPLICES · CHANNEL LIMIT
- 22
WUE — Water Usage Effectiveness
Estimate annual site water use per unit of IT energy, with a gallon conversion and qualified industry-reference comparison.
SITE WATER · IT ENERGY · WUE · US GALLONS
Facilities & Space
- 20
Floor loading
Compare average rack floor loading with a rated capacity using the rack’s rectangular footprint.
RACK WEIGHT · FOOTPRINT · FLOOR RATING
- 21
Rack Layout & Floor Space Planner
Estimate rack rows and positions from room dimensions, rack size, aisle widths, and perimeter clearance.
ROOM · RACK SIZE · AISLES · PERIMETER CLEARANCE
- 24
Clean-agent fire suppression quantity calculator
Estimate FK-5-1-12 or HFC-227ea quantity from room volume and minimum temperature with a visible NFPA 2001 total-flooding calculation.
ROOM LENGTH · WIDTH · HEIGHT · AGENT · CONCENTRATION · TEMP
Comparing tools? LoadCairn vs vendor UPS calculators · vs spreadsheets · vs generator sizing software
Install once, work anywhere
Take LoadCairn into the data hall
Install LoadCairn on your phone or laptop. The available calculators work offline, including floor loading and rack layout, so you can size a UPS or check a rack plan in an electrical room or basement with no signal.
- Works offline: UPS runtime, UPS battery sizing, transformer sizing, available fault current, cooling load, cooling unit count and redundancy, airflow, dew point, humidification load, rack power, generator sizing, generator fuel burn, kW/kVA/amps, PUE, chilled water flow, availability, fiber loss budget, voltage drop, cable tray fill, conduit fill, MPO fiber breakout planning, energy cost, floor loading, rack layout, WUE water usage effectiveness, power factor correction, and clean-agent fire suppression quantity
- Opens full-screen from your home screen, like a native app
- Free, no app store, no account needed
A result you can review
No black-box numbers.
Each calculation keeps its working in frame, so a planning estimate can be understood in context and checked against the source information that matters to your facility.
Every result is traceable to its formula and its published source (ASHRAE, NIST or manufacturer data), so you can defend the number in a design review.
- 01 / TRACE
Inputs
The values and conditions behind the estimate stay visible.
- 02 / TRACE
Formula + source
See how the calculation is formed and where its method comes from.
- 03 / TRACE
Assumptions
Review the limits and assumptions before using the result for planning.
Every result is an estimate for planning only—not a substitute for a licensed engineer’s design or manufacturer data.
Calculator FAQsIndividual or team
Calculate freely. Keep the work when you need to.
Every calculator is free for individual, unsaved use. Paid plans add the tools to retain work, share it, and bring it into project records.
Included with paid plans
- Saved projects
- Branded PDF reports
- CSV exports
- Shared team workspaces
Your saved estimates stay yours: export them as CSV at any time. Read the privacy policy.
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