Megavolt-ampere to megawatt conversion

In Con Edison's own hosting capacity map FAQ section, one question reads, “does the stated capacity in MVA need to be multiplied by a power factor to get MW?” ConEd’s answer is no, despite the MVA label written on the field, the values given are already in MW.

The confusion on ConEd’s labeling is understandable because MVA and MW aren't measuring the same thing. A megawatt is power doing work. A megavolt-ampere is apparent power, the total electrical load a piece of equipment has to be sized to handle, including the portion that oscillates between source and load without doing useful work. The two are connected by the power factor, typically around 0.9 for most industrial and commercial loads, but that's a characteristic of the load, and it’s not a universal constant.

In practice, a transformer rated 100 MVA doesn't deliver 100 MW to loads. It delivers closer to 90, depending on the power factor of whatever load is connected. If a utility treated the two units as interchangeable, every MVA-labeled site would look 10% roomier than it is next to a MW-labeled one. For many candidate sites, a 10% swing is the difference between a comfortable margin and a tight one.

Power factors

The power factor assumption isn't always the same across sites. Older industrial equipment including motors, HVAC compressors, fluorescent ballasts tend to have a power factor of 0.85 to 0.9 because they’re drawing reactive as well as real power.

Modern data center IT load is different, since their server power supplies use active power-factor correction and therefore typically run close to unity. This means that the MVA and MW numbers converge. Some utility interconnection tariffs even set a minimum power factor, often 0.9 to 0.95, as a condition of service.

The right conversion factor to apply depends on the prospective tenant, so the only way to convert MVA to MW with confidence is to go to the utility directly for which convention its published number uses and what power factor to use.