Solar PV battery & inverter compatibility

[LaChab]

 

Hi,

 

I'd welcome some guidance on buying a solar inverter.

We've recently fitted the panels, so we now need to buy an inverter. (possibly Solax X1 Hybrid G4, as we've had one of their string inverters fitted to our current system for >10 years with no problems)

It seems a bit short term to get a string inverter, as I'm sure we'll be getting batteries some time. (Fogstar Seplos look interesting)

I don't really understand AC charging (Victron?), so I was going to go for a simple hybrid inverter, but I'm worried about compatibility with the battery. 

I've no wish to be restricted to batteries of the same make as the inverter. 

Is that a problem? Does the inverter need to interface with the batteries? Are there common standards? Does it matter?
So many unknowns!
 

[Gone West]

1 hour ago, LaChab said:

So many unknowns!

I'll follow this with interest.

[cjsparkey]

If you are thinking of going with Fogstar battery they have a list of hybrid inverters that are compatible with on their website.

 

 

Plug-and-Play Inverter Compatibility: The PACE Battery Management System (BMS) seamlessly integrates with a wide range of popular inverters, including (but not limited to) Victron, Pylontech, Growatt, Sofar, Solis, and Goodwe

Edited March 20 by cjsparkey

[SteamyTea]

3 hours ago, LaChab said:

Does the inverter need to interface with the batteries? Are there common standards? Does it matter?

Interesting question.

 

Basically, you cannot just pump energy into a battery, even at the correct levels i.e. volts and amps, though should really be talking about coulombs.

 

When batteries are charged, and discharged, there is an optimal temperature range the ensure a reasonable charge/discharge rate. Keeping within the correct range will give maximum longevity.

As battery chemistries vary, even within a similar group i.e. lithium, lead, sodium, different charging and discharging regimes are needed.

This is usually taken care of by a battery management system (BMS) which may be a dedicated unit for a particulate battery chemistry, or programmable for different chemistries. They can often sense battery state of charge (SoC), temperature and individual cell conditions. But not all will.

BMS are often built into the battery packs.

 

Now an inverter is, in basic form, a controller that can take in a 'wild' voltage and amperage, process it to output a very controlled alternating current i.e. 230V at 50 Hz as a true sine wave.

Then it gets complicated. Safety features are added on i.e. grid impedance for automatic disconnection, voltage regulation for local standards i.e. 230V -6% +10%, power limiting i.e. 16 A per phase, frequency matching to stop poor power factors and unwanted harmonic, maximum power point monitoring (MPP) which is an interface between the inverter input and the load that it is supplying, there is often a minimum load that an inverter will supply i.e. 200W.

 

So even before one starts thinking about charging batteries, an inverter is doing a lot of work.

 

Chemical batteries do not like to be worked too hard, especially when charging. You can think of a discharged battery as an empty carpark. Initially it is easy and quick to park a car (high voltage and current) as it gets fuller, finding a free parking spot is harder, especially if the cars are not slowing down (electrons move at close to light speed in a pure vacuum, bit slower in copper wires).

What the BMS does is control the rate that the electrons can enter the battery. This is part of the reason why batteries are often charged between 20% to 80%, it is faster and preserves the battery's chemistry.

 

Expecting a single inverter to do all the above i.e. legal requirements (grid disconnect), power management, variable load delivery and battery management is a big ask, especially for under £1000.

 

So does it matter, yes, is it best to buy into one system, for reliability, probably yes. Is there an alternative, yes, but takes a lot of homework and fiddling.

The way around this is by having an inverter and BMS that plays it a bit safe, so not the fasted charging or discharging, may compromise the overall efficiency, and can reduce the effective life of the batteries.

It is a bit of a minefield really, but clever engineers do their best to get the optimal systems all working together, at an acceptable cost.

So either don't overthink it, or overthink it too much.

[LaChab]

@cjsparkey Thanks for that.

[LaChab]

@SteamyTea Thanks for the explanation. That's really helpful. Althougfh a bit worrying as well!

[SteamyTea]

20 minutes ago, LaChab said:

Althougfh a bit worrying as well

Just don't over think it.

Just do costings of a hybrid system and a separate component systems.  Base it on 6 years of life.

Things will have changed by then.

[LaChab]

@SteamyTea Thanks for the reassurance! Could you possibly give me an idea of what I would need for the separate component system.

[JohnMo]

40 minutes ago, SteamyTea said:

Just don't over think it.

Exactly - just do AC coupled. One box does it all.

 

Leave the PV on string inverters to do it's thing. Good thing about AC coupled it runs the whole house (ours does 6kW output until battery flat, if you want), so a power cut comes along the house just carries on as normal, string inverters also stay online.

[SteamyTea]

4 minutes ago, LaChab said:

Could you possibly give me an idea of what I would need for the separate component system.

Not easily as it all depends on which components you choose and how they interact with each other.

 

Take @JohnMo's advice, fit and forget, when it breaks or looses too much performance, salvage what you can and replace what you cannot.

[LaChab]

@SteamyTea @JohnMo Thanks. I'll try and cut down on the over thinking!

[MikeSharp01]

1 hour ago, JohnMo said:

Good thing about AC coupled it runs the whole house (ours does 6kW output until battery flat, if you want), so a power cut comes along the house just carries on as normal, string inverters also stay online.

How do they manage the switch over and back again when the AC mains returns is it automatic?

[JohnMo]

26 minutes ago, MikeSharp01 said:

How do they manage the switch over and back again when the AC mains returns is it automatic?

The basics of how I operate 

 

Charge to 100% on cheap rate. So at 7.30am it's fully charged. At 7.30am house is provided by battery. If sun's out the PV feeds house first and then tops up battery if not at 100%. Any short fall in PV output then battery tops up house. It continues providing power to house until at 4% charged, irrespective of what occurs to mains. Everything occurs automatically.

 

When It gets to 00.30, battery charges again and in cheap periods mains supplies house, unless there is a power outage and then the battery takes over automatically.

 

I just set the tariff rates and how much to charge to, then leave it to it. Can do stuff with home assistant like set charge % based on solar forecast, worked most the time, but not always, so gave up on it.

 

Last week or so has been good for PV, but still cold (-3 last night) but run ASHP during day on excess PV to charge the floor, basically heating almost for free at a CoP of 4.5 to 5. At 4.30pm battery generally at 95%. 

 

Energy costs about £1.70 a day this week including heating and hot water. Last week was an average of £1.90 a day. Equivalent of 6.5 to 7.5kWh at standard rates.

 

On a bad day I will export 3kWh, good days less than 1kWh - I don't get paid for export 

 

 

[Dillsue]

23 hours ago, LaChab said:

 

Hi,

 

I'd welcome some guidance on buying a solar inverter.

If you're not sure on the config you want/need, get yourself a secondhand inverter off ebay to get the panels up and running. No pressure to make uninformed decisions and fair chance youll be able to sell it again when you know what you want.