Inverter Sizing

[Matto]

Hi

I have been provided a quote for:
10 off 420W JA Solar panels on a roof with west elevation
2 off EP5 batteries and junction box
1 off H1-3.7-E Hybrid inverter

I can't quite wrap my head around the sizing of the inverter so I wonder if you guys can point me in the right direction.

Here are my questions and any input would be appreciated:

• Does the 3.7kW relate only to the AC side of the inverter or is it the entire power path together?

• If the answer is yes its only the AC side then does that mean if I was somehow generating the absolute 4.2kW peak from the panels can I take 3.7kW out of the AC connection and dump the remaining 500W into the battery?

• What would the maximum charge rate be for the batteries if the panels were generating 4.2kW and nothing was being taken from the AC output?

• Are there any disadvantages to using two EP5's rather than an EP11, the contractor has said they are having to supply EP5's and JB's due to the lack of availability of EP11's?

• Is it better to under or oversize the inverter and does the inverter specified seem correct for the panels listed?

• If the panels were changed to Jinko 480W items or Aiko 455W would that require shifting up to the 5kW version of the inverter or better to stay as we are?

• Am I best off sticking to the West elevation only or would it be wise to split the panels between east and west?

[Dave Foster]

Yes only the AC side

Yes, you could be taking 3.7kW from the AC side and the remaining .5kW would be used to charge the batteries (dc)

The maximum charge rate would be approx 3.4kW (3.7kW less losses)

On this size of inverter not many disadvantages, the EP11’s are higher voltage and on larger inverters would be better as the max dc current limit would be lower and hence better at dealing with lower temperatures.

It’s better to oversize and the panel / inverters are well sized to work together - but the maximum AC you will ever be able to provide is 3.7kW so if a kettle and toaster were on together you will be drawing from grid the excess.

The larger panels would clip on sunny days, for the 3.7 better to stay with Aiko

On the proposed system a west elevation is good if all panels are facing West. An east /west split would give your more power in the morning - see my final comment.

As a final point, whilst the 3.7kW inverter is easy to get G98 for, you would be much better getting a larger inverter - firstly the price difference is marginal, secondly you will be more likely to meet all your demands from solar / battery than having to use grid to top up.

With a bigger 6kw inverter you can have more panels, and the bigger panels and also a west / east split - I can’t stress enough that the bigger inverter would be considerably better for you - I know so many people that have installed the 3.7 and suffered regret and/or changed to a 5 or 6 kw inverter after a short period - if you can get panels on the east and west do so, install as many panels on both but don’t exceed the inverters single string limit (16A).

With a larger 6kw inverter you would be better with the EP11’s as they are higher voltage with a lower operating dc current needed to operate - particularly if the batteries are exposed to lower temperatures.

[Matto]

Dave, thanks for the awesome reply. That all makes sense

One other question I have is around the EPS connection. I plan to have a separate CU connected to the EPS output configured in UPS mode. This will run lights, the fridge, IT equipment and boiler in the event of a grid outage.

I understand that as part of this the installers will convert my TNCS earthing system to TT.

Looking at the install guide for the H1-6.0-E it implies that I need a 10mm2 supply cable to the inverter and a 63A breaker. This is to cope with the peak battery charge current from the AC of 26.1A and a potential further 26.1A being supplied to the EPs output which totals 52.2A max. Have I read this correctly?

If so are these inverters usually connected by an MCB or RCBO? It would appear that 63A single pole MCBS are available for my consumer unit but 63A single or double pole RCBOs are not. Is this likely to be a major snag?

[Dave Foster]

You have read that correctly but it is a worst case example based on the EPS running the maximum possible scenario whilst the battery is charging at peak power.
In reality you won’t have that much of a load on your EPS, from what you’ve said probably nearer 16A so you would size it to suit your maximum potential load.

Usually the inverter will be connected via its own small 4 way consumer unit which would be protected by both MCB and an RCD to suit your system size - that way you don’t have to worry about bending your normal consumer unit out of shape.

[reef]

Further to what Dave mentions above, definitely get the larger inverter and as many panels as possible on both of the east and west roofs. If you get the larger inverter then all of your wiring installed will be uprated for this too.

I made the mistake of getting only one roof done initially (fortunately with a 5kW inverter, but it took some convincing as a lot of installers won't want to do a G99) and then had to get a second inverter and array later which cost more. It wasn't worth going for one large inverter at that time as the wiring would have had to be redone leading to further expense.

Panels are cheap and installation/scaffolding are expensive, so its most cost-efficient to do it all first time.