I have an on-grid PV system generating 2.6kW tops.
Planning to install a Multiplus with LiFePo4 batteries for a backup to essential loads.
The factor 1.0 rule tells me that I need a 3kW inverter. But my essential loads are below 1kW! I’ll never use 3kW!
Can I install a low power inverter?
Thanks.
Mark Jordan
No. The inverter needs to be able to take 100% ac pv production and squeeze it into the batteries at ANY time.
Therefore, also the battery must be able to take 2.6kW for as long as it takes to shutdown the ac inverter.
But this only applies for pv on acout. Pv on acin will be shutdown on grid loss immediately, hence doesn’t need to be considered.
So, the Multiplus 48/3000/35 would satisfy the rule.
But wait, 48V x 35A = 1680W ! The PV can generate 2600W!
Do I need to install two in parallel?
The Multiplus 3000 can provide a peak rate of 5000W according to the datasheet.
Without knowing the details here, I would assume that for AC-DC as well. But maybe that someone with more knowledge about that detail should answer.
If that’s the case, it would be sufficent time to shutdown or throttle acpv in such an overload situation.
If this is not the case, you’d better get a 5000, running one unit is easier and smoother than paralleling 2 units.
The factor 1.0 rule clearly says:
VA rating of the MultiPlus = max. PV inverter power
For a short time the MultiPlus can push more power from AC to DC.
In that time it also increases the frequency to throttle the PV inverter down.
If the PV is connected to ACIN, can I use a timer relay to switch it to ACOUT during the grid power fail?
When the grid returns, the timer relay would reconnect the PV to ACIN.
Something like this:
That way I could use a 2000W Multiplus for my essential loads.
That doesn’t make sence, amount of PV stays the same, and it would require reconfiguration of the system, when the location of the PV changes.
But if you want to keep the inverter power small, you have other options.
Best would be having DC-PV, because that would also make your system able to black start in the morning, when the batteries run empty over night during a grid-loss. acpv can’t do that.
Also the efficiency of DC-MPPTs is way better, when it comes down to primary charging a battery.
What kind of reconfiguration would be necessary?
When the PV is connected to the grid and ACIN the inverter would be in stand-by.
When the grid fails, PV is disconnected and the inverter would start feeding the load.
When the inverter is running feeding the loads, PV would be connected to ACOUT and gradually increasing power generated, until the inverter raises frequency to limit the power.
It makes perfect sense to me and avoid the full power situation switching.
The functionality that reports the acpc production to the gx would need to be reconfigured, so the system then knows, that pv production moved from acin to acout.
It avoids nothing, the situation would be exactly the same: You would have a violation of the factor-1 rule after switching to acout. (When you undersize the inverter)
If your system is correctly sized, there is not even a need to switch back to acin, when grid is present. The multi can passthrough pv power from acout to acin, whenever it doesn’t need it.
Only when grid is lost, the multiplus needs to be able to consume the acpv on acout at any time, cause else that excess energy has nowhere to go.
It could be configured to PV on both sides, ACIN and ACOUT.
Half true, because the inverter wouldn’t allow the PV power rising above load. And when grid power returns it would be disconnected.
In my case there would be the problem of PV generating more power than the inverter.
Exactly! When grid is down the PV would switch off automatically if connected to ACIN. Later it would be switched to ACOUT and starts generating again, controlled and limited by the inverter.
Another solution would be dividing my PV in two and connecting half in ACIN and the other half in ACOUT an using a 2kW Multiplus.
Sure it will, if there are clouds, it will unthrottle ACPV, allowing 100%, and the very next second acpv may spike up to maximum power.
But I don’t understand the discussion about this topic. There is a rule for reasons. Obey it, if you want a save system. Period.
Whether that makes sence in your opinion or not is up to you.
I would disagree that running 1 unit is smoother than running 2 in parallel.
Also, if running 2 inverters in parallel, if one fails, it can be removed from the system, which can then operate at a lower capacity, instead of a total system failure. This is a factor in off grid design…
There are a lot of issues that can happen with two parallel’d units that won’t happen with one inverter.
Fail-safety is the only reason when 2 in parallel make sence. (But it is no automatic-failover, requires a system reconfiguration as you said)
The best of both worlds is running a 3 phase system with one unit per phase to avoid all concurrency issues, but also beeing able to re-configure that system to a Split-Phase 120 or 240 in case one inverter fails.
I’ve been running a parallel system for more than 10 years with no issues. This also gives a much higher instantaneous peak current (in the mS time frame) than a single inverter. For me the ability to reconfigure on fault, and the above is important.
The Multiplus need a second to rise frequency? Is this the response time to a rising PV power?
I don’t know the reaction time of the multiplus on this, and “the second” was just “spoken language”.
But you can replace second with millisecond if that makes more sence 
The event of detecting “too much power” has to come first (obviously), so by the time, the frequency shifting starts to kick in and the PV Inverter starting to react on it, there is a timespan where the system eventually has to deal with the “100% production case”. (It is not like PV could deliver 100% at any time ofc.)
If there won’t be a possibility for this to happen, there wouldn’t be a safety rule targeting this.
Also, consider you are currently having 5000 Watts load, AC PV is therefore running at 100% (say 2500) and the multi discharges by 2500 Watt as well. Now that consumption stops. It’s inevitable that the system now has to switch to “charge the battery with 2500 watts” for a short amount of time - but it has to be able to do so.
So, the best solution for me will be splitting my PV system in two. One half connected directly to the grid and ACIN of the Multiplus. The other half connected to ACOUT of the Multiplus. Both generating 1,3kW peak power each.
This way I can use a Multiplus 48/2000/25 with a 5kW LiFePO4 battery.
As my load would be less than 1kW, the PV can supply that power to help when sun is shinning.