Solar & Battery Install Remote From House

Qyburn

Hi,
We didn't look at solar originally as we wouldn't qualify for FIT, however now prices look as if could easily be cost effective. I'm awaiting some opinions an quotes, but wondered if there are any issues with our specific electrical layout.

The panels would go onto a metal Dutch barn with a huge S facing (190 Deg to be precise) roof and no obstructions or shading. The abnormality is that we wouldn't be able to wire the panels back to the house or to the meter cabinet. The inverter and battery would need to housed in a building next to the barn, which has its own DB fed by SWA split off at the house from between the meter and the house CU. The DB has spare ways and the SWA is 25sq.mm.

Any possible issues we should look out for? Or limitations due to this layout? We would want battery backup during power cuts but would be happy if this was manual by switching off the isolator at the meter and doing "something" to bring the battery online. 

Ta in advance.

Ectophile

Your electrician will need to check the voltage drop on the SWA cable to make sure it's acceptable when the panels are generating at their maximum.  That may depend on how big you make the solar installation.

The tricky bit may be the islanded mode during a power cut.  There needs to be a single switch-over point from mains to inverter when it goes into islanded mode, to prevent any possibility of back-feeding onto the grid.  On my installation, that means a separate wire run from the inverter to a big rotary switch in the meter cupboard  The switch changes the house over from mains to inverter.

Your problem is that the inverter is going to be at the wrong end of a long length of SWA.  To do it the way my installation is done, would need running in a separate SWA from the inverter to the house, and a signal wire between the switch and the inverter to tell it that it's safe to change over to islanded mode.

Qyburn

Thanks. Do you know what the voltage drop limit is? And presumably this would assessed at the worst case of both solar and battery at their max.

Regarding isolation and power cut operation that is my biggest concerns, and also an important justication for the battery.  I mentioned that a manual fail over would be OK, but only if that is possible and is of course. I believe the Tesla batteries have their own inverters, does that mean they can be installed remote from the solar inverter? Maybe we could install the battery alongside the meter cabinet, with changeover as you describe.

The problem at this stage is that all installers are backed up at the moment so working off telephone calls and user-provided site information. It may be weeks before anyone would actually be on site. That's why I'm trying to think it through myself in advance.

Ectophile

The voltage drop on the cable should be less than 3% (about 7V) at the maximum current.  To work it out, you need to know the maximum current, the length of the cable, and the ohms per metre of that cable (and remember to add the lengths of the live and neutral, so double the measured length).

AC-coupled batteries do exist and don't have to be anywhere near the inverter.  You can put them conveniently close to the mains supply if that helps.

My system is DC-coupled, which means that the battery has to be next to the inverter.

Heedtheadvice

Interesting  that you mention 3% voltage drop. Nothing  wrong with your message principles.

 

Where does that 3% come from as I would expect  it needs to be much less than that? 

Consider as an example the grid voltage at the premises and the max inverter output voltage. Typical values might be 247v and 254 which is a difference of seven. That allows your nominal 3% drop on the line that could be ok but if the grid voltage rises then either the generated current must fall ( giving poorer performance ) or the Inverter requires to increase it's output voltage and could well exceed it's max voltage and therefore trip out.

Much depends upon actual ( variable grid voltage ) voltage and inverter spec, does it not?

Qyburn

The SWA to the outbuildings is 25sq.mm with a route length of no more than 50m. That would be good for around 50A before exceeding 3%.

Or 2.7V drop at 20A from a 5kW system.

But I guess this is another point in favour of an AC coupled battery located at the house if that can be arranged.