Two hybrid inverters sit on a quote in front of you. One says 97.6% max efficiency. The other says 97.6% max efficiency. The price gap between them is several hundred euros, and the number that was supposed to settle the argument has just refused to do its job.
That is the actual situation with the Fronius Primo GEN24 and the Solinteg Integ M MHS series. Identical headline figures, wildly different documentation. So what do you compare when the comparison metric collapses? The interesting part of this comparison turns out not to be which brand converts more sunlight, because on the published record they convert almost exactly the same amount. It's how much each company is willing to tell you about the conditions under which that number holds.
I went through four PDFs and three manufacturer spec pages for this piece. What follows is a comparison of published records, not of hardware I've bolted to a wall.
One Number, Measured at One Operating Point
Max efficiency is the single best result the converter can produce. It happens at one load level, at one DC input voltage, at one temperature, usually somewhere between 40% and 70% of rated power with the string sitting in the sweet spot of the MPPT window. Your roof does not live there. It lives in the mornings and the cloud edges and the 8% load afternoons of a Belgian November.
European efficiency exists to fix that. It's a weighted average that samples conversion at 5, 10, 20, 30, 50 and 100 percent of rated power, then weights each by how much of a Central European year is realistically spent there. The 50% bucket carries the heaviest weight. The 100% bucket carries almost none, which is exactly why the max-efficiency figure is the one that ends up on the box. Guess which figure gets set in bold on the brochure?
So far, so standard. Here's where Fronius does something the rest of the industry mostly does not.
On the technical data page for the Primo GEN24 6.0 Plus, the European efficiency isn't given as a number. It's given as three numbers: 96.1, 97.1 and 96.7 percent, at 230, 400 and 480 Vdc respectively. Same inverter. Same standard. One volt-dependent full percentage point of spread between the worst and best case.
::cts A single max-efficiency figure is a marketing artifact, and the proof is that Fronius and Solinteg both claim 97.6% on their single-phase hybrids while only one of them shows you how that number moves when your string voltage does.
What does that percentage point mean in string design terms? A 400 V string and a 230 V string on the same 6 kW inverter are not the same system. The low-voltage configuration gives up roughly 1% of everything the array produces, forever, and no datasheet headline anywhere would have warned you.
The Published Record, Side by Side
Four series, all currently sold, all hybrid, all documented in English. Figures below come from the manufacturers' own datasheets and technical data pages.
| Specification | Fronius Primo GEN24 (1-phase) | Solinteg MHS-30 (1-phase) | Fronius Symo GEN24 (3-phase) | Solinteg MHT-25 (3-phase) |
|---|---|---|---|---|
| Rated power classes | 3.0 to 6.0 kW | 3.0 to 8.0 kW | 3.0 to 10.0 kW | 4 to 12 kW |
| Max efficiency | 97.6% (all classes) | 97.6% (all classes) | 98.1 to 98.2% | 98.10 to 98.20% |
| European efficiency | 96.8 to 97.2% (varies per class) | 97.0% (same for all six classes) | 96.7 to 97.9% (varies per class) | 97.30 to 97.40% |
| Euro efficiency resolved by DC voltage | Yes, on the 6.0 Plus page | Not published | Not published | Not published |
| MPP adaptation efficiency | Greater than 99.9% | Not published | Greater than 99.9% | 99.90% |
| MPP trackers | 2 | 1 on 3K and 3.6K, 2 from 4.2K up | 2 | 2 |
| DC input voltage range | 65 to 600 V | 80 V start, 600 V max | 80 to 1000 V | 135 V start, 1000 V max |
| MPPT operating window | 65 to 530 V usable (65 to 480 V on the 6.0) | 100 to 550 V | 80 to 800 V usable | 120 to 950 V, or 200 to 950 V from 8K up |
| Battery voltage range | 150 to 455 V | 85 to 450 V | 160 to 700 V | 135 to 750 V |
| Max battery charge current | 22 A | 30 A | 12.5 A on 3.0-5.0, 22 A on 6.0-10.0 | 25 A |
| Backup switchover | Under 10 s full backup, under 15 s PV Point | Under 10 ms | About 10 s full backup | Under 10 ms |
Read the three-phase columns again. Solinteg's MHT-10K-25 posts a 98.20% max and a 97.40% European efficiency. The Fronius Symo GEN24 10.0 posts 98.2% and 97.9%. On the weighted figure that actually predicts annual yield, Fronius is ahead by half a point, while the headline numbers are identical to the first decimal. Half a point on a 10 kW system running 11,000 kWh a year is about 55 kWh annually. Not nothing. Not a reason to double your budget either.
Where Each Vendor Stops Publishing
Neither company comes out of this clean. They fail in different directions, though, and the directions matter. Which failure would you rather live with: a number you cannot find, or a number that contradicts itself two pages later?
Solinteg Keeps Efficiency Out of Its Own Spec Tables
Here's the thing that genuinely surprised me. Go to Solinteg's website, open the spec table for the MHS 3-8 kW series, and there is no efficiency row. None. Not max, not European. Same for the MHT 4-12 kW page. Same for the industrial MHT 25-50 kW page, which lists MPPT voltage windows, battery ranges and charge currents in detail and simply omits how well the thing converts power.
The numbers exist. They're in the downloadable PDF datasheet for the MHT series, and in the technical parameters appendix on page 78 of the MHS user manual, which is where I found them. But a buyer comparing spec tables in two browser tabs will see efficiency for Fronius and a blank for Solinteg, and will have to go looking through a manual to fix that. For a company whose actual efficiency figures are competitive, that's a self-inflicted wound.
There's a second gap, subtler. Solinteg quotes one European efficiency for all six MHS classes: 97.0% for the 3K and 97.0% for the 8K alike. Fronius resolves it per class, and the spread across the Primo range is 96.8 to 97.2. Is a single value across six power classes with different thermal and switching behaviour an approximation, or a class-worst figure carried across the whole family? Solinteg doesn't say, and that ambiguity is worth a note in any tender document.
Fronius Contradicts Itself About Backup
Fronius publishes more, and some of what it publishes disagrees with itself. In the Symo GEN24 datasheet, page 2 states Full Backup is not available for the 3.0 to 5.0 Plus units. Page 4 footnote 2 says it's available on 6.0 to 10.0 Plus. Page 6 footnote 2 then says "3.0-6.0 Plus", which is the Primo range and reads like a copy-paste that nobody caught.
The spec rows themselves settle it: the 3.0, 4.0 and 5.0 columns list PV Point only and leave Full Backup blank. So the reading the data supports is that Full Backup on the Symo family starts at 6.0 kW. But an installer quoting a 5 kW three-phase system with whole-home backup off that footnote is going to have an unpleasant conversation later. Documentation quality is a product attribute, and this is a miss on an otherwise unusually transparent datasheet.
Solinteg Contradicts Itself About Switchover
Before that reads as a one-sided complaint, Solinteg has the same disease in a different organ. The MHT 25-50 kW datasheet specifies backup switchover in under 20 ms. The product page for those same models advertises 10 ms. Both are Solinteg's own words about one product, and they differ by a factor of two.
The datasheet wins, as it always should. But notice what just happened to the comparison: I've now caught both vendors publishing figures that disagree with their own other figures, and in both cases the resolution was to trust the document with the revision stamp on it over the page written by marketing. That's the actual lesson of this whole comparison, and it's worth more than any decimal place above.
What Actually Changes Your Yield
Efficiency is one term in the yield equation and rarely the decisive one. So what does move the needle? Three things, and none of them made the brochure.
The MPPT window comes first. The Symo GEN24 tracks from 80 V and works up to 1000 V DC input; the MHT-25 starts feeding at 135 V and accepts up to 950 V with a battery attached. That difference decides how early in the morning your strings wake up and how short a string you're allowed to build on an awkward roof face. If you want to see what that looks like in real production data rather than in theory, it shows up clearly in reading your inverter's own logging.
Second, tracker count on the small single-phase units. The MHS-3K and MHS-3.6K ship with one MPP tracker. Every Primo GEN24, down to the 3.0, ships with two. On a simple south-facing roof this changes nothing at all. On a house with an east and a west face, a single tracker forces both orientations onto one voltage and quietly costs more than the entire efficiency gap being argued about above.
Third, and least discussed: the Primo GEN24 Plus datasheet states that AC power derating occurs at a DC battery input voltage of 419.7 V and higher. That's a real operating constraint on a real product, published plainly, and I've never seen an equivalent disclosure in a Solinteg document. Does Solinteg's hardware genuinely have no such behaviour, or does it simply not document it? I can't tell you, and neither can the datasheet.
Which One I'd Specify, and Why
Here's my opinion, and plenty of installers will disagree with it: on efficiency alone, Solinteg has closed the gap far enough that paying a Fronius premium purely for conversion is no longer defensible. A 97.40% European figure on the MHT-10K against 97.9% on the Symo 10.0 is half a percentage point. If your quote difference is 15%, can half a point of weighted efficiency really carry that? It cannot.
What does carry it is everything around the number. Fronius tells you how efficiency moves with voltage, states its derating thresholds, and documents a 10-year registered warranty on inverters under 50 kW with a paid path to 15. Solinteg gives you a wider battery window, 30 A of charge current on a single-phase box where Fronius gives 22, and 10 ms backup switchover against the 10 seconds a Fronius Full Backup takes. That last one is not a rounding difference. It's the difference between your desktop rebooting during an outage and not noticing the outage happened.
So the honest split looks like this. Backup-critical single-phase home, tight budget, straightforward roof: the Solinteg case is strong and the efficiency penalty is a rounding error. Complex roof, three-phase service, or a system somebody will still be diagnosing in 2040: the Fronius documentation is worth paying for, and that's a different argument than the spec sheet was having. Systems on three-phase supply have their own constraints worth understanding before either brand is chosen, which is a topic covered in how solar works with three-phase power.
One closing note on legacy hardware, because it comes up constantly. If you're replacing an older unit, the Fronius Primo 8.2-1 is discontinued and superseded by the GEN24 Plus line discussed here, and the larger commercial Fronius Eco 25.0-3-S sits in a different class entirely. Don't compare a GEN24 European efficiency against a spec sheet from a 2018 SnapINverter and conclude anything. And if you want to run the arithmetic on what any of these percentages mean for your own array before you commit, the solar panel efficiency calculator guide walks through the math with your own numbers instead of a vendor's.
Summary
Fronius and Solinteg post nearly identical headline efficiency on comparable hybrid inverters, so the headline is not the deciding number. On the weighted European figure that actually predicts annual yield, Fronius leads by roughly half a point on three-phase units, worth about 55 kWh a year on a 10 kW system. The real separation is documentation: Fronius resolves efficiency by DC input voltage on the Primo GEN24 6.0 Plus page and states its 419.7 V battery derating threshold plainly, while Solinteg omits efficiency from its website spec tables entirely and publishes it only inside datasheet PDFs and a user-manual appendix. Both vendors contradict themselves somewhere, and in both cases the revision-stamped datasheet beats the product page. Specify Solinteg where budget and fast backup switchover matter on a simple roof, and Fronius where a complex array or a long service life makes the documentation itself part of what you are buying.