Solar panels make direct current. Your house runs on alternating current. Something has to convert one to the other, and there are three ways of doing it.
A string inverter does the whole job in one box. Optimisers add a small electronic unit behind each panel and still send DC down to one inverter. Microinverters put a complete little inverter behind each panel and send AC down instead.
This article compares the three. It does not cover sizing an inverter or what to do when one fails, which have their own pages: inverter sizing and selection in the UK and repair or replace the inverter.
Key Points
- All three are normal in the UK. Grid connection certification belongs to each product, not to the type, and it is printed on the datasheet.
- The real difference is where the electronics sit. One box at low level, or one unit per panel on the roof.
- Optimisers and microinverters help when panels perform differently to each other. On a clear roof facing one way there is much less for them to fix.
- Every string inverter we fit has two independent MPP trackers, so a plain string system can already be split across two roof faces.
- Roof level electronics mean roof level repairs. That is scaffolding, not a ladder.
- Battery choice follows inverter choice, so decide about storage at the same time.
Common Misconceptions
"Microinverters and optimisers stop shading losing you power." They reduce the effect of one panel dragging on the others. They cannot make a shaded panel generate what an unshaded one would.
"A string inverter means one shaded panel kills the whole array." That is the old picture. Panels have bypass diodes, and the inverters we fit have two independent MPP trackers.
"More electronics means more monitoring, so it must be better." Better monitoring is genuinely useful for finding a fault. It is not generation. Decide what you are buying.
"Microinverters mean no single point of failure." There is still a gateway, one AC circuit and a consumer unit. What changes is that a failure takes out one panel instead of all of them.
"Panel level electronics are maintenance free because they are sealed." Sealed is not unreachable. The Enphase unit is rated IP67 and the SolarEdge optimiser IP68, but a failure is still under a panel on a roof.
Real-World Advice
Our published installations are all string systems. All eighteen published case studies use a single hybrid string inverter with a battery, and none carries optimisers or microinverters. Fifteen use the Anker SOLIX X1-H6K-S; the rest use the Fox ESS H1-6.0-E-G2, KH7 and KH10. See our published work.
Both hybrids publish the same shape on their datasheets: two MPP trackers, one string per tracker. That matters more than it sounds. A house with an east face and a west face can put one string on each and let the inverter track them separately, with no electronics on the roof at all.
We do not normally fit optimisers. In our view they are a weak point on the roof and likely to fail well before the panel behind them would. On a roof with real shade we design around it first, using the inverter's second tracker and the panel layout, and we say so at survey.
UK-Specific Considerations
The comparison
| String inverter | Optimisers plus a string inverter | Microinverters | |
|---|---|---|---|
| How it works | Panels are wired in series into a string. The whole string feeds one inverter | A small DC to DC unit sits behind each panel and conditions its output before the string reaches the inverter | A complete inverter sits behind each panel |
| Where DC becomes AC | In the one inverter, normally in a loft, garage or on an outside wall | In the one inverter, same as a plain string system. The roof units stay DC to DC | On the roof, behind each panel |
| Shading behaviour | Panels in a string share a current path, so the weakest panel holds the others back. Two MPP trackers let you split the array in two | SolarEdge states its optimisers mitigate all types of module mismatch loss, from manufacturing tolerance to partial shading | Each panel has its own MPP tracker, so nothing is shared between panels |
| Monitoring | System level. The Fox ESS datasheet offers remote monitoring by app or web portal | The datasheet we could open states module level power optimisation and module level voltage shutdown. It does not publish a monitoring row, so ask what the app actually shows | Each microinverter talks to a gateway over power line communication, so reporting is per unit |
| What fails, and where | One box at low level. It can be reached and swapped without going on the roof | The inverter at low level, plus one optimiser per panel on the roof | One inverter per panel, all on the roof |
| Roof level electronics on a 13 panel system | None | 13 | 13 |
| Warranty on the datasheets we opened | Anker SOLIX X1 power module states 10 Years Limited. The Fox ESS H1(G2) datasheet publishes no warranty row | SolarEdge S series states a warranty of 25 years | Enphase IQ8 states a warranty of up to 25 years, valid provided an internet connected IQ Gateway is installed |
| Battery | The hybrid units we fit take a high voltage LFP battery directly on the DC side | Optimisers are stated as specifically designed to work with SolarEdge inverters, so the battery follows that inverter | The Enphase battery is described on the same datasheet as an AC coupled storage solution |
Those warranty terms are the manufacturers' own words for their own products. The warranty on the equipment we fit is stated in your quote.
Two rules that catch people out
Optimisers have a minimum string length. The SolarEdge S series datasheet gives a minimum of 8 optimisers per string on a single phase residential design with the S440 and S500, and a maximum of 25. So a very small array cannot simply have optimisers added.
You also cannot mix optimiser generations. The same datasheet says S series and P series optimisers may not be mixed in new installations, so an addition to an older SolarEdge system has to match what is there.
Grid rules
Certification is by product, not by type. The Fox ESS H1(G2) datasheet lists G98 and G99, the Anker SOLIX X1 datasheet lists G99, and the Enphase IQ8 UK datasheet lists G98, G99 and G100. Check the datasheet for the exact model quoted.
One detail worth knowing. The Fox ESS 3.7 kW model is listed at 4048 VA maximum output apparent power, footnoted as 3680 for G98. That is the software setting that keeps the unit inside the 16 A per phase connect and notify route. Ask which setting yours is commissioned on.
When This Isn't Right For You
Panel level electronics are hard to justify on a simple roof. One face, one direction, no chimney and no trees, and you are paying for an answer to a problem you do not have. That is also why we do not normally fit optimisers: in our view they add a failure point on the roof that is likely to give up before the panel does.
They are also not the fix for a roof that is genuinely too shaded. If a panel spends the middle of the day in shadow, the honest answer is to move it or leave it off. See why is my solar generating less. And they do not solve a roof problem: if the roof needs work, do the roof first. See roof suitability by roof type.
Before You Spend Money
Ask which of the three the quote is for, in writing. It should not be buried in a model number.
Ask what the shade actually is, and when. A survey should tell you which panels are affected and for how long, not just that there is a tree.
Ask what a plain two string design gives on your roof first. Then ask what the panel level option adds, and what it costs.
Ask who replaces a roof level unit if one fails in year eight, whether scaffolding is included, and whose warranty pays for the labour.
Ask what the monitoring screen will show. Per panel, per string, or per system.
Ask about the battery at the same time, not afterwards. It is the same decision.
Frequently Asked Questions
Q: Which is best for a shaded roof?
A: Something that treats panels separately. Optimisers and microinverters both do that, and a two string layout on a string inverter does a limited version of it for free. Which one is right depends on where the shade falls, so it is a survey question.
Q: Do microinverters generate more electricity?
A: Not on their own. On a clear, single direction roof the difference is small. The gain comes from removing mismatch between panels, so it depends on how mismatched your panels are in the first place.
Q: What happens when a roof level unit fails?
A: You lose one panel rather than the array, but reaching it means getting on the roof. Ask before you buy whether the manufacturer's warranty covers the labour and the access, or only the part.
Q: Can I have a battery with microinverters?
A: Yes, but it is an AC coupled battery. The Enphase datasheet describes its own battery as an AC coupled storage solution. See AC coupled and DC coupled batteries compared.
Q: Are optimisers tied to one inverter brand?
A: The SolarEdge datasheet states its optimisers are specifically designed to work with SolarEdge inverters. So choosing optimisers usually also chooses the inverter and, with it, the battery.
Q: Does the choice change my grid application?
A: Not by itself. The route depends on your connected capacity per phase, not on which type you pick. The certification the network operator needs is printed on each product's datasheet, so check the model quoted. See the G98 application process.
Summary
A string inverter puts everything in one box at low level, simple to live with and simple to replace. Optimisers keep that box but add a DC to DC unit behind every panel to deal with mismatch and shading. Microinverters move the whole conversion onto the roof, one per panel.
All our published installations are string systems with hybrid inverters and batteries, and every one of those inverters has two MPP trackers, so a split roof is already handled. Panel level electronics earn their place when the roof is broken up or genuinely shaded. On a plain roof they are electronics to maintain for a problem you do not have. And whichever you choose, the battery decision comes with it. See inverter replacement and solar installation.