Carbon and water calculator
An excellent PUE on a coal grid is an efficient way to burn coal.
PUE says how much energy a building spends supporting IT. It says nothing about where that energy came from or what it cost in water. This converts the same facility into annual CO2e and cubic metres, and reports carbon usage effectiveness and water usage effectiveness beside the absolute numbers.
The ratios matter less than the totals. A facility can improve CUE by adding IT load, exactly as it can improve PUE, so the emissions figure is the one that has to fall.
Worked example — 250 kW at PUE 1.5 on a 300 g grid
Evaporative cooling at 1.8 L/kWh, with no clean-energy contracts in place.
- IT load — 250 kW. Annual-average power drawn by IT equipment.
- PUE — 1.5. Facility energy divided by IT energy, over the same year.
- Grid carbon intensity — 300 gCO2e/kWh. Emissions per kilowatt-hour delivered by the grid serving the site.
- Contracted clean energy — 0 %. Share of consumption matched by purchase agreements or certificates. This changes market-based reporting, not what the grid emitted.
- Site WUE — 1.8 L/kWh of IT. On-site water consumed per kilowatt-hour of IT energy. Closed-loop and air-cooled plant sits near zero; evaporative cooling does not.
Annual emissions: 986 t CO2e — Location-based, from 3,285 MWh of facility energy
| CUE Total emissions over IT energy — the carbon twin of PUE | 0.450 kgCO2e/kWh |
|---|---|
| Market-based emissions After matching 0% of consumption | 986 t CO2e |
| Facility energy | 3,285 MWh/yr |
| IT energy | 2,190 MWh/yr |
| On-site water 10,800 litres a day | 3,942 m³/yr |
| WUE Site water over IT energy — excludes water used to generate the electricity | 1.80 L/kWh |
| Emissions per kW of IT | 3.94 t CO2e/kW/yr |
A site WUE above roughly 1.2 L/kWh indicates evaporative or adiabatic cooling. That trade is often the right one for energy, and it is the trade PUE cannot see: water spent to lower the ratio.
Formula
CUE = facility energy × grid intensity ÷ IT energy · WUE = site water ÷ IT energy
Facility energy is IT load × PUE × 8,760 h. Location-based emissions use the intensity of the grid actually serving the site; market-based emissions subtract contracted clean energy. Both are normally reported, because only the first describes what was emitted.
Grid intensity, very approximately
Orders of magnitude for orientation, not reporting figures. Real intensity varies by hour, season and year, and an annual average hides the fact that a facility runs around the clock through both the clean hours and the dirty ones.
| Grid | Rough gCO2e/kWh | What dominates it |
|---|---|---|
| Iceland, Norway | Under 50 | Hydro and geothermal |
| France, Sweden | 50 – 100 | Nuclear and hydro |
| United Kingdom, Spain | 150 – 250 | Wind and gas |
| United States average | 350 – 400 | Gas, coal and a growing renewable share |
| Germany, Japan | 350 – 450 | Gas, coal and imports |
| Poland, India | 600 – 750 | Coal |
Location-based and market-based are both true
Location-based emissions use the intensity of the grid physically serving the site. Market-based emissions apply the contracts a company holds — power purchase agreements, renewable certificates, supplier-specific rates. The difference between them is an accounting boundary, not a physical one, and a facility reporting only the market-based figure is reporting the half of the story that flatters it.
Annual matching has the same problem at a smaller scale. A facility that buys as much wind over a year as it consumes still draws from whatever is generating at three in the morning in January. Hourly matching exists precisely because the annual version can reach zero on paper while the plant next door keeps running.
Water moves rather than disappearing
Evaporative and adiabatic cooling consume water on site and reduce electricity, which improves PUE. Closed-loop and air-cooled plant consume almost no site water and generally raise energy use. Neither is simply better: the right answer depends on whether the site is short of water, short of power, or short of both.
On-site WUE also excludes the water used to generate the electricity itself — thermal generation is water-intensive, and a facility with near-zero site water on a thermoelectric grid still carries a substantial indirect footprint. Source WUE captures that, and it is the figure most reports quietly leave out.
What this calculation does not include
- Embodied carbon in servers, the building, the concrete and the plant — increasingly the dominant share on a clean grid.
- Refrigerant leakage, which carries a global warming potential hundreds to thousands of times that of CO2.
- Backup generator runtime and fuel, including the testing schedule.
- Hourly grid variation, which is the whole basis of carbon-aware scheduling.
- Useful work. Emissions per unit of computation is the question that actually matters, and no facility-level metric answers it.
Frequently asked questions
What is CUE in a data center?
Carbon usage effectiveness is total facility CO2e emissions divided by IT energy, reported in kilograms of CO2e per kilowatt-hour. It is the carbon twin of PUE: a facility with a PUE of 1.5 on a grid at 300 gCO2e/kWh has a CUE of about 0.45 kgCO2e/kWh.
What is a good WUE for a data center?
Air-cooled and closed-loop facilities report site WUE near 0.1 L/kWh or below; evaporative cooling typically lands between 1.0 and 2.0. A low figure is not automatically better — it usually means water was traded for electricity, and whether that is the right trade depends on the site.
How do you calculate a data center carbon footprint?
Multiply IT load by PUE and by 8,760 hours to get annual facility energy, then multiply by the grid carbon intensity serving the site. A 250 kW facility at PUE 1.5 consumes about 3,285 MWh a year; on a 300 gCO2e/kWh grid that is roughly 985 tonnes of CO2e before any embodied carbon is counted.
Does buying renewable energy make a data center zero carbon?
It makes the market-based figure zero. The grid still emitted whatever it emitted at the hours the facility drew power, which is why location-based emissions are reported alongside, and why hourly matching is treated as a stronger claim than annual matching.
Why is PUE not a sustainability metric?
Because it is a ratio of energy to energy. It is blind to the carbon intensity of the supply, to the water spent lowering it, to the embodied carbon of the equipment, and to whether the IT load is doing anything useful. Two facilities with identical PUE can differ by an order of magnitude in emissions.
How much water does a data center use?
At a site WUE of 1.8 L/kWh, a 250 kW facility consumes roughly 3,900 m³ a year — about 10,800 litres a day. Air-cooled facilities consume a small fraction of that on site while typically using more electricity, which moves part of the water footprint upstream to generation.
Where this calculation stops
This is an operational-energy estimate at one boundary. It excludes embodied carbon, refrigerant leakage, generator fuel and the water used to generate the electricity, and any of those can dominate the total.
Grid intensity figures are approximations that move every year and every hour. Reporting requires the supplier-specific or published regional factors for the years in question, not a rule of thumb.