How to size a hybrid inverter for battery storage only?

[rsmiele]

I would like to add battery storage to my existing 7-year-old 7.85kw system (7.85kw system: 24 Sunpower SPR-E20-327 panels, SolarEdge SE7600A inverter, and SolarEdge 400/405 optimizers.) Separately, I'm in the process of installing 5 additional E20-327 panels with optimizers to increase the overall system to 9.5kw (no change to the existing inverter).

Given the existing SolarEdge optimizer architecture, it looks like an AC-coupled hybrid inverter is the recommended method to add battery storage. Assume I will be adding 5 EG4 LifePower4 48V 100AH (5120wh each) rackmount battery subsystems; I haven't decided if I will start with 3 or go with a higher number of battery units. The hybrid inverter will not have any direct solar panels attached; it will only manage the battery subsystem.

Questions:

1. Is the AC-coupled hybrid inverter the preferred solution for adding battery storage to an existing PV system with optimizers?
2. How is the hybrid inverter sized for battery storage only?

Thanks!

 

[zanydroid]

SolarEdge optimizers are not reusable for other inverters without semi-heroic efforts. So AC coupling seems the way to go.

There are 2 main sizing considerations which come in only when off grid.
1. Charging power. This can be bottlenecked by either the hybrid or battery
2. Inverting power. Again bottlenecked by either hybrid or battery.

The simple guideline is that both should be equal or greater than the AC coupled output

The reason for (1) is to be able to always safely absorb the AC coupled power when off grid. If you read some of my recent discussions with lux power on the 18kpv thread I mention that charger surge power rating may allow this limit to be exceeded. But 1:1 or lower is simpler.

The reason for (2) is more of a guideline. It means that your off grid system is guaranteed to be able to maintain the output level of (2) even if all solar drops off at once.

In practice the hybrid portion is often symmetric between charger and inverter, but the battery/BMS is less likely to be symmetric.

There is also a lesser consideration in terms of how much AC the inverter can accept on the port the solar inverters are supposed to connect to. Typically this is the breaker rating of the generator AC port. This has some minor on grid implications too in that you might need to attach some AC solar on a different location to bypass the gen port if you have too much.

Did you consider PowerPro? It is cheaper than those server racks, but may have lower charge current by default etc. IMO server rack form factor is probably going away in residential in favor of stackable and wall batteries. And server racks will end up being for data centers ?, since utility scale already has other form factors

 

[Ampster]

How is the hybrid inverter sized for battery storage only?

It should be based on the power you need when the grid is down and/or how much power you need for load shifting. Also if you want to utilize AC coupling to your existing GT system and get the full benefit of that system you need to understand the limits on AC coupling of any hybrid inverter. That may require a deep dive into the specs and it often requires a battery pack of sufficient size to buffer the output of the GT system when AC coupled.
I would also consider not adding capacity to you existing GT system, That will only make the AC coupling more difficult. Consider running those panels to the hybrid inverter since that will give you the same capacity and will at least give you some workable solar until to get the kinks of AC coupling worked out. I know that my SolArk system integrates much better with AC coupling a GT system if it has some of its own DC coupled solar. Not all hybrids are the same and as I mentioned it is helpful to see the real world limitations of AC coupling for each hybrid.

 

[garetwo]

There are 2 main sizing considerations which come in only when off grid.

What if we are grid tied?

 

[garetwo]

I would like to add battery storage to my existing 7-year-old 7.85kw system (7.85kw system: 24 Sunpower SPR-E20-327 panels, SolarEdge SE7600A inverter, and SolarEdge 400/405 optimizers.) Separately, I'm in the process of installing 5 additional E20-327 panels with optimizers to increase the overall system to 9.5kw (no change to the existing inverter).

Given the existing SolarEdge optimizer architecture, it looks like an AC-coupled hybrid inverter is the recommended method to add battery storage. Assume I will be adding 5 EG4 LifePower4 48V 100AH (5120wh each) rackmount battery subsystems; I haven't decided if I will start with 3 or go with a higher number of battery units. The hybrid inverter will not have any direct solar panels attached; it will only manage the battery subsystem.

Questions:

1. Is the AC-coupled hybrid inverter the preferred solution for adding battery storage to an existing PV system with optimizers?
2. How is the hybrid inverter sized for battery storage only?

Thanks!

Have you progressed with this? Have you modified the additional panels to feed the hybrid inverter?

 

[zanydroid]

What if we are grid tied?

If you have a hybrid that grid forms and keeps the inverters up when you are grid down then you still (in principle) need to respect the limits set by the manufacturer on the max AC coupling size

Among other options you can somehow automatically disable AC coupling when grid down