Join Inverters Growatt SPH5000 and Growatt SPH6000 possible?

MikeUK

I was installed two days ago with two inverters and two batteries.

I was led to believe this was the configuration I would bv

MikeUK

This is the config.

At the moment the system has produced nothing.  No power has been received. Though I am told by the engineer that I should receive power as from tomorrow when the Sum comes up, 5 days later.

I have since been advised that

• I should have been sold a single larger inverter
• That the number of panels is too much for the South
• That the East and North will never charge the battery

Should I be demanding the inverters be switched for a larger inverter.... Can the inverters be 'joined' so that the extra sunlight of the 17 panels on the south can somehow be passed to the battery on the North?

Advice appreciated.

BB.

Welcome to the forum MikeUK.

We would need to know a bit more information... Such as the roof angle/pitch and your location. Guessing that the 3.16 kWatt north roof array is at 20 degree angle from flat and you are around Portsmouth/London UK, using PV Watts to estimate harvest (using defaults for the rest of the questions--Assuming a simple Grid Tied (no battery) inverter setup for now):

https://pvwatts.nrel.gov/pvwatts.php

RESULTS

Print Results

Month	Solar Radiation ( kWh / m2 / day )	AC Energy ( kWh )
January	0.39	31
February	0.73	51
March	1.61	128
April	2.91	224
May	4.29	337
June	4.37	330
July	4.40	337
August	3.59	274
September	2.26	167
October	1.10	82
November	0.58	45
December	0.36	28
Annual	2.22	2,034

If you had the same panels facing 180 degrees (south) at 20 degrees pitch instead:

RESULTS

Print Results

Month	Solar Radiation ( kWh / m2 / day )	AC Energy ( kWh )
January	1.08	88
February	1.59	117
March	2.45	202
April	4.07	316
May	5.15	401
June	5.03	376
July	5.22	395
August	4.76	362
September	3.44	260
October	2.26	180
November	1.31	103
December	0.77	62
Annual	3.09	2,862

Or an increase of 2,862 kWH / 2,034 kWH = 1.4x more harvest (40% more) from south facing vs north facing roof assuming 20 degree pitch from horizontal over a 1 year period. Obviously a North Facing array vs South Facing array--The North is going to be seriously hampered by the poor angle to sun during the winter.

More or less, I suggest 3.0 or more hours of sun per day is a "good amount" of sun. Less than that amount of sun, solar really struggles to produce a "useful/cost effective" amount of harvest...

Regarding a larger inverter/solar MPPT unit... Typically we assume that a solar array will generate around 77% or less of its "name plate" rating (basically as the panels get warmed up in sunlight, the output voltage falls, and since Power=Voltage*Current, the output wattage also falls). "Everyone" advertises the "marketing wattage for solar panels", but in real life, you would only begin to approach the marking output in subfreezing climates.

For a 6,750 Watt array * 0.77 panel and controller derating = 5,197 Watt realistic peak array output power (like a few times a year in cold/clear weather.

I do not really know any details about the Growatt system (looks interesting)... But for a simple estimate of monthly/yearly harvest, I would probably use (for Lithium batteries):

• Daily Watt*Hour harvest = Hours of Sun per day (month or yearly average) * 0.61 panel+system derating = average daily harvest (by month or by year)

Use the PV Watts kWH per day per sq meter "hours of sun per day" numbers to make your calculations (or we can play with the NREL numbers to change to off grid/hybrid power harvest numbers)....

Note that grid tied systems are probably closer to 77% rating of solar power harvest, and when running from the battery bank, closer to 61% overall efficiency (sort of "off grid" mode).

Again, I am not your system designer/installer and are making a huge number of guesses here... But some guesses to your questions:

I have since been advised that

• I should have been sold a single larger inverter

• That the number of panels is too much for the South

• That the East and North will never charge the battery

Should I be demanding the inverters be switched for a larger inverter.... Can the inverters be 'joined' so that the extra sunlight of the 17 panels on the south can somehow be passed to the battery on the North?

For battery powered systems--You can have separate battery banks, but that is something I suggest to avoid. You are forced to "balance" the loads between systems and no easy way to transfer power from one bank to the other such as you discharge your "north bank" and have extra energy in the "south bank"... The "north bank" is sort of out of commission until it gets recharged from Utility or Solar power--Or you physically transfer some AC loads from North to South system. Single bank systems don't have that extra management issue.

However, you have 3 different planes of solar panels... Ideally, each plane (north, east, south) should have its own MPPT (maximum power point tracking) input). These inverter/mppt units only have 2 MPPT inputs each--So I can see why they may have done this.

With your setup--I probably would have leaned more to just a North and South only arrays (skip the small east array), one (larger) AC inverter/mppt controller, and one (possibly larger) battery bank.

At this point, as far as I can tell, the south array is appropriate for the 6 kWatt inverter/mppt unit.

Have you reviewed the amount of storage/size of the inverter/charger vs your off grid (utility fail/backup power) usage? A 6 kWatt*hour battery bank is not that large. For example, if you wanted two days of backup power (utility fail, stormy weather and near zero sun):

• 6,000 WH storage * 0.7 (rough guess) useful storage * 1/2 days = 2,100 WH per day of 230 VAC energy

A typical refrigerator uses around 1,000-2,000 WH per day (depending on how large and how efficient) by itself. Leaving enough for some LED lighting, laptop computer, cell phone, LED TV, etc...

Not to say that the N/E array will never charge the battery bank... But as an example (using my PVWatt guesses above) here in November (I am skipping the East array, it is 1/3rd the size and just more math/numbers/PVWatts data to toss around--Not worth the additional confusion at this point in the posts):

• November North Array harvest = 3,160 Watt array * 0.61 off grid Lithium AC system eff * 0.58 hours of sun per day = 1,118 WH = 1.1 kWH per average Nov day

If you assume you have something like 6 hours of "useful light" per day:

• 1,118 WH per / 6 hours of decent Nov Sun = 186 Watts of "average" charging to battery bank across the whole day  

It is possible that the North AC inverter/charger consumes 80 Watts or more of energy when "operating"--So it is even possible that you would effectively have near zero energy harvest on a dark November day in the UK.

We can get more detailed in the actual harvest numbers (your location, roof pitch, your expectations and/or what you were told by your designer/installer)...

And you have to decide if the amount of system harvest and costs are "worth it" to you... For example, (just guessing about UK power costs and how p vs Pound work):

Just the north array system harvest (assuming grid tied operation):

• 45 kWH per month (45,000 WH) * £0.51/kWH = £22.95 worth of solar power from North Array for month of November

We can do the same for the South and East arrays (which will be better with larger array(s) and facing "not north").

My disclamers... The above is a lot of guesswork from across the pond. I have no information on the details of your installation or how the Growatt system is configured/performs). 

The above is as much to sort of "level set" your expectations and show you how the math and tools work. I can argue that a north facing array in the UK is not ideal--But only you can decide if it is cost effective for your needs/expectations (such as South roof is "full" of panels--Need additional roof space if you want more panels).

Also, I only showed the math for the North Array production in November (again based on my guesses)... Overall, you should be getting quite a bit more from your south array.

My method of calculating harvest is simplified but will usually get you within 10% of so of the "real numbers"--And remember just variation in weather for a few days or even a month can be 20-50% (more or less) vs the long term averages used in these tools (typically 20+ year averages). 

Also location can have large effects too (marine layer vs reference site, shading from overheat power lines, buildings, chimneys, trees, etc.). Solar Electric Panels only do will in direct sun... Any even minimal shading (such as power line shadows across array) can easily kill the harvest by 50% when shading is present.

As always, start asking more questions of your installer and compare to what they "predicted" the performance to be vs what you are seeing. From here, I cannot tell if there is a "north array" issue, or it is simply operating as expected (when there is little sunlight for the last few days).

Any corrections/updates to my guesses and/or further questions?

-Bill