We are just about to commence the installation of solar PV panels on top of a block of flats in Southwark SE16.
These panels will cover most of the available roof space. In the daytime there will be constant DC generation going to inverters.
If there is a fire, should we treat as an electrical fire, and what equipment should be used IE not water?
Any advice would be useful

Because there will be constant DC generation going to inverters you must use a non conducting medium and because the ancillary equipment is likely to be located in the loft space (indoors) you should consider CO2 because dry powder is not recommended indoors.

Because there are areas that you can isolate and there is likely to be a class "A" risk you could consider foam or dry water mist that have passed the 35kV dielectric test which can be use close to electric equipment.

Because the loft is not inhabited you should also consider a fire alarm to warn the residents if a fire should occur. 

What voltages are involved? Some water spray & foam extinguishers can safely be used directly onto electrical fires of up to 1000V as well as accidental contact up to 35,000V.

CO2 is probably most appropriate although you need to consider:

a) Who is going to be able to use the extinguisher and is trained to do so?

b)How are the occupiers going to know there is a fire and be able to access it quickly enough for a first aid attack?

Some fire types and locations are such that suitable alarm, compartmentation & escape are more appropriate with fire brigade intervention especially in domestic premises

The biggest problem with solar panels seen from a firefighter perspective, is the inability to turn off the electricity. Only darkness will do the trick. Foam has been tried, but on a bright day, plenty of light gets through to generate dangerous voltages.

If you have an electric fire in the panels, before the first circuit breaker, you may be able to extinguish it briefly, but it is likely to re-ignite as soon as oxygen returns.

I think fire prevention is the first priority here, working with vendors that can provide the safest products.