Optimisers or microinverters? A fair comparison for UK installers

Optimisers and microinverters both solve the same basic problem: in a plain string, one shaded or mismatched panel holds back the rest. Both give you panel-level MPPT, panel-level monitoring and low voltage on the roof when the system is shut down. So which should you fit?

We make optimisers, so we're not neutral. But the honest answer is that both are good technologies, and the right choice depends mostly on the system around them.

The core difference

• Microinverters convert DC to AC at every panel. The cable coming off the roof carries AC, and there's no central inverter.
• Optimisers condition the DC at every panel, then send it down the string to a central string or hybrid inverter, which does the DC-to-AC conversion in one place.

Everything else follows from that.

What both do well

• Shade and mismatch. Each panel runs at its own maximum power point, so one shaded panel doesn't drag down the others.
• Panel-level monitoring. You can see each panel's output, which makes fault-finding much easier.
• Low voltage on the roof when shut down. Microinverters do this by design. Optimisers with rapid shutdown, like ours, drop each panel to about 1 V.

Where optimisers have the edge

• Batteries and hybrid inverters. In the UK, many new systems go in with a hybrid inverter and a DC-coupled battery. Optimisers work with that set-up directly: the hybrid inverter stays at the centre of the system. Microinverters produce AC at the panel, so a battery is normally AC-coupled with its own separate inverter.
• One conversion point. The DC-to-AC conversion happens once, in an inverter that's easy to reach, rather than in a separate unit behind every panel.
• Simpler electronics on the roof. An optimiser is a DC-to-DC converter, not a full grid-connected inverter.
• Keeping the inverter you know. Installers can keep using the string or hybrid inverters they already specify, service and support.

Where microinverters have the edge

• No central inverter to fail. If one microinverter fails, only that panel stops. With optimisers, the central inverter is a single point of failure for the array.
• Very small or scattered arrays. For a handful of panels spread across several roof faces, a microinverter system can be the neater solution.
• AC on the roof cable. Some designers prefer that the cable coming off the roof carries AC rather than DC.

So which should you fit?

• New system with a hybrid inverter and battery, or likely to add one: optimisers usually fit better.
• Existing string or hybrid inverter that's working well, but the roof is shaded: optimisers let you add panel-level performance without replacing the inverter. Check compatibility first.
• Very small, scattered array with no battery plans: microinverters are worth a look.

Whatever you choose, check compatibility with the inverter and battery before you quote. For Archai, that means Solis inverters or Fox ESS single-phase inverters today. See where we stand.

If you'd like a second opinion on a particular design, get in touch.