What are PV micro inverters and what are their advantages and disadvantages?

Solar photovoltaic (PV) panels are a hot topic this summer, especially since many expect that the currently very generous financial support under the feed-in tariff scheme will be drastically cut for anyone who installs their PV roof after 1 April 2012, as the government will introduce new tariffs for systems installed after that date.

Apart from the panels themselves, the other major component of a grid-connected PV roof is the inverter.

Modern grid-tied inverters fulfil three roles:

  1. they convert the direct current (DC) electricity produced by the PV panels to the alternating current (AC) electricity the grid and our appliances need
  2. they ensure compliance with regulations about feeding electricity into the grid, e.g. by immediately disconnecting if there is a power cut – otherwise there would be a risk of sending electricity down the wires while repairs are carried out
  3. they maximise electricity production by constantly varying their resistance (load) in a way that ensures that PV panels always operate under conditions optimal for the level of solar radiation at that moment – this is called maximum power point tracking (MPPT)

The most common type of inverters for domestic PV installations in the UK are central “string” inverters: The PV modules on your roof are connected in one or more “strings” and wired to a single central inverter somewhere inside your house.

However, there is another, currently less common approach: Having a small dedicated inverter for each individual PV module, a so-called micro-inverter. Theoretically, this approach should have three main advantages:

  1. this allows the inverter to optimize the operation of each panel individually, which should increase performance in cases where some panels on the roof receive more sunshine than others. This could be the case because of shading from a tree or chimney, or because the PV panels are mounted on parts of the roof facing different directions (e.g. SE and SW)
  2. micro-inverter systems should be a bit simpler and easier to install, as they avoid the need for high-voltage DC wiring and the installation of a separate inverter box inside your house
  3. because each inverter only has to deal with one inverter, the power passing through it is lower. Some manufacturers argue that this makes it possible to build systems that last longer. Indeed, most micro-inverters have warranties between 15 and 25 years.

However, all this is bought at the cost of currently higher capital costs – using micro-inverters will add somewhere between £500 and £2,000 to the cost of a domestic PV roof. Perhaps more critical, though, is the uncertainty about lifetime maintenance cost: While each individual micro-inverter may last much longer than a central inverter, the high number of individual inverters (10-20 in a typical domestic system) may still mean it’s quite likely that there will be dome failures during the life of the PV roof. And while the warranty could cover the cost of replacing the micro-inverter itself, the labour and scaffolding required to change micro-inverters on a roof may still make replacements very expensive.

At the moment, all of this is largely speculation. There simply aren’t enough micro-inverter systems, especially of the latest generation, which have been around for long enough on UK roofs for us to know how they pass the test of time. Anyone installing this technology at the moment should consider themselves an “early adopter” – with all the opportunities and risks associated.

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