Solar sizing 4kw Vs 4.8kw

SolarSoGood

Hi all,

I'm looking to get solar panels and have had a few quotes so far. It's a South facing bungalow in East Anglia with no shading.

Most have quoted for 4kw which is 10 panels, one has quoted for 4.8kw which is 12. 12 almost fill the roof space and you couldn't fit anymore.

So, my question is which is best? 

The inverter in both cases is 3.7kw.

Would the 4.8kw system improve the output in winter etc? 

What happens in summer when the system produces 4.8kw but the inverter only inverts 3.7kw? What happens to the excess? Is this an issue?

Does installing a battery (currently undecided on this) change which would be a better size?

Sorry for the all the questions and thanks for any advice received.

Martyn1981

Hiya, and welcome to MSE.

Sometimes going bigger pushes up the cost per Wp, due to more expensive, higher efficiency panels, but I assume in your case that 'they' are simply adding 2 more panels, as you state. So .... bear with me here .... the cost per Wp may be similar or slightly lower for the larger system?

Personally, unless you are entering insane territory, I'd always goes with bigger, way too expensive to retro-fit later due to duplicating costs. 4kWp and 4.8kWp are both good sizes, but not extreme.

Yes, larger system will generate more in the winter (around 20%+ in this example), and in the summer the inverter will simply cap generation to 3.7kW whenever it's higher. But expect hot panels in sun all day to lose about 20% efficiency, so summer day gen may top out at around 3.84kW anyway. 

More capping in April/Oct when the sky is clear and air temps are low, but the length of the sun day is shorter.

You will need to check that the inverter has a max DC capability of 4.8kW+, even if its AC max is 3.7kW.

You could get a DC side battery where the inverter will only allow 3.7kW AC through, but can also direct DC straight to the batts, but this is a much bigger discussion.

Also you could ask your DNO (via the installer) for permission to exceed the nominal 3.68kW export limit. An inverter of around 4kW to 4.2kW may be the sweet spot for efficiency for a 4.8kWp system.

[I've used 3.7kW as you mentioned that, but for pedantry purposes, it's probably 3,680W, as per the DNO limit, but 3.7kW will make sense to all on here.]

Screwdriva

If I were you, I'd max the # of panels (vs. getting a battery) if costs are a factor, and go for the 12 panels. This will help in the colder months when solar energy is scarce. 

Then, I would turn my attention to the quality of the brands quoted and ensure I chose the best option available - I would only go for Sharp, Hyundai, Solarwatt, REC, Panasonic or Sunpower 400W panels. I would also chose a SolarEdge or Enphase or GivEnergy inverter with 20+ year warranty only. 

QrizB

Another vote for taking 4.8kWp rather than 4.

The number of hours per year where you achieve the rated output is small, and so the extra panels will help to fill in the shoulders of the day (and the seasons).

Solarchaser

Martyn has covered the points well, and so I'd just personally say yeah 4.8 is better, winter gen is what you really care about for your own home bills