Solar Inverters Explained: Which One Do You Need?

Most people choosing solar spend their time on the panels. Which brand, how many watts, how they will look on the roof. The inverter gets a single line on the quote, which is a shame, because it is the component that decides how the system behaves day to day and the one most likely to need replacing before the panels do.

Here is what that line on the quote actually means.

What the inverter does

Panels produce direct current. Your house, and the grid, run on alternating current at 230 volts. The inverter converts one to the other.

It also does the thinking. Its maximum power point tracker (MPPT) constantly adjusts the electrical load on the array to squeeze the most out of whatever light is falling on it, which changes minute to minute as cloud passes over. It handles the safety side too, disconnecting automatically if the grid goes down so nobody is working on a supposedly dead cable that your roof is quietly energising. And it is what feeds the app on your phone.

String inverters

The standard arrangement. Panels are wired in series into one or more strings, and the strings run down to a single inverter, usually in the loft or garage.

It is the simplest design, the cheapest, and for a straightforward roof it is the right answer. One box to maintain, one box to replace eventually, and nothing on the roof that needs electronics. Most inverters have two MPPT inputs, so an east and west array, or a main roof and a garage roof, can be tracked independently on the same unit.

Hybrid inverters

A hybrid inverter handles solar and battery storage in one unit. Given that most systems we install now include storage, this is where the majority of quotes land, and the FoxESS and Sigenergy kit we fit is built around it.

The question worth asking is what happens if you are not having a battery yet. A hybrid inverter costs more up front but leaves you able to add storage later by connecting a battery to the unit that is already there. Fit a plain string inverter and adding a battery later means either an AC coupled battery with its own inverter, or replacing the one you have. Neither is a disaster, but if there is any chance of a battery within a few years, paying the difference now is usually the cheaper route.

Worth knowing: batteries are stackable from 5 kWh up to 60 kWh, so the capacity decision can wait. The inverter decision is harder to postpone.

Microinverters and optimisers

These move the electronics up to the panels. Optimisers sit behind each panel and let it work at its own best point; microinverters go further and do the DC to AC conversion at the panel itself.

Both make sense on genuinely awkward roofs: persistent shading that cannot be designed around, or several small roof faces at different angles. Both add cost per panel and put more components in the one place that is inconvenient to reach. On a clear, uncomplicated roof they add expense without adding much output, so if they appear on a quote for a plain south facing roof, ask what problem they are solving.

Why the inverter is smaller than the array

A 12 panel array of 460W panels is 5.5 kWp, and it will often be paired with a smaller inverter. That is deliberate. A UK array rarely hits its full rated output, so slightly undersizing the inverter costs almost nothing in lost generation and saves money.

There is a regulatory reason too. Up to 3.68 kW of inverter output per phase, your installer simply notifies the network operator after the job under G98. Above that, a G99 application goes in beforehand and approval can take several weeks. A good installer will tell you which route your design needs and factor the timing in, rather than discovering it later.

Lifespan

Panels generate for 25 years or more. Budget for replacing the inverter once in that time, typically after 10 to 15 years, and check the warranty on the quote: 10 to 12 years is common now.

If you would like someone to look at your roof and explain which inverter arrangement it actually needs, our survey is free and there is no obligation.