48V MPP and Growatt Recommended Battery Ah Sizing

[Henderson]

I've noticed that the MPP 6000 series and Growatt 5000ES and a few other Growatt models recommend a battery sizing of 200Ah (10.24KWh = 200ah*51.2v) per inverter. Does anyone know what is the basis of this recommendation? When I run the numbers, the 200Ah recommendation seems oddly low. For example, assuming 5 hours sunlight and a 5000ES inverter, assuming 90% overall efficiency that's a total of 5000w*5*0.9 = 22.5kw which equates to approx 400Ah. Is MPP's and Growatt's assumption that the amount of solar I am generating, that 50% of it is being used for my load and the other 50% used to charge the batteries? And if that's the case, if during sunlight hours my load only consumes 10% of solar generated, will it be safe to have a higher Ah battery on these inverters seeing that 90% of solar generated is being unused and can therefore go to charging the batteries (given the charging amps capacity of the inverter of course)? For my power utilization scenario (and I suspect for alot of other folks) my power usage is much higher at night than during the day. Thanks in advance.

 

[MichaelK]

Battery recommendations are based on a couple of different factors, primarily based on amp draw out of the battery. As a general rule of thumb, you don't want to draw more than 1/8C out of a lead-acid battery, 1/5C out of an AGM, or 1/4C out of a Li battery. So, it's load specific.

A second factor is what is known as "ripple current", which is pulsating DC into the battery at the same frequency as your inverter. It is supposed to be damaging to both the batteries and the electronics? I believe that the amount of "ripple" can be measured by setting your multimeter to AC volts and measuring the voltage of the battery? A bigger battery will withstand ripple better than a smaller one.

Yes, I would agree with your numbers that a battery should be in the 400Ah range. In fact, that's what my 6848 manual states as the lower limit. I think that the mentality of MPP and Growatt is that their units are budget oriented economy units, and a budget oriented battery will go with it? So, basically, they are going cheap!

To answer your question, I'd say it's always better for the system to have a bigger battery, IF that sized battery can be adequately charged. Many, many solar enthusiasts have discovered over the years that big batteries and small solar results in inadequate charging that has ruined their lead-acid batteries, though this is not really a problem with Li.

I think a very good rule of thumb though for any battery chemistry is if your battery can't be fully charged by noon, then your solar capacity needs to be increased.

 

[Henderson]

Thanks for the feedback.