Single phase vs. 3-phase UPS: what’s the difference?

You just want to source the right UPS to protect your data centre but electrical installer geeks are spouting nonsense about ‘three-phase power’. Read on to discover all you need to know about choosing the right solution for your needs.

Why we have single-phase and 3-phase power

Electricity utility companies produce 3-phase power as standard because this is what the large-scale industrial energy consumers require. Rather than having one ‘wave’ of alternating current (AC), 3-phase power produces three. Like a three-cylinder engine, this creates more reliable and constant power output at higher quantities. It is also more efficient; delivering more power over fewer wires.   

Households and other domestic consumers use far less power and so transformers are situated in public power distribution networks to provide them with only one phase of the 3-phase power generated on the grid. This is called ‘single-phase’ power. It isn’t much use to big energy consumers but does a perfectly good job for most home and office needs.

Hence, in homes and offices, the power distribution system (the in-built infrastructure of sockets and wires) is geared up for single-phase power. In industrial settings, specialised power distribution systems will be in place for handling 3-phase power.

The kind of phase power governs the kind of systems you can use inside

3-phase power is the norm for heavy users of electricity, including data centres. In these instances, the systems within them have to be built to accommodate this form of electrical input. A standard electrical appliance for the home would not be suitable in such an environment as it will be manufactured to accommodate single-phase power only. 

As far as 3-phase power-ready systems in a single-phase environment is concerned, there is a level of compatibility. The 3-phase system will work, but there is no advantage to using it. The equipment you deploy should be optimised to the power distribution system in place.

Why 3-phase power is relevant to data centres

Having barely registered on the scale 10 years ago, data centres now consume more than 3% of all global electricity output – more than 400 terawatt hours of power a year. That’s around 50% more power than the entire UK uses up annually. 

Individually, the biggest data centres in the world require very large quantities of electricity and draw this in the form of 3-phase power. Some even have their own local electricity generation capabilities onsite or their own dedicated electricity substations provided by the utility company. 

Today’s data centres have reached ‘industrial’ levels of energy consumption.

The big question for data centre owners is, do my electrical consumption needs make it appropriate for me to use single-phase or 3-phase power distribution? This is especially pertinent to office-based server rooms and comms cabinets drawing standard single-phase power that might presently be overlooked because they are not seen as being ‘serious’ data centres. Just because these facilities are not in the same league as a Google mega data centre, it doesn’t mean they don’t share the same heavy-duty need for 3-phase power.

How to find out if you are on the right kind of phase power for your needs

If you don’t know already, find out whether your data centre environment is served by single-phase or 3-phase power. Your facilities/estates manager should be able to tell you, or you could ask your utility provider or a qualified electrical contractor. 

A general rule of thumb is, if your data centre currently consumes in excess of 20kW then single-phase power may not be appropriate to your needs (another measure would 20kVA; the unit of measurement is similar but not the same thing).

Ultimately, you might be drawing significantly less power than 20kVA and it could still be more appropriate to use 3-phase power. This would be if the systems supported within your data centre were deemed sufficiently business-critical to warrant the more stable attributes of 3-phase power, if not the higher capacity capabilities.

UPS solutions need to match your underlying power distribution

Choosing between single and 3-phase UPS relies heavily on whether you already have single or 3-phase power distribution within your data centre or comms room. There may even be some of each if you have multiple sites and infrastructures. 

If you are looking to replace an existing UPS then the most obvious path will be a like-for-like swap (i.e. single phase for single phase; 3-phase for 3-phase). The problem with this is that circumstances change and it might not make sense to stick with the same approach anymore.


  • Having single-phase power distribution in place will make single-phase UPS simple and 3-phase UPS difficult
  • Having 3-phase power distribution in place will make 3-phase UPS simple and single-phase UPS difficult
  • Even though it means more work, changing your underlying power distribution as well as your UPS might still be a smart move because your existing setup could be bad for your needs

Size your power load to check if a big enough single-phase UPS solution even exists

If you are on single-phase power and intend to stay that way, bare in mind the lack of applicable UPS solutions that exist beyond a certain scale. Single-phase UPS modules are rarely manufactured beyond 20 kVA (20,000 VA) so even using multiple modules in an overall solution for N+1 redundancy would restrict size to 60 kVA maximum.

Also think about what your future power load is likely to be, not just what you are presently using. Future power load requirements can be influenced by many factors including:

  • Upgrades to existing servers
  • Higher density equipment deployment
  • Increased IT hardware in the data centre
  • Increased cooling load to reduce temperature to safer levels
  • Overall business and data growth
  • Desire to increase UPS resilience through enhanced redundancy configuration  

You also need to remember that newer and more enterprise-oriented data centre IT equipment is geared for 3-phase power, not single-phase. This could be a major factor in your long-term decision making for power distribution and UPS.

If your power load is set to increase toward the maximum capacity allowed by single-phase UPS solutions, then even if your existing power distribution is single-phase it could be wise to change to 3-phase instead.

Four catalysts for switching up to 3-phase power distribution

The needs of some organisations will dictate that they remain on single-phase power, for which existing power distribution systems and UPS solutions are robust and in line with industry best-practice. However, it is unlikely than any organisation currently using 3-phase power and associated systems would move to a single-phase regime. This would entail a number of electrical engineering challenges to achieve a retrograde step.

The migration from single-phase to 3-phase power is a far more likely scenario in the context of a business-critical data centre with concerns about data growth and maximum uptime availability. The catalyst for this change is typically one of the following:

  1. A technology refresh of UPS solutions
  2. A strategic review of energy efficiency (PUE) and/or environmental sustainability within the data centre environment
  3. A downtime event caused by failure/s in electricity supply
  4. Investment in new data centre servers and/or other IT equipment