One decision that can eat away your solar ROI

It was supposed to be a quick return on investment.

A small 99 kW PV farm, set up by a farmer in a rural part of Europe, was expected to pay for itself within five years.

Everything checked out: the location, the panels, the inverters, the grid connection terms. Everything except one detail.

The transformer. A cheap, “universal” model that, on paper, could handle any system.

In reality? Excessive no-load losses, incompatibility with the medium-voltage grid, unstable voltage during peak hours, and months of frustrating back-and-forth with the distribution operator. Now, 18 months later, energy output still fails to meet expectations.

This blog is a remedy for mistakes like that. Written by engineers, for engineers — and for anyone building a PV farm with the help of a friend on the weekend.

If you're wondering which transformer to choose for a 50 kW, 100 kW or maybe 150 kW PV farm — you're in the right place.

You will learn which parameters actually matter, how to avoid mistakes that can cost thousands, and what questions to ask your substation designer before it's too late.

In this article, you’ll learn:

• When 50 kW is still a micro-installation and when it becomes a professional PV plant

• What parameters matter when selecting a transformer for 50, 100 or 150 kW PV systems

• Why a standard transformer might not work well with solar

• Whether it's possible to build a PV system without a transformer — and when

• How to select a PV transformer step by step, using real examples

• What mistakes investors and installers often make when choosing a transformer

• Dry vs. oil transformers — what pays off in agricultural vs. industrial settings

• How to stay compliant with your grid operator and deliver quality energy

Reading time: 12 minutes

What transformer for a 50, 100 or 150 kW PV farm?

It might not look like much — a PV installation rated at 50, 100 or 150 kW. It’s not a utility-scale solar plant, but it’s not residential either. Often it’s a private, agricultural or small business project with one goal in mind: not just to save money, but to earn it.

And this is exactly the power range where things often go wrong — in ways that are hard to reverse. The common denominator? One simple but high-stakes question: what transformer is actually right for a small PV farm like this?

On industry forums, in project documentation, in investor meetings, we keep hearing the same doubts:

• Is a 100 kVA transformer enough for a 100 kW PV plant?

• Should I oversize to 200 kVA “just in case”?

• Can I just use a stock transformer from the warehouse?

And that’s where the trouble starts. Because when it comes to PV systems in the 50 to 150 kW range, a transformer cannot be an afterthought. It’s not just about power. It’s about compatibility with the MV grid, resilience to voltage fluctuations, and understanding that at 50 kW, you are already playing in the professional league — not at home.

Is 50 kW still just a “system,” or is it already a PV farm?

From an investor’s perspective, 50 kW might still feel “small” — a few panel strings on a warehouse roof or a field near the main building. But in the eyes of energy law and the distribution network operator (DNO), 50 kW marks a turning point.

In practice:

• It is the upper limit of a micro-installation

• Anything beyond falls under the category of “small-scale renewable installation” (MIOZE)

Which means:

• No more simplified connection procedures

• A full design and approval process is now required

• Strict technical criteria apply, including harmonic distortion (THDi), voltage compliance, and galvanic separation

That’s why a transformer for a PV system in this category is not just a voltage adapter. It’s a fully integrated component of the electrical infrastructure. It must be compliant with MV grid specifications, resilient to variable load conditions, and designed with future upgrades or energy export in mind.Common mistakes? Unfortunately, all too familiar

Investors often limit the declared capacity to 49.9 kW to avoid the regulatory burden of MIOZE procedures. Yet they still order a 100 kVA transformer “just in case.” Or they install inverters that, at peak generation, push up to 110 percent of nominal power. The result?

• Higher no-load losses – the transformer operates outside its optimal efficiency range

• Increased harmonic distortion (THDi) – standard transformer cores are not designed to handle PV inverters

• Voltage spikes on the MV side – without voltage regulation at ±2.5 percent, synchronization and compliance issues start to appear

What was supposed to be “extra headroom” becomes a bottleneck. Good intentions turn into unexpected faults, performance drops, and delayed settlement with the grid operator.

What parameters define a good transformer for a 50 to 150 kW PV system?

It depends on the configuration, but the core rules are consistent:

• MV grid voltage – most commonly 15.75 or 20 kV, depending on region and local utility

• Transformer ratio – typically 0.4/15.75 kV, though 0.8/15.75 kV is required for 800 V inverter outputs

• Grounding – defined by the operator’s requirements: isolated neutral point, resistor grounded, or directly grounded

• Usage profile – rooftop PV for five-day operations or ground-mounted for seven-day continuous exposure

A 63 kVA transformer is usually sufficient for a 50 kW installation. But if you plan to scale up, it is better to consider 80 to 100 kVA. The condition: proper insulation rating (at least class F), cooling method (ONAN or AN), and a matching voltage ratio for the inverters.

Conclusion

If you're asking yourself which transformer to choose for a 50, 100 or 150 kW PV system, remember there’s no room for guesswork. It’s like choosing the foundation for a building. It might not draw attention at first, but everything else will depend on it. And the cost of getting it wrong stays with you long after the invoice is paid.

What kind of transformer does a PV system really need?

At first glance, a transformer seems like a simple component. Two windings, voltage conversion, an iron core. What could possibly go wrong?

Plenty. That assumption is one of the most common reasons why PV systems underperform. Using a standard transformer for an application it was never meant for creates a mismatch that’s invisible until energy losses, overheating or grid compliance issues show up.

Because solar is not like industrial power supply. There is no steady consumption around the clock. Instead, there are rapid surges at noon, near-zero flow at night, and high levels of harmonic distortion caused by inverters. As a result, the operational environment for PV transformers is fundamentally different.

A PV transformer plays a different tune

So what sets a PV transformer apart from a conventional one?

• Load profile
  In solar, the transformer faces highly asymmetrical conditions. No generation at night, peak output midday. Standard units are not built for such swings.

• Power direction
  In a PV system, power flows from the inverters into the grid – the opposite of traditional setups. This affects thermal behavior and winding design.

• Harmonics
  PV inverters produce current distortion, typically 6 to 10 percent THDi, sometimes more. A transformer for PV must have a suitable core, larger winding cross-sections, and often oversizing to prevent overheating under harmonic load.

• No-load conditions
  On cloudy days or low-irradiance periods, inverters may generate little to no power, but the transformer remains energized. In such cases, no-load losses become a real cost driver.

All this means that a standard transformer might "work" in theory, but in practice leads to reduced efficiency, higher bills, and frustrated technicians.

What are the minimum specs for a PV transformer?

• Insulation class: at least F (155°C), ideally H (180°C), for thermal safety under overload

• Cooling type: ONAN (natural oil and air cooling), ideal for outdoor transformers up to 250 kVA

• Low-voltage winding: matched to the inverter output (0.4 kV or 0.8 kV) – the wrong ratio can trigger failures

• Harmonic tolerance: windings and core must handle THDi levels up to 10 percent without excess losses

Example from the field:
A 150 kW PV farm using 800 V inverters was fitted with a 0.4/15.75 kV transformer. After just three months, problems emerged: overheating, inverter shutdowns, lost output. Diagnosis? A mismatch in voltage ratio. The transformer was replaced with a 0.8/15.75 kV unit with an amorphous core. Production rose by 11 percent, and the system finally delivered as promised.

Can a standard transformer be used in a PV system?

This question comes up surprisingly often. Can I use a regular transformer for a solar farm?

Technically, yes — if efficiency, durability and grid compliance are not your priority.

But if you expect your system to perform reliably for 15 to 20 years, the answer is simple: it’s not worth the risk.

Can you build a PV system without a transformer? When it works and when it’s asking for trouble

This is one of the most commonly searched questions among individual investors and small business owners. Does a PV installation really need a transformer? Especially in the 30 to 50 kW range, where the line between a micro-installation and a small PV plant is blurry and every additional component, including the transformer, adds to the cost. So the question arises — could you skip it?

PV without a transformer — wishful thinking or real option?

Let’s start with theory. A transformer in a photovoltaic system is not absolutely required from a physics standpoint. In certain technical conditions, it is possible to build a PV system without a dedicated transformer station. But those cases are the exception, not the rule.

When can a PV system operate without a transformer?

• Installed capacity is up to 50 kW — still qualifies as a micro-installation, so direct low-voltage (LV) grid connection may be allowed

• You have access to an internal LV switchboard (not part of the DSO infrastructure) — for example, expanding a factory’s existing internal grid

• Low-voltage inverters (3x400 V) — so no galvanic isolation or voltage step-up is required

• DSO accepts direct connection — which is often the most difficult part. Operators usually require isolation and voltage compliance with the grid

In such a configuration, instead of a transformer station, you must ensure:

• proper protection devices

• reactive power compensation

• harmonic filtering (e.g. active filters)

• continuous energy quality monitoring

But here’s the catch — very few installations meet all of these criteria simultaneously.

What could replace a transformer in a PV system?

In theory, a transformer can be "replaced" with a carefully configured system of inverters and filters. In practice, though, this is not really a substitution but a complete redesign. The inverters would need to ensure:

• output voltage matches the grid (e.g. 3x400 V, ±10%)

• harmonic distortion remains low (THDi < 4%)

• operation without galvanic isolation (which requires DC-side grounding)

• adaptation to variable load and reactive power demands

All of this increases the system’s complexity and cost. And often, it turns out that building a transformer station is actually the more economical choice. It’s not a paradox — it’s the result of the many roles a transformer plays in a PV system: voltage regulation, galvanic separation, harmonic filtering and protection against disturbances.

When is a transformer absolutely necessary?

• When capacity exceeds 50 kW — the system qualifies as a small-scale installation and falls under strict grid rules

• When connecting to a medium-voltage grid (15 or 20 kV) — a transformer is required, without exception

• When galvanic separation is required by the operator — which is the case in most countries

• When the system is far from the load center — for example, in a ground-mounted PV plant with no existing LV infrastructure

A transformer is not just a voltage step-up device. It is also a safety buffer that protects inverters from overvoltage and grid-side noise. It is what allows the system to meet the technical connection conditions — and without that, no grid agreement will be signed.

Conclusion — can you build PV without a transformer?

Yes, but only in specific setups. And usually only for smaller capacities, up to 30 to 40 kW. In every other case, a transformer is essential — not just because "rules say so," but because it determines:

• user safety

• grid operator approval

• the quality of injected power

• the long-term durability of your inverters

What transformer for a 50, 100 or 150 kW PV plant? Technical specs and real-life examples

You walk onto the construction site. PV tables are mounted, inverters are wired, the foundation for the substation is in place. Everything looks great — until you look at the transformer. It’s a stock 160 kVA unit with a 0.4/15.75 kV ratio. Sounds good? Maybe — but if your inverters output 800 V, that transformer could be a time bomb.

At Energeks, this is not theory. This is our daily reality.

What transformer for a 50 kW PV system?

For a 50 kW installation with 3x400 V inverter output, the typical transformer setup is:

• 63 kVA

• 0.4/15.75 kV or 0.4/20 kV ratio

• ONAN cooling

• Voltage regulation ±2 x 2.5%

• Insulation class F

• No-load losses up to 350 W

This configuration meets MV grid requirements, enables safe connection to the DSO switchgear, and helps compensate basic inverter-generated harmonics. And let’s be clear — even in small PV farms, the transformer is not just a “step-up box.” It stabilizes the entire system.

What transformer for a 100 kW PV system?

This is where things get serious — especially because of the increased peak current levels. For a 100 kW PV plant, we recommend:

• 125 kVA

• 0.4/20 kV or 0.8/15.75 kV ratio depending on inverter specs

• Core rated for THDi up to 8 to 10%

• Insulation class H for improved thermal endurance

• No-load losses up to 600 W, load losses around 1.5 kW

A common question is: is 100 kVA enough for a 100 kW system? The answer is — only under ideal conditions. In practice, a 20 to 25 percent oversize margin helps maintain efficiency and system life, especially for projects expected to operate 15 to 20 years.

What transformer for a 150 kW PV system?

At this scale, any mismatch in specs can quickly compromise safety and grid compliance. A typical configuration:

• 160 to 200 kVA (most commonly 200 kVA)

• 0.8/15.75 kV ratio — necessary for 800 V inverters like SolarEdge or SMA CORE2

• Amorphous or oversized conventional core

• ONAN or AN cooling, depending on indoor or outdoor mounting

• Voltage regulation ±2 x 2.5% or even ±5%

• Harmonic resilience: THDi up to 12%

A frequent mistake? Using a 0.4/20 kV transformer with 800 V inverters. The result: inverter overheat alarms, voltage mismatch, and a drop in output by 8 to 10 percent versus expected production.

Does the transformer have to be bigger than the PV capacity?

This comes up almost as often as “can I save money on cables?”

In theory, the transformer can match inverter output exactly. In practice:

• it should be oversized by 10 to 15 percent

• account for cable losses

• allow for short-term overloads on sunny days

• give room for future expansion

So for a 150 kW PV plant, a 200 kVA transformer is not overkill. It’s standard good practice that ensures stability and compliance.

Step-by-step transformer selection for a PV farm

• Check inverter output voltage — is it 400 V or 800 V?

• Choose the right transformer ratio — based on grid voltage (15.75 or 20 kV)

• Account for THDi — if above 8 percent, choose a unit with reinforced low-voltage windings

• Verify short-circuit level of the MV grid — transformer withstand must match it

• Select insulation and cooling — class H and ONAN are a solid baseline

This is not a spreadsheet. It’s a construction site. A transformer for a 50, 100 or 150 kW PV plant has to withstand 365 days of work per year, with variable loads, under real grid conditions. A poor choice can cost you not only your warranty — but the profitability of the entire system.

Why your PV transformer overheats: 5 mistakes that only show up after commissioning

On paper, everything looked perfect. Inverter output: 100 kW. Transformer: 125 kVA. Manufacturer efficiency: 98.4 percent. Sizing margin: 25 percent. Spreadsheet says the return on investment is five years. The investor is happy. The installer too.

Then comes real life. Inverters start disconnecting around noon. Voltage at the low-voltage busbar swings unpredictably. Transformer temperature hits 95°C on a warm afternoon — and that’s not even at full load. What went wrong?

A transformer is not a number — it’s a behavior in a system

A PV transformer is a dynamic component. It operates in a system where everything changes hourly — irradiance, load, grid voltage, harmonic content. And a spreadsheet knows nothing about clouds, surges, or inverter behavior.

Here are the five most common mistakes that do not show up on the drawing board — but appear after the PV plant goes live.

1. Transformer too small for real-world overproduction

A 100 kW PV system can easily generate 110 to 115 percent of its nominal power on sunny days. That’s normal — panels are often rated above STC and optimized for extra output. But a 125 kVA transformer with no headroom for overloads? That’s a bottleneck.

Symptoms:

• inverter disconnections during peak sun

• transformer overloads and thermal alarms

• higher-than-expected load losses

What to do: if you’re asking should the transformer be larger than the inverter power, the answer is yes — smart oversizing (10 to 15 percent) is an industry standard, not a luxury.

2. Wrong voltage ratio

One of the most frequent field errors. Your inverters output 800 V, but someone orders a 0.4/15.75 kV transformer because “that’s what we always use.” The result? Voltage mismatch, inefficient operation, overheated windings, and inverter faults.

Fix: always verify your inverter AC output. SMA CORE2 and SolarEdge SE100K require 0.8/15.75 kV, not 0.4 kV.

3. No resilience to harmonics

PV inverters generate non-sinusoidal current. THDi levels can easily hit 8 to 10 percent, especially at partial load. Standard transformers rated for <3 percent THDi cannot handle this distortion.

Consequences:

• overheating of core and windings

• higher iron and copper losses

• shorter insulation life span

What to look for: choose a PV-specific transformer with low-loss core material, reinforced windings, and thermal headroom for harmonics.

4. Ignoring the short-circuit level of the MV grid

Designers focus on transformer size and ratio but forget to check short-circuit levels at the point of connection. If the MV grid can deliver 16 to 20 kA and your transformer is only rated for 12.5 kA, it may fail on the first switching surge.

Risk: winding deformation or insulation breakdown due to undervalued withstand strength.

Pro tip: always ask your DSO for fault level data and confirm that your transformer’s mechanical and dielectric specs match.

5. No voltage regulation on the primary side

MV grid voltage is not a constant. It fluctuates — especially in regions with high renewable penetration. If your transformer has no primary-side regulation taps (±2 x 2.5 percent), matching inverter output to grid voltage becomes guesswork. Inverters do not play well with guesswork.

Outcome: inverters disconnect due to overvoltage, poor power quality, rejected compliance tests.

Recommendation: voltage regulation on the MV side is low-cost insurance for long-term grid compliance and uptime.

What to verify before you switch on your PV transformer

• Is the transformer rated with enough margin for real-world peaks?

• Is the voltage ratio compatible with the actual inverter output?

• Can the core and windings handle high THDi?

• Does the withstand rating match the fault level of the MV grid?

• Is there voltage regulation on the MV side?

Because a PV transformer that looks fine on paper can fail in real life by week one. And instead of ROI, you’re looking at RMA.

Dry or oil transformer? What pays off — in the field, in a container, or inside a facility

If there is one question that keeps coming up in PV investment discussions, it is this one: “Should I go with a dry or an oil-immersed transformer for my solar farm?” It sounds simple enough. But the answer depends on many variables — and what seems cheaper at first is not always better in the long run.

Although datasheets for both technologies may look similar, real-life working conditions tell a different story. Ambient temperature, humidity, installation location, cooling capacity, and daily load profile all shape performance. And the wrong choice here? It will show up not on day one, but in year two — when your inverters start to complain.

Oil-immersed transformer — the workhorse of containerized and field-mounted PV

Let’s begin with the classic solution: the ONAN (Oil Natural Air Natural) transformer. This is the most common choice for containerized substations and pole-mounted systems used in open-air PV farms.

Why it works:

• Superior cooling performance — the oil bath stabilizes temperature during sustained output

• Better tolerance to overloads — ideal for high midday peaks

• Lower cost at higher power levels — especially above 160 kVA

• Greater harmonic resilience — oil-immersed cores handle non-linear loads more effectively

An oil transformer is a long-term, outdoor-ready solution, especially in regions with wide temperature swings from winter to summer. It fits perfectly in prefabricated container stations, ensures galvanic isolation, and allows for relatively easy servicing.

Field example:
A 150 kW ground-mounted PV installation using SMA CORE2 inverters (800 V AC) was paired with a 200 kVA ONAN transformer, 0.8/15.75 kV ratio, insulation class H. After two full seasons, the system remained stable, cool, and fully compliant — no shutdowns, no alarms, no complaints.

Dry-type transformer — clean, quiet, and safe for indoor solar systems

The dry-type resin-insulated transformer (AN) is the go-to choice when the substation is located inside a building — a warehouse, a manufacturing hall, or a commercial facility with rooftop PV.

Key advantages:

• No oil, no risk of leakage — no containment basin needed

• Environmental safety — easier to pass fire safety inspections

• Lower noise levels — typically 50 to 55 dB, ideal near offices or equipment

• Compact footprint — can be installed in technical rooms with limited space

However, dry transformers are not perfect. They do not handle overloads as well, are more sensitive to humidity, and rely entirely on passive cooling, which can be insufficient in higher power classes unless additional ventilation is installed.

Case study:
A rooftop PV system of 100 kW on a production facility used a 125 kVA dry-type transformer, 0.4/20 kV. Thanks to the quiet operation and lack of oil, the unit was installed just a few meters from occupied office space, with no special fire separation required. The result? Fast commissioning and zero complaints from facility management.

Oil or dry? Choose based on where it lives

Here is how to compare the two, not on paper — but where they will actually operate:

• Installation site
  Oil transformer: outdoors, in container stations
  Dry transformer: indoors, in technical rooms or warehouses

• Cooling performance
  Oil transformer: very efficient, natural circulation
  Dry transformer: moderate, passive cooling only

• Overload tolerance
  Oil transformer: high
  Dry transformer: medium

• Containment needs
  Oil transformer: yes — spill basin or protective barrier
  Dry transformer: none

• Noise levels
  Oil transformer: 60 to 65 dB
  Dry transformer: 50 to 55 dB

• Humidity resistance
  Oil transformer: high
  Dry transformer: lower

• Cost above 160 kVA
  Oil transformer: lower
  Dry transformer: higher

Don’t ask “which is better” — ask “where will it work?”

If your PV installation is located in an open field or a prefabricated container substation, an oil-immersed transformer is the better option. It offers flexibility, strength, and better thermal performance.

If you are building inside a facility or near office areas, and environmental or acoustic limits are a factor, then a dry-type transformer is often the only viable solution.

Both have their place. What matters is selecting the right one for your project’s specific context, not just what’s in stock.

A transformer is a strategic choice — not just an electrical detail

A transformer may not be the most visible part of your PV system. But it is one of the most consequential. It affects energy quality, uptime, compliance with the grid operator, inverter durability, and — ultimately — the financial performance of your investment.

Whether you are designing a 50 kW micro-installation or scaling up to a 150 kW rooftop or ground-mounted PV plant, choosing the right transformer is a decision that pays off for years. It is not just about matching ratings. It is about building a system that works — every day, every season, with zero surprises.

At Energeks, we work with designers, installers, and investors across Europe who want smart, field-tested energy solutions — not catalog copy.

If you want to:

• consult a transformer selection with one of our engineers

• check the availability of PV-ready dry or oil models

• compare setups for rooftop, field, or container-based stations

visit our current offering here:
🔗 energeks.com/offer

And if you value honest engineering stories, real-life case studies, and technical wisdom that goes beyond datasheets — we’d love to connect with you on LinkedIn.

🔗 Follow Energeks on LinkedIn

Let’s keep building solar the right way — with focus, care, and a long-term mindset.

Thank you for reading. If you found this helpful, feel free to share it or reach out.
We’re always happy to exchange ideas with those who treat energy like it matters.

Sources:

C57.110-2018 - IEEE “Recommended Practice for Establishing Liquid-Immersed and Dry-Type Power and Distribution Transformer Capability When Supplying Nonsinusoidal Load Currents”

NREL.GOV: Inverters: A Pivotal Role in PV Generated Electricity

IEC 60076-1:2011, Power transformers - Part 1: General

Photo Cover: Trinh Tran pexels/191284110-14613940