Transformer Selection for Solar, Battery Storage and EV Charging Projects

Solar, battery storage and EV charging all load a transformer in ways a conventional building never does.

In a normal commercial installation the power flows one way, the current is roughly sinusoidal, and the load follows a predictable daily pattern. In a renewable or charging installation none of those assumptions hold. Power flows backwards when the sun is out or the battery discharges. Inverters and chargers draw non-sinusoidal current that heats a transformer more than the same apparent power of ordinary load. Equipment is sold in kilowatts while transformers are sold in kilovolt-amperes, and the two are not the same number.

This article explains what changes when the load is an inverter, a battery or a charger, and how to size and specify a distribution transformer for these projects.

Why these projects are different

Four differences drive every specification decision that follows.

Power flows in both directions. A solar or battery system exports as well as imports. The tap range, protection settings and vector group must all work for reverse power flow, not just for a load drawing from the grid.

The load is harmonic-rich. Inverters and DC chargers are not linear loads. They draw current in short pulses, which adds harmonic content, and harmonic current causes extra eddy-current loss in the transformer winding that a standard design does not account for.

Load profiles swing hard. A charging hub can sit near idle overnight and pull heavily at once. Cyclic loading makes the winding expand and contract, and repeated thermal cycling ages insulation faster than a steady load at the same average.

Equipment is rated in kW, transformers in kVA. This single mismatch causes more sizing errors than anything else. A transformer must be sized on the apparent power the equipment exchanges, not on the useful power it delivers.

Solar and PV projects

On a photovoltaic site the transformer’s load is the inverter, and grid codes usually require the inverter to operate across a power factor range, commonly 0.95 leading to 0.95 lagging, so that it can support voltage.

The kVA trap. Take an inverter rated 1,000 kW operating at 0.95 power factor. The apparent power it exchanges is 1,000 / 0.95, which is 1,053 kVA. A 1,000 kVA transformer would therefore be overloaded even though the inverter’s rating and the transformer’s rating look like the same number. Always size on the inverter’s kVA rating.

Voltage rise. Where a long medium voltage collector run separates the inverter from the point of connection, the voltage at the far end rises when the plant is exporting at full output. That is a reason to specify a wider tap range than a standard distribution unit carries, so the ratio can be adjusted once the plant is commissioned.

Site conditions. A PV transformer usually sits outdoors in a kiosk or pad-mounted enclosure, exposed to dust, sun and wide temperature swings, and often at a site where nobody visits daily. Sealed construction and a cooling design that does not depend on frequent attention are worth more here than a marginal gain in efficiency.

Battery energy storage

A battery system is the only common application where the transformer is genuinely bidirectional as a matter of routine operation, several times a day, every day.

Design for reverse flow explicitly. Protection must be coordinated so that a fault on the grid side is cleared correctly when the plant is exporting, and the tap changer must not be left in a position that only suits import. State the requirement in the specification rather than assuming it.

Cyclic duty. Charging and discharging on a daily cycle means the winding temperature rises and falls repeatedly. Insulation ages by the number and severity of hot cycles, so a storage transformer benefits from a design with lower hot-spot rise than a purely cost-driven unit.

Idle losses still run. A storage transformer is energised continuously even when the battery is neither charging nor discharging, which means no-load loss is paid for around the clock with no revenue attached. On a project where the plant may stand idle for long periods, that makes no-load loss a significant lifetime cost, worth evaluating on total cost of ownership rather than purchase price alone.

EV charging installations

AC charging points are a mild load. DC fast charging is a different problem, because a fast charger is a large power electronic converter that draws a highly distorted current.

Harmonic content. A modern DC fast charger may have a current total harmonic distortion of 25 to 30 percent at the point of common coupling. The harmonic currents do not carry useful power, but they do flow through the winding and generate additional eddy-current loss concentrated near the conductor surface. A transformer sized only on the fundamental current will run hotter than its designer intended.

Diversity and peak. Chargers are idle for most of the day, so it is reasonable to apply a diversity factor rather than adding every charger at full rating. But the peak must be respected honestly: six chargers starting together in the evening is a real event, not a statistical average.

Noise. Urban charging sites are frequently close to housing, and cooling fans are the loudest part of a transformer installation. That is a reason to prefer a design that meets the rating without forced cooling where the site is noise-sensitive.

Sizing for harmonic loads: a worked example

Six 60 kW DC fast chargers, charger efficiency 94 percent, displacement power factor 0.97, current total harmonic distortion 27 percent, and a standard transformer with no special harmonic rating.

Step 1. DC output: 6 x 60 = 360 kW

Step 2. AC real power at 94 percent efficiency: 360 / 0.94 = 383 kW

Step 3. Fundamental apparent power at 0.97 displacement power factor: 383 / 0.97 = 395 kVA

Step 4. Include harmonic current. A current distortion of 27 percent multiplies the rms current by the square root of one plus 0.27 squared, which is 1.036: 395 x 1.036 = 409 kVA

Step 5. Apply harmonic derating. Because the eddy losses in a standard winding are much higher under this harmonic spectrum, the usable capacity of a standard unit is commonly taken as 0.75 to 0.85 of nameplate. At 0.80: 409 / 0.80 = 511 kVA

Result: the project needs approximately 511 kVA of properly rated capacity. The next standard size is 630 kVA, or a 500 kVA unit built with a harmonic rating.

Now compare that with sizing on the fundamental alone. Step 3 gives 395 kVA, which rounds naturally to a 400 kVA transformer, and that unit would be running beyond its capability from the first evening of operation, with a hot winding, accelerated insulation aging and a warranty argument waiting to happen. The gap between 395 and 511 is the cost of ignoring harmonics.

What to put in the specification

A specification for these projects should state, in writing:

The required rating in kVA, calculated from apparent power and not from equipment kW

Whether the load is harmonic-rich, and the harmonic rating or derating factor required

The expected direction of power flow, so the tap range and protection suit both import and export

Maximum and average ambient temperature at the site, and the altitude

Cooling method and the rating required at each cooling stage

Sound level limit in decibels at the nearest sensitive boundary, if the site is near housing

The expected load cycle, so thermal cycling is considered rather than average load alone

Enclosure type and ingress protection for the site environment

Mistakes that appear after commissioning

Sizing on the inverter or charger rating in kW rather than the kVA it exchanges

Ignoring harmonic eddy loss, so the transformer overheats while apparently within its rated current

Selecting a vector group and protection scheme that only works for power flowing into the site

Specifying a standard tap range for a plant with a long collector run and significant voltage rise

Choosing forced cooling for a noise-sensitive urban site, then discovering the sound limit is exceeded when the fans run

Frequently Asked Questions

Why can I not size a transformer on the kW rating of an inverter or charger?

Because a transformer carries current, and current is set by apparent power, not by the useful power delivered. At a power factor of 0.95, 1,000 kW corresponds to 1,053 kVA of current-carrying duty. Sizing on kW understates the load the winding actually sees.

Do I need a special transformer for EV fast charging?

Not always, but the harmonic content must be assessed. Where current distortion is significant, either specify a transformer with a harmonic rating or apply a derating factor to a standard unit. Sizing on the fundamental current alone is the common error.

Does a battery storage transformer need different protection from a normal one?

Yes. Because power flows in reverse when the battery discharges, protection must be coordinated for both directions, and the tap position must suit export as well as import. This should be stated explicitly in the specification.

Is no-load loss important for a solar or storage transformer?

It is more important than usual, because the transformer is energised continuously while the plant may generate or discharge only part of the day. No-load loss is paid around the clock with no output to offset it, so it should be evaluated over the service life rather than ignored.

Related Products, Applications and Support

Sizing a transformer for a solar, storage or charging project? Send us your single line diagram and load list and we will check the rating for you, or email info@xsdfftransformer.com, call +86 158 6789 7761 or WhatsApp +852 5416 2620.