People will happily spend three weeks comparing solar panel brands.
Then they'll let the installer pick the inverter without asking a single question.
Which is a bit odd, when you think about it. Your panels will almost certainly outlive their 25-year warranty. Your inverter probably won't see 15.
It's the one part of the system you're most likely to have to replace. And it's the part nobody talks about.
What it actually does
Your panels produce direct current. Your kettle, your fridge and the grid all want alternating current.
The inverter sits in the middle and converts one to the other.
That's it. That's the job.
But it's also the brain of the system. It decides how much power to pull from each string of panels, it shuts things down safely when something goes wrong, and it's what your monitoring app is actually talking to when it tells you what you generated today.
It's doing that conversion every daylight hour for the next decade and a half. Which is why it wears out before the panels do.
String or hybrid?
A standard string inverter takes the DC from your panels and turns it into AC. Done.
A hybrid inverter does the same, but can also charge and discharge a battery.
Here's the bit worth knowing: if you think you'll add a battery later, a hybrid inverter now is much cheaper than replacing a perfectly good string inverter in three years' time. If you're certain you never will, you're paying for a feature you won't use.
That's genuinely it. Don't let anyone make it more complicated.
Why your inverter will be "too small" (and that's fine)
This one surprises people.
If you've got 5kW of panels on the roof, you might well be quoted a 4kW inverter. It looks like you're being short-changed.
You're not.
Your panels only hit their rated output in perfect conditions — clear sky, right angle, cool day. In the UK, that happens rarely. Sizing the inverter to a peak you'll hit a handful of times a year means paying for capacity that sits idle the rest of the time.
On those few peak days you lose a sliver of generation. Every other day, you've spent less. It's the right trade.
The UK grid rule that shapes all of this
There's a threshold worth knowing about, because it quietly drives a lot of quotes.
If your inverter's output is 3.68kW or less on a single-phase supply, your installer can fit it and simply notify the network operator afterwards. That's the G98 route, and it's straightforward.
Go above that and they have to apply for permission before anything goes on the roof — the G99 process — and approval can take weeks.
Most domestic systems sit at or below that threshold. So if you're wondering why your quote stops at 3.68kW when there's clearly more roof available, that's usually why. It's not the installer being lazy.
If you want a bigger system, it's still perfectly doable. It just needs planning, and an installer who starts the paperwork early rather than the week before.
What to ask
Three questions, and you'll know more than most people who've had solar fitted:
- What's the warranty, and who honours it? Manufacturer warranties vary far more than panel warranties do.
- Is it hybrid, and if not, what happens when I want a battery?
- What's the output, and does that need a G99 application?
Want to see how this plays out on a real brand? Our look at Fox ESS walks through one manufacturer's range in detail.
And if you would rather not work all this out yourself:
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