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Inverter Selection

A project log for Solar Gas Station

Off-grid solar & battery shed for EV Charging and RV Power.

Bryan WilliamsBryan Williams 01/22/2022 at 15:210 Comments

After comparing a great many recommended units, I settled on the GroWatt 12 kW 48V 150 VDC 120A Off-Grid Inverter, which is actually a rebranded Sunrino SP Series low frequency split phase inverter. GroWatt has their own designs that use high frequency conversion technology, but have their own quirks. Decoding the part description, the primary inverter in my system is 12 kW continurous output with split-phase 120/240 VAC output as is the standard residential service voltage in the United States. This allows 120V or 240V circuits at the output. Battery nominal voltage of 48V (Lead and Lithium are supported), 150 VDC solar input (open circuit voltage of string), and 120A battery charging circuit. 

The 150VDC input limitation is one of the biggest drawbacks to this particular model. My bifacial 410W panels are 49.9V open circuit maximum. This means I am maxed out at 3 series panels, which is unfortunate as it means using combiner boxes and paralleling strings. This model supports two independent MPPT tracker inputs.

The battery charger of 120A x nominal 48V battery works out to 5760 Watts maximum being pushed into the battery storage. The maximum PV array power for this model is 7000 W. Recall I have a full pallet of 30 x 410W panels. It took reading this spec to realize that I wouldn't be able to get by using a single inverter with my 12,300 W panel array! I went back to Signature Solar's website to find the 12 kW models were sold out, but an 8 kW was available. I figured this would not be too much of a compromise, as this could be dedicated to my 32 Amp EV charger. Both units have the 7000 W solar input limit, 120 A battery charger circuit. Both units have the ability to charge the battery from the AC utility, with the significant difference between the units being a 70 A limit on the 8 kW model vs a 100 A limit on the 12 kW. 

I don't intend to use AC charging unless absolutely necessary, but it is a nice-to-have that I can use a relay control to allow charging if the battery gets low at night and control the time of charge so only "super off-peak" 12am to 6am rates are used. In case of a cloudy week with poor solar output, I can still use grid power and use the battery to time shift the load. This is still an off-grid use, as the inverters cannot supply power back to the grid.

The Lithium Iron Phosphate cells I am using are EVE 280 Ah cells, with a standard charge rate of 0.5C, or 140 A. The charge rate of the battery is really what drives a lot of the ratings specs it appears, from 7000 W limit to the solar array to the 120 A, 5760 W charging circuit.

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