In brief
- “100 MW” is a measure of electrical power: how much electricity a site can draw at any one moment.
- It does not tell you how big the building is, how many jobs it brings, or how much water it uses.
- Four different “MW” figures are often mixed up. Always ask which one you are being given.
Power and energy are different things
- Kilowatt (kW), megawatt (MW), gigawatt (GW) are rates of power. 1 MW is 1,000 kW, like a thousand one-kilowatt electric heaters switched on together. 1 GW is 1,000 MW.
- A megawatt-hour (MWh) is an amount of energy: power multiplied by time. A terawatt-hour (TWh) is a million MWh.
- In household terms: a typical home in Britain uses about 2.9 MWh of electricity a year.[1] So 1 MW running all year is roughly the electricity of 3,000 homes, and a 100 MW site running flat out for a year would use about 876,000 MWh, roughly what 300,000 homes use. Real sites usually run below their maximum, so treat this as a ceiling, not a forecast.
Four different “MW” figures that get mixed up
| Figure | What it means | Watch for |
|---|---|---|
| IT load (or “rated IT load”) | The power available for the computers themselves.[2] This is the figure operators usually quote, and the one the UK Government uses for its 100 MW threshold in England.[3] | It leaves out cooling, so the site’s real demand is higher. |
| Total site power | The computers plus cooling, lighting and losses. | Always higher than the IT load. With a PUE of 1.3 (below), a 100 MW IT load needs about 130 MW in total. |
| Grid connection (sometimes in MVA) | The supply capacity reserved from the electricity network. | It can be larger than what the site will use, and it may hold up other connections in the queue. |
| Backup generators | Standby engines, usually diesel, used in a power cut and for testing. At Bro Tathan, for example, 72 generators totalling 120 MW.[4] | Not extra electricity use, so do not add them to the others. But they matter for air quality and fuel storage. |
Ask also whether a figure describes what is operating today, what has permission, or the whole campus when finished. Different sources quote different ones, which is why the same site can appear with two or three figures.
Other ways data centres are described
- Floor area (square metres or square feet): the size of the buildings. Two buildings of the same size can need very different power, especially if one is built for AI.
- Racks: the cabinets that hold servers. A traditional rack uses a few kW; AI racks can use 40 kW or more. “150 racks” is a small site; “100 MW” is very large.
- PUE (Power Usage Effectiveness): total site power divided by IT load. 1.0 would mean no overhead; most sites are between about 1.1 and 1.6. Lower is better. The draft policy for England expects 1.3 or better.[3]
- WUE (Water Usage Effectiveness): litres of water used for each kWh of computer energy. 0 would mean no water for cooling. Check whether a figure covers cooling only or all water on site, and whether it is an annual average (which hides the summer peak).
- Size labels: “edge” means small and local. Senedd Research describes an “enterprise” data centre as one run by an organisation for its own needs, with more than 10 MW of IT load, and “hyperscale” data centres as averaging 20 to 100 MW.[2]
- Tier: a resilience grade (how well the site keeps running through failures). It says nothing about size or impact.
- Behind-the-meter power: electricity made on or next to the site (for example by gas engines) that does not pass through the public network. In England the draft policy allows it before a grid connection only for “exceptionally strategic” projects, and only temporarily.[3]
- “Waterless”, “closed-loop” and “adiabatic” cooling: see how data centres use water. “Waterless” usually means little water on site in normal weather, not none at all.
Three questions to ask whenever a number is quoted
- Which MW is it: computers, whole site, grid connection, or generators?
- Is it running now, permitted, or planned for the full campus?
- What is the figure for water, does it cover cooling only or everything, and what is it on the hottest day?
Household comparisons are approximate. Industry water figures are mostly self-reported.[5]
Sources for this page
- Ofgem, ‘Average gas and electricity use explained’ (a typical household in Britain uses about 2,900 kWh of electricity a year). https://www.ofgem.gov.uk/information-consumers/energy-advice-households/average-gas-and-electricity-use-explained
- Senedd Research, ‘Data centres in Wales: How will Wales manage a rapidly growing industry?’ (July 2026): IT load, enterprise and hyperscale. https://research.senedd.wales/media/eytgh5cx/data-centres-in-wales.pdf
- UK Government, draft National Policy Statement for Data Centres (October 2026): “rated IT load”, the 100 MW threshold and the Power Usage Effectiveness of 1.3. https://assets.publishing.service.gov.uk/media/6ae1c5937fe45a08a4036e4e/draft-national-policy-statement-for-data-centres.pdf
- Vale of Glamorgan Council, report on application 2023/00780/FUL (Bro Tathan), released through WhatDoTheyKnow: 72 standby generators totalling 120 MW. https://www.whatdotheyknow.com/request/screening_of_datacentre_at_bro_t
- techUK, ‘Understanding data centre water use in England’ (August 2025; an industry report, free sign-up needed to read it). https://www.techuk.org/resource/techuk-report-understanding-data-centre-water-use-in-england.html