Solar PV quotes - could you sense-check and advise on Sofar vs Growatt

tomkaier

Hi everyone,

thought I'd ask the hivemind what you thought of the solar PV quote I've received from a company called Skylar Solar. For our east/SE facing roof, they've proposed 

• 12 x 410W Hyundai panels incl bird guard
• 3.6 kW Growatt hybrid inverter with 6.5 kWh battery

For £10.4k plus scaffolding. It's not the cheapest quote I had so far, but from what I gather the Growatt system is better than e.g. GivEnergy.

What I found surprising is that the inverter is smaller than the entire system, and my understanding would have been that the panels, on a sunny summer day, would be able to generate ~4.74 kW but the inverter would only make use of 3.6 kW at any time. Having spoken to the company about this, they argued that a larger inverter would come on later in lower light conditions - e.g. a 5kW inverter would not allow us to generate as much in winter, where it arguably mattered more to squeeze out any last bits of efficiency. An additional factor would be the DNO of course. In their opinion, the efficiency gains in lower light conditions would make up for the restriction in peak output.

Then Green Energy Together UK quoted a 3.8 kWp system using 

• 10 Jinko Solar 380 W panels incl bird guard
• Sofar 3.6 kW hybrid inverter
• Sofar GTX5000 5 kW battery

for £10.6k incl scaffolding, but £8.6k if we participate in a group buying scheme to be installed in May this year.

Curious to hear everybody's thoughts, and if you had experience with Growatt or Sofar systems then even better! 

Btw, having plugged my details into PVGIS (great recommendation), I got the following info:

Provided inputs:Location [Lat/Lon]:51.389,0.095Horizon:CalculatedDatabase used:PVGIS-SARAH2PV technology:Crystalline siliconPV installed [kWp]:3.8System loss [%]:14

Simulation outputs:Slope angle [°]:45Azimuth angle [°]:-90Yearly PV energy production[kWh]:3015.14Yearly in-plane irradiation[kWh/m2]:994.21Year-to-year variability [kWh]:134.00Changes in output due to:Angle of incidence [%]:-3.66Spectral effects [%]:1.73Temperature and low irradiance [%]:-5.32Total loss [%]:-20.19PV electricity cost [per kWh]:0.171

arty688

They are rightly about the inverter I think the general consensus is the inverter should be undersized as that will make it more efficient. 

Magnitio

I am assuming this applies to the Growatt hybrid inverter: if the panels are generating 4.6kw and your battery is not full, then 1kw can be used to charge the battery and 3.6kw converted to AC for use in your house or for export. If anyone knows different, please advise.

QrizB

Magnitio said:

I am assuming this applies to the Growatt hybrid inverter: if the panels are generating 4.6kw and your battery is not full, then 1kw can be used to charge the battery and 3.6kw converted to AC for use in your house or for export. If anyone knows different, please advise.

Maybe.

I've had a quick look through the Growatt SPH3600 documents (link) but can't see anything specific about it's ability to use input PV power greater than 3680W. That doesn't mean that it can't.

I would want to have a discussion with the installer about this, as you would think they've experience with that situation.

1961Nick

I can't see the slightly quicker pick up of the smaller inverter making up for losing 1kWh for long periods in summer. The SPH4600 has the same 150v start up voltage as the SPH3600 anyway.

tomkaier

Thanks guys. Any thoughts on Growatt vs Sofar vs….? I’m finding remarkably little on the app access and general maintenance of the system - are all of them easy to program e.g. with regards to energy imports on an off peak tariff? How would one program one of these systems for octopus agile, is there one better than the other? I’ve also received quotes for a Fox Hybrid inverter, I gather they’re probably in the same ballpark as the other two? 

1961Nick

The sofar is a good basic workhorse. It communicates with the Solarman app & provides very comprehensive data at 5 minute intervals for everything except the SOC which is virtually a live feed. The downside is that it's one way communication & you have to program the inverter using the front panel. That's ok for Octopus Go but clearly impractical for Agile. There may be ways of controlling it using a raspberry Pi & a degree in geek, but those things are outside of my limited ability.

I don't know anything about the Growatt.

arty688

1961Nick said:

I can't see the slightly quicker pick up of the smaller inverter making up for losing 1kWh for long periods in summer. The SPH4600 has the same 150v start up voltage as the SPH3600 anyway.

Not based on any real world experience just google but it says smaller inverters are more efficient with smaller loads.
Also the panels performance is based on idea conditions so I'm not sure a SE facing array in the UK will be subjected to ideal conditions too often and is much more likely to be subjected to low light conditions most of the time.
The idea ratio is around 1.2 so the inverter should be 3.8kw in this case 
Again I have no idea what I'm talking about just what I researched.

2nd_time_buyer

I think it will depend on your elevations and shading. We have 6.7kw panels on a 5kw inverter. The panels are split over SE and SW so they are never all in optimum position at the same time. Even so it has clipped a few times this month already and we have lost a free kWh. 

Personally, I think undersizing is overrated and in hindsight I would have gone for an inverter to match the panel output:

- With optimisers and solaredge inverter, efficiency is still very good even at low wattage
- Having a larger inverter allows you to add more panels later and often the cost difference between inverters is small.

On the opposite argument, the energy lost at low power of likely to be more valuable than the energy lost at high power.

1961Nick

My experience of a 3.7kWh inverter, a 4kWp array & a SSE orientation is that the output flatlines indicating the inverter has reached it's peak.

I accept that a SE orientation is less ideal than the SSE I have, but the OP is located much further south & that should offset some of the difference.

I really do think 3.6kWh is too small for a 5kWp array. If 1.2 is the ideal ratio (?), then that suggests at least 4.1kWh.

Am I being cynical for thinking that a 3.6kWh inverter makes DNO approval simpler & this could be the reason for it's inclusion in the quote?

2nd_time_buyer

"Am I being cynical for thinking that a 3.6kWh inverter makes DNO approval simpler & this could be the reason for it's inclusion in the quote?"

I am pretty sure you are right and that it is a factor. I have not seen any compelling evidence for the often quoted 120% rule you mentioned above. But as I said above, I am pretty sure it does not hold true for panels with optimisers.