Solar Panel Types Explained: Mono, Poly, HJT & TOPCon

Last year we pulled a set of polycrystalline panels off a roof in Pearl City. They were 14 years old, faded to a milky blue, and producing about 60% of their original rating. The homeowner thought solar had failed him. It had not. His panels had.

The technology inside a solar panel matters more than most installers admit. A panel is not a panel is not a panel. On Oahu, where roof space is tight, salt air is constant, and HECO power costs over $0.40 per kilowatt-hour, small differences in heat performance and degradation turn into real money fast. That is why the cheapest panel on a quote is often the most expensive one to own.

Here is the straightforward version: polycrystalline is obsolete, standard mono PERC is still perfectly serviceable, TOPCon is the current value leader, and HJT is the premium choice if you want the best long-term production on a hot Hawaii roof. The details below explain why.

The Five Panel Technologies You Will Encounter

Panel Type	Typical Efficiency	Heat Performance	Best Use
Polycrystalline	15–18%[1]	Weak	Do not install on a modern residential roof
Mono PERC	20–22%[1]	Good	Budget-conscious residential systems
HJT	22–23.5%[2]	Best	Tight roof space, highest long-term output
TOPCon	22–23%[3]	Very good	Best value for most homeowners
Thin-film	11–15%[1]	Excellent	Commercial or utility-scale, not typical homes

If you only remember one thing from this article, remember this table. For most homes in Hawaii, the real choice is TOPCon versus HJT.

Polycrystalline: The One We Stopped Installing

You can spot poly panels by their blue, speckled look — multiple silicon crystal fragments melted together into a single cell. They run 15–18% efficient^[1] with a temperature coefficient around -0.40% to -0.45% per °C on datasheets, or about -0.22% to -0.25% per °F, annual degradation of 0.5–0.7%, and typical wattages of 300–350W.^[6]

A decade ago, poly was the industry standard. Not anymore. On a 500-square-foot roof section in Kailua, poly panels might produce 4.5 kW. The same footprint with modern monocrystalline panels gets you 6.5 kW. On an island where most homes have compact roofs and tall trees casting afternoon shadows, that efficiency gap is not academic. We stopped installing polycrystalline years ago.

Monocrystalline (Mono PERC): The Workhorse

Standard mono PERC panels — the black or dark blue rectangles you see on most roofs today — use a single silicon crystal structure with Passivated Emitter and Rear Cell technology. They hit 20–22% efficiency^[1], carry a temperature coefficient of -0.35% to -0.40% per °C on datasheets, or roughly -0.19% to -0.22% per °F, degrade at 0.4–0.55% annually, and come in 390–430W modules.^[6]

Solid technology. Still the majority of installations worldwide. If your roof is large, mostly unshaded, and the quote difference is meaningful, mono PERC can still make sense. But the next two options have pulled ahead in ways that matter specifically for Hawaii: better heat behavior and slower performance loss over time.

HJT (Heterojunction): The Best Performer in Heat

HJT panels sandwich thin layers of amorphous silicon around a crystalline core. The REC Alpha Pure-R series is the flagship example: 22–23.5% efficient, 430–470W per panel, and annual degradation of just 0.25% or less.^[2] But the number that matters most in Hawaii is the temperature coefficient: -0.24% to -0.26% per °C, which works out to about -0.13% to -0.14% per °F.^[2]

Here is why that matters. Solar panels are rated in a lab at 77°F (25°C). Your roof in Ewa Beach on a July afternoon? Cell temperature can hit 149°F (65°C). At that temperature, an HJT panel loses roughly 9.6% of its rated output. A standard mono panel loses 14–16%. That is not a rounding error. Over 25 years, on a 10 kW system, the HJT advantage adds up to thousands of extra kilowatt-hours — energy you would otherwise buy from HECO at Oahu's painful power rates.

REC guarantees 92% output at year 25.^[2] No other panel technology comes close to that warranty number.

TOPCon: Premium Efficiency at a Better Price

TOPCon adds an ultra-thin tunnel oxide layer to improve electron flow. Hyundai is one of the leading manufacturers using this architecture.^[3] The specs land between mono PERC and HJT: 22–23% efficiency, 420–450W per panel, -0.29% to -0.34% per °C temperature coefficient on the datasheet, or about -0.16% to -0.19% per °F, and 0.35–0.4% annual degradation.^[3]

TOPCon panels are slightly easier to manufacture than HJT, which keeps the price competitive. Their heat performance is better than standard mono but not quite as strong as HJT. For a homeowner in Mililani or Kapolei comparing quotes, TOPCon often hits the sweet spot between performance and budget. In plain English: if you want premium output without paying for the absolute best panel on the shelf, this is usually the answer.

Thin-Film: Not for Your Roof

Thin-film panels (CdTe, CIGS) deposit photovoltaic material on glass or flexible substrates. They achieve 11–15% efficiency^[1] with excellent temperature coefficients of roughly -0.20% to -0.25% per °C, or about -0.11% to -0.14% per °F, but each panel only produces 80–150W. You would need to cover an enormous area to match what crystalline panels do in a fraction of the space. Thin-film belongs on utility-scale ground mounts and commercial warehouses, not on the 600-square-foot roof of a three-bedroom house in Kapolei.

What Actually Moves Your Electric Bill

Most homeowners focus on efficiency because it is the biggest number on the brochure. That is understandable, but incomplete.

Efficiency matters because roof space is limited. A higher-efficiency panel lets us fit more watts on a chopped-up roof in Hawaii Kai or a narrow roof face in Kaimuki. But once you are comparing two modern premium panels, the bigger differences are heat loss, annual degradation, and warranty strength. Those are the numbers that determine what your system is still doing in year 12, year 18, and year 25.

If a salesperson only talks about wattage, ask one question: "What is the temperature coefficient, and what does the warranty guarantee at year 25?" That question separates people who design systems from people who just move boxes.

The Number That Matters Most on a Hawaii Roof

Temperature coefficient. Full stop.

Panels are rated at 77°F (25°C) in a lab. On your roof, cell temperatures routinely hit 131–158°F (55–70°C). For every degree above that test point, you lose a percentage of output — and that percentage varies dramatically by technology. At a typical midday rise of 54°F above STC, which is the same as a 30°C rise, a poly panel at -0.43% per °C loses 12.9% of rated power. Mono PERC at -0.37% loses 11.1%. TOPCon at -0.30% loses 9.0%. HJT at -0.25% loses just 7.5%.

The gap between worst and best is over 5 percentage points of real production, every single day your system operates. Over 25 years, that gap compounds into a meaningful pile of kilowatt-hours — and at Hawaii electricity rates, a meaningful pile of money.

Technology	Typical Temp Coefficient	Loss at 149°F Cell Temp
Polycrystalline	-0.43% per °C	About 17.2%
Mono PERC	-0.37% per °C	About 14.8%
TOPCon	-0.30% per °C	About 12.0%
HJT	-0.24% per °C	About 9.6%

149°F cell temperature equals 65°C, a realistic black-roof summer condition in leeward Oahu. Datasheets publish temperature coefficients in °C, so that is the convention used in this comparison.

What We Recommend for Hawaii Homes

For most homes, we do not think this is a five-way debate. It is a two-way debate.

If your roof is small, chopped up by dormers, or expected to carry rising electric loads from an EV, pool pump, or new air conditioning, buy the best panel you can justify and stop chasing the lowest quote. More watts per square foot and slower degradation are worth paying for on a constrained roof. That is where HJT shines.

If your roof has decent space and you want strong economics without buying the top shelf option, TOPCon is the better play. It is meaningfully better than older mono PERC, close enough to HJT in performance to satisfy most homeowners, and usually easier on the project budget.

We have seen too many homeowners in Ewa Beach and Waipahu approve a cheap quote, save a few thousand dollars up front, then spend the next decade wondering why the system never quite hits the rosy production estimate. Panel choice is part of that story. So is installer quality. But when you start with better equipment, the margin for disappointment gets much smaller.

What We Install

At Alternate Energy Hawaii, we carry two panel lines, and we picked them for specific reasons.

The REC Alpha Pure-R 460W (HJT) is our premium choice. It delivers 22.6% efficiency with one of the best heat coefficients available for a residential panel: -0.24%/°C, or about -0.13%/°F.^[2] REC is a Tier 1 manufacturer with a 25-year product warranty and a 25-year performance guarantee of 92% output.^[2] The all-black aesthetic looks clean on any roofline. We put these on a south-facing roof in Hawaii Kai recently and the homeowner's monitoring app showed production exceeding the design estimate in the first week because the real conditions were slightly cooler than our conservative model assumed.

The Hyundai HiE-S440VG (TOPCon) is our high-value option. At 22.0% efficiency and -0.30%/°C, or roughly -0.17%/°F, it delivers strong performance without the HJT price premium.^[3] It also carries a 25-year warranty.^[3] For homeowners who want quality panels but need to keep the total project cost under control, this is the one we recommend most often.

We pair both panels with Enphase IQ8 microinverters^[4] for panel-level optimization and monitoring, so each module operates at its own maximum regardless of shading or orientation differences across your roof.

If your roof space is tight, go HJT for maximum watts per square foot. If budget is the priority, TOPCon delivers excellent performance at a lower cost per watt. If you are near the coast in Ewa Beach or Kaneohe, both hold up well — but proper maintenance matters more than panel type in salt air. And if long-term production is what you care about most, HJT's 0.25% annual degradation rate means your system should still be producing strong power when your kids are in college.^[6]

Either way, you are getting panels that produce 30–50% more energy per square foot than what was available ten years ago, with dramatically better long-term retention. Do not overthink this. Just avoid the cheapest option on the quote. The panels are the part of the system working for you every sunny day for 25 years. Spend the extra money there.