tas6 avatar image

Off grid 1 to 1 rule for PV ac coupled solar

Off grid installation

I have currently 8.7kw solar through a Fronius 8.2 primo ac coupled to AC1 out on a 10000va Quattro +
Us2000 Pylontech X 5 batteries. 28kw single phase generator and Color GX to monitor and control with a lynx dc
I have a further 8Kw of solar still to be fitted

Current settings

Fronius limited to 6kw output max (just to be safe - I hope to increase this to 8.2kw after input from this thread)

Generator configured to come on when SOC falls to 20% to protect batteries

Plan - questions

I am considering placing to 8 kw on the dc side with a Victron charger (could you recommend one)

Using the 1 to 1 rule am I right in saying i can safely go up to 10kw of ac coupled solar (possibly by adding a second smaller ac coupled Fronius) note that the actual charger on the Quattro states 140amp as the charging max.
if I put the other 8 Ish solar on the dc side will this have any effect on what I should limit the ac solar too?

I am thinking to add a further 3 pylon tech to the system to bring me up to the 8 max they allow without a separate controller but I read somewhere on Victron that I should have 4.8kw of storage for each 1.5kw of solar - this is an awful lot of storage !? Is there a reason for this ?

I intend to program the Fronius to reduce its output to 50% when the SOC is above 90% to protect the batteries and attempt to stop them going all the way to100%

I am having great results currently running completely off grid - tumble dryer, washing machine, dishwasher electric ovens etc - AND charging an Audi etron but I am trying to organise myself for winter

Thank you for your replies

PylontechFroniusquattro 10kva
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2 Answers
Mike Dorsett avatar image
Mike Dorsett answered ·

Hi, The 1 :1 Rule is to protect the inverter/ Charger from overload when there are no other AC loads to take power from the AC coupled solar. Adding DC charging does NOT affect this.

Assuming your system voltage is 48V (Not explicitly stated), 8kW at 48 V is 167A. As Victron don't produce an MPPT with a 175A rating you will need 2 off 150/85 VE.Can chargers, giving you max string voltage of 150V and a total charge current up to 170A.

The ratio of storage battery to solar is determined by several issues.

1) How much energy do you get from your solar in a day? This depends on where you live. Some energy is used directly, and it is good to have a little extra to produce power on a cloudy day.

2) Autonomy: to operate successfully off grid , you should be able to store about 20 to 24 Hours energy in 80% of your battery capacity as a minimum. This takes you through the evening (usually highest demand) and the night (lowest demand). for about 4/24Hours, all your power comes direct from Solar. This ratio also depends on where you live (How many sun hours you get ).

3) Charging capacity: Your battery must be rated to absorb the charge at the rate your system can produce it. Some batteries are rated at C/2 max charging current, some are rated at 1C, some are low at C/4. This includes all charging sources from AC and DC.

Generally once you achieve 20 Hours of autonomy, rule 3 is achieved also. Adding more batter will help increase the solar fraction by storing excess energy for cloudy days. There are on line resources that give you a map of annual solar resource in kWh/kW, which is a good starting point for determining how much solar you need.

1 comment
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I would use the 450/200 instead of 2x 150/85. A lot less/smaller wiring and 4 trackers.
tas6 avatar image
tas6 answered ·

Thanks Mike (yes 48v)
on the 1:1 situation

Am I right in saying I can ramp up my cap on the ac coupling Fronius to 10kw (as a combination of the 8.2 I have + possibly another one rated at 2) ?

As I understand it the pylon tech are normally 25amp charging but with a max of 100amp for 100 sec so this should be long enough for the Quattro to wind the Fronius back using frequency shifting.

I have the gen also programmed to kick in on 85amps draw from the batteries so this should protect them from large ac loads ?


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