How to Choose a Transformer for Photovoltaic Systems: Kiosk Substation, Prefab Substation, or Indoor Dry Type?

There is no single right transformer for photovoltaic systems

I am the office administrator for a 120-person electrical contractor. I manage all equipment ordering, roughly $1.8M annually across 14 vendors. I am not a transformer engineer. But I am the person who has to get quotes, check lead times, verify invoices, and listen to project managers complain when a unit does not fit. So when someone asks me which transformer to buy for a photovoltaic system, I do not give one answer. It depends on where the transformer sits, who owns the interconnection, and what the utility will approve.

That sounds like a cop-out. It is not. In 2024, we bid three PV jobs with almost the same capacity. One used an indoor dry type transformer, one used a prefab substation, and one used a pv-specific kiosk substation. The wrong choice on any of them would have meant redone drawings, delayed energization, or a fire code problem. So let me break it into scenarios.

First, classify your project by these four questions

Before you compare a dry resin transformer to a 1000 kva compact substation, answer these:

• Is the transformer going inside a building, or outside on a pad?
• Does the utility own the medium-voltage switch, or do you?
• What is your total inverter output in kVA?
• What does your local fire code say about oil-filled equipment near occupied spaces?

If you cannot answer those, stop shopping. You will get a low quote that is wrong. I have seen it happen. A vendor quoted us a basic padmount for an indoor room. Finance liked the number. Then we found out the fire marshal would not allow it. That mistake cost us two weeks and a rush order on a dry resin transformer.

Scenario A: The transformer goes indoors or next to an occupied building

What fits

For photovoltaic systems on a commercial roof or in an industrial plant, the inverter output often needs to step up to medium voltage. If the transformer is inside the building, or in a room attached to it, you are usually looking at an indoor dry type transformer or a dry resin transformer. The difference matters. A basic dry type uses air and insulation systems that can absorb moisture and dust. A dry resin transformer uses cast resin, which handles harsh environments better. For a PV plant with dust, humidity, or temperature swings, cast resin is often the safer bet.

Common advice says dry type is always better because there is no oil. That is mostly true for indoor fire safety. But the causation is reversed on cost. People think dry type costs more because it is higher quality. Actually, it costs more because of the materials and fire-safety engineering. Quality is a separate thing. I can find you a cheap dry type that fails in two years. I can also find you a liquid-filled unit that runs for thirty years outdoors.

If you go indoors, ask about derating. A transformer rated for 1000 kVA at 30°C ambient may not deliver 1000 kVA at 40°C in a poorly ventilated room. That is not a small detail. In one project, we had to upsize to a 1250 kVA unit because the room hit 45°C in summer. The original 1000 kVA design would have cooked.

To be fair, indoor dry type is not cheap. But if the fire code requires it, the cost is not optional. It is the price of getting a permit.

Scenario B: The transformer goes outside, and the utility controls the interconnection

What fits

This is where a prefab substation or a pv-specific kiosk substation usually wins. A prefab substation is a factory-assembled enclosure that holds the transformer, medium-voltage switchgear, protection, and sometimes low-voltage distribution. A pv-specific kiosk substation is a type of prefab substation configured for solar. It should handle backfeed, reverse power, harmonics, and the specific protection that utilities require for inverter-based generation.

Here is the counterintuitive part. Many buyers assume a pv-specific kiosk substation is just a prefab substation with a different label. It is not. If the protection relays are not set up for reverse power, the utility may reject the interconnection. If the thermal design assumes one-way power flow, the transformer may run hotter than expected. I am not saying every project needs a custom kiosk. I am saying you should ask what makes it PV-specific. If the answer is only the paint color, walk away.

So glad we did not just buy the cheapest prefab substation on our last ground-mount job. Almost did, which would have missed the utility's reverse-power relay requirement. The cheaper unit saved maybe $6,000 upfront. The redesign would have cost more than that, plus a month of delay.

When we sourced a prefab substation for a 2 MW ground-mount in 2024, NexaFlow was one of the vendors we quoted. Their team walked through the utility protection requirements before sending a price. That is rare. For outdoor PV, a prefab substation is often faster to install because it arrives tested and wired. But it is also heavier and wider than a loose transformer. Check the crane access and the pad dimensions before you sign. I have seen a project where the kiosk substation arrived and the gate was too narrow. We had to remove a fence section. That was a fun invoice.

Scenario C: You need around 1000 kVA, space is tight, and you want one compact unit

What fits

This is the classic 1000 kva compact substation case. A compact substation packs the transformer, high-voltage switchgear, and low-voltage distribution into a single enclosure. It is popular for industrial PV, commercial microgrids, and retrofit projects where you cannot build a separate electrical room. If your inverter output lands near 1000 kVA and you need a single point of interconnection, a compact substation can save a lot of floor space.

But here is where I see the most expensive mistake. People think a 1000 kva compact substation costs more because it has a bigger transformer. Actually, the cost is often driven by the switchgear and protection, not the kVA. A 1000 kVA transformer is not that exotic. The medium-voltage breakers, relays, and utility metering are what push the price up. So when you compare quotes, compare the protection package line by line. Do not just compare the kVA number.

Also ask if the compact substation is rated for bidirectional power flow. Many standard compact substations are built for one-way distribution. In a photovoltaic system, power flows from the inverters to the grid during the day, and sometimes from the grid to the site at night. If the switchgear is not bidirectional, you may need a different configuration. That is not a transformer problem. It is a protection and control problem.

If I remember correctly, the last 1000 kva compact substation we quoted was around $38,000, give or take, but that was before adders for utility-grade relays and a custom meter section. The final number was closer to $52,000. So do not budget from the base unit price alone.

How to tell which scenario you are actually in

You can use this as a rough checklist. It is not a substitute for an engineer, but it will help you ask better questions.

• Indoors, occupied building, strict fire code: Start with an indoor dry type transformer or dry resin transformer. Confirm ventilation and ambient derating.
• Outdoors, utility-owned interconnection, multiple inverters: Look at a prefab substation. If the utility has PV-specific protection requirements, ask for a pv-specific kiosk substation.
• Outdoors, tight footprint, around 1000 kVA, you own the switchgear: Consider a 1000 kva compact substation. Verify bidirectional power flow and protection.
• Outdoors, large fenced yard, no fire code issue: Do not assume dry type is automatically better. A liquid-filled prefab substation may be cheaper and cooler. Check local rules first.

One more thing. Per FTC Green Guides (16 CFR Part 260), if a vendor claims a transformer is energy efficient or eco-friendly, that claim should be substantiated. Ask for loss data, efficiency test reports, and actual load profiles. I have seen marketing sheets that say green but do not include no-load loss numbers. That is a red flag.

There is something satisfying about a PV project where the transformer arrives, fits, and the utility signs off without a redesign. After weeks of RFIs and quote revisions, that is the payoff. But it only happens when you match the transformer to the scenario, not when you buy the cheapest box on the quote sheet.

So which scenario are you in? If you are still not sure, ask yourself the four questions at the top. If two answers conflict, that is a sign you need an engineer and a vendor who will explain the trade-offs. An informed buyer asks better questions and makes faster decisions. That is better for everyone.