Methodology
Every number this tool produces comes from the formulas below. Nothing is hidden, because a sizing estimate you cannot defend is not worth having. Calculator version 1.1.0.
Load and heat
IT load from a device list
IT load (kW) = Σ(qty × nameplate W) ÷ 1000 × utilization
Nameplate ratings describe the maximum a power supply can draw, not what the machine actually pulls. Real draw commonly lands at 40–70% of nameplate, so the utilization factor is applied to avoid oversizing every downstream component. Metered PDU or UPS output data beats this estimate every time.
Apparent power
kVA = kW ÷ power factor
Electrical infrastructure is sized on apparent power, not real power. Modern server supplies run at 0.95–0.99 power factor.
Heat rejection
BTU/hr = kW × 3412.142 tons = BTU/hr ÷ 12,000
Every watt delivered to IT equipment leaves the room as heat. There is no envelope, solar, or occupancy term as in conventional HVAC, which makes the calculation simpler and the absolute numbers much larger. One ton of refrigeration equals 12,000 BTU/hr.
Facility power
Facility kW = IT load kW × PUE
PUE captures everything that is not IT load: cooling, distribution losses, lighting. A well-run enterprise room lands near 1.4–1.6. Liquid-cooled AI halls run lower, often 1.2–1.3.
Annual energy and cost
Annual kWh = facility kW × 8,760 Annual cost = annual kWh × $/kWh
Assumes the facility runs at the modeled load continuously, which is the right assumption for infrastructure sizing and a conservative one for a cost estimate. Real utility bills add demand charges and time-of-use rates that this does not model.
Electrical
Circuit capacity
Single phase: kVA = V × A ÷ 1000 Three phase: kVA = V × A × √3 ÷ 1000
Nominal capacity before any derate is applied.
Continuous-load derate
Usable kVA = circuit kVA × 0.8
A load running three hours or more is continuous, which every production rack is. Conductors and overcurrent devices must be rated for at least 125% of the continuous load, which works out to loading a circuit no higher than 80% of its rating. This is the single most common finding in a rack power audit.
NEC 210.20(A), 215.3
Circuits required
Circuits = ceil(rack kVA ÷ usable circuit kVA)
When you enter the circuits a rack already has, the tool measures your load against that instead and flags it if the load does not fit.
UPS
Required capacity
UPS kVA = load kVA ÷ target load fraction
Sizing a UPS to run at 100% of its rating leaves no headroom for growth or for the inrush of a restart. A target of 80% is a common design point.
Redundancy multiplier
N = 1.0 N+1 = (n + 1) ÷ n 2N = 2.0
N+1 depends on how many modules carry the load. Four modules plus a spare installs 1.25× the required capacity; two plus a spare installs 1.5×. 2N means two complete independent systems, each able to carry the full load.
Battery runtime is deliberately not calculated. Runtime depends on the specific battery string, its age, its temperature, and the manufacturer’s discharge curve. Any tool that gives you a runtime number from load alone is guessing. Take your kVA figure and your runtime target to a vendor and ask for the curve.
Thermal environment
ASHRAE TC 9.9 defines a recommended envelope, which is the same across classes, and an allowable envelope, which widens by class. Operating inside allowable but outside recommended is a deliberate trade, not a free one: it raises fan power and shortens the ride-through window when cooling fails.
| Class | Recommended | Allowable | Typical use |
|---|---|---|---|
| Class A1 | 64.4–80.6°F | 59–89.6°F | Enterprise servers, storage arrays. Tightly controlled. |
| Class A2 | 64.4–80.6°F | 50–95°F | Volume servers, networking gear. Most common enterprise class. |
| Class A3 | 64.4–80.6°F | 41–104°F | Equipment rated for wider ambient, often edge deployments. |
| Class A4 | 64.4–80.6°F | 41–113°F | Widest allowable envelope. Verify every device supports it. |
Battery life at elevated temperature
Remaining life ≈ 100% ÷ 2^((ambient°F − 77) ÷ 15)
VRLA battery life halves for roughly every 15°F above the 77°F reference. A UPS room held at 92°F is on track to replace batteries at about half the rated interval.
Cooling technology thresholds
Per-rack density, not total room load, is what decides cooling approach. These bands are industry convention rather than a published standard, and they have moved every year since 2023 as AI deployments pushed density up. ASHRAE TC 9.9 classifies anything above 20 kW per rack as high density. Well-contained air realistically tops out near 15–20 kW; containment plus rear-door heat exchangers stretch that to roughly 40–50 kW; above about 50 kW there is no air-only option. For reference, an NVIDIA GB200 NVL72 rack draws roughly 120 kW and GB300 NVL72 lands between 132 and 142 kW, both shipping liquid-cooled with no air variant.
- ≤ 5 kWPerimeter CRAC/CRAH
Room-level cooling handles this comfortably. Blanking panels and basic airflow hygiene are enough.
- ≤ 12 kWPerimeter cooling with containment
Hot or cold aisle containment becomes necessary. This band covers most enterprise rooms — Uptime put modal density at 11 kW in 2026, up from 9 kW the year before.
- ≤ 20 kWContainment with in-row cooling
Room-level air is at its practical limit. In-row units placed beside the load are the usual answer. ASHRAE TC 9.9 classifies anything above 20 kW as high density.
- ≤ 50 kWRear-door heat exchanger or in-row
Beyond well-contained air. RDHx moves heat exchange to the cabinet and stretches air-based designs to roughly 40-50 kW, at rising cost and complexity.
- ≤ 150 kWDirect-to-chip liquid cooling
There is no air-only option above roughly 50 kW. This is the AI training band: GB200 NVL72 draws about 120 kW and GB300 NVL72 lands at 132-142 kW, and those ship liquid-cooled with no air variant.
- 35+ kWImmersion or rack-scale liquid, with high-voltage DC
Above 150 kW you are in announced-roadmap territory — Rubin-class racks are specified at 190-230 kW and Rubin Ultra near 600 kW. Expect 800 VDC distribution, structural floor loading review, and factory-integrated pods rather than rack-by-rack design.
Receptacle reference
What this tool does not do
- Size conductors or specify breakers. Ampacity depends on conductor type, insulation, bundling, and ambient — that is an engineered calculation.
- Model airflow. CFM, containment effectiveness, and recirculation need CFD or measurement, not arithmetic.
- Calculate battery runtime, for the reason given above.
- Model demand charges, time-of-use rates, or utility tariffs. The annual cost figure is load × hours × rate, nothing more.
- Size coolant distribution units, flow rates, or facility water for liquid-cooled deployments. Above 50 kW per rack the mechanical design is a specialist exercise.
- Replace a licensed professional engineer. Use this to frame the problem and check an assumption, then have the design stamped.