How can I use an on-grid solar inverter in an off-grid setup? - Page 1
Forum section: electronics / general

author: mrnams
author_rank: Newbie
timestamp: April 17, 2025, 01:11:48 pm
content: I have a solar system with an on-grid (grid-tie) inverter, which only works when there is an active grid supply. Unfortunately, I live in an area where there's no grid power for most of the day, so my solar setup goes unused during that time.Buying a new hybrid inverter is quite expensive and would make my existing on-grid inverter a waste, which I want to avoid.Here's what I’m thinking:1) I have a 12V 150Ah battery bank connected to a pure sine wave inverter, which generates a stable 230V AC output.2) I'm considering disconnecting my solar inverter from the actual grid and connecting it instead to the output of the battery inverter.( In image connecting Point S-B)3) This would act as a “fake grid” to simulate grid presence, tricking the on-grid inverter into syncing and starting solar power production.4) Then I would run my AC loads using the combined output from both the solar inverter and the battery inverter.My questions:1) Is this approach safe and practical?2) What are the risks of backfeeding from the on-grid inverter into the battery inverter?3) How can I protect the battery inverter from overcurrent or overvoltage due to solar surplus?4) Are there proven dump load controller circuits or open-source solutions to balance this kind of setup?

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: April 17, 2025, 03:18:57 pm
content: In another forum, a member claimed that he had done something similar to your proposed setup, but instead of using a pure sine inverter as the “pacemaker” he used a gasoline generator instead.Don’t recall whether the generator was of the inverter type, which would make it almost identical to your proposal, but with gasoline producing the actual power generation, or whether it was a simpler stator winding-generated sinewave.Did a quick search but couldn’t find the actual post.Hopefully someone else will chime in.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: April 17, 2025, 03:24:27 pm
content: Thinking about it a little more, what would happen if the grid-tie inverter attempts to feed back excess energy into the sinewave inverter/ generator?Unlike a real grid, which would absorb any or almost any amount of energy fed back, how would a much less compliant source behave?I don’t know.

author: kripton2035
author_rank: Super Contributor
timestamp: April 17, 2025, 04:43:22 pm
content: there was a post here about the same problem, I can't find it either.but I remember someone made it and it did not work well I don't recall why.

author: rteodor
author_rank: Frequent Contributor
timestamp: April 17, 2025, 06:19:17 pm
content: Thefactor 1.0 ruleapplies. It says that "The max PV power must be equal or less than the VA rating of the inverter/charger". Also the inverters must support frequency shifting (so they can "communicate") and the hybrid inverter must be "reversible".Do read the link to understand it better.QuoteYes it isifthe inverters and battery are properly sized and configured.QuoteDescribed in the link: You can destroy a lot of your equipment.QuoteBy making sure that- your hybrid inverter and battery are able to absorb shocks that appear when large loads are disconnected.- your grid-tied inverter can be controlled by the hybrid inverter via "frequency shifting".QuoteDon't know about such solutions. As described above there are 2 situations to protect against: large load disconnect and overproduction.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 18, 2025, 03:59:49 am
content: How about mod some cheap surplus switching PSUs to run from the solar DC? Just wire to the solar string for backup power.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 18, 2025, 11:54:31 am
content: There are at least two differences between "Grid Tied" converters and "stanadalone" converters.The Grid Tied versions need to sense the mains AC voltage before they even turn on (this can be circumvented quite easily)But the second (already mentioned by UnijunctionTransistor), the grid tied converter optimizes power delivery back to the AC power. This is in fact the sole purpose of these things. While standalone converters (presumably) output a steady AC voltage, and then adjust the delivered current to whatever your load needs (within it's capabilities of course).Here in the EU, the grid tied converters turn themselves off if the mains voltage gets too high (I think the limit is 252Vac for a 230Vac mains net). I don't know whether this cut off is abrupt, or whether delivered current is gradually reduced when this maximum voltage is approached. This may depend on the manufacturer / model. I also have not looked into regulations.

author: Jeroen3
author_rank: Super Contributor
timestamp: April 18, 2025, 12:18:37 pm
content: On-grid pv inverters want to output all their power. This is a problem if there is no place for it to go.If you have a 4-quadrant battery inverter, it can take it's power until the batter is full. Hence why victron has the 1:1 rule mentioned above.So on-grid pv inverters need to ramp down their output if there is no way for the energy to go, victrons inverters do this by increasing the frequency and relying on the frequency drooping that is built in in pv inverters.Just as with rotating producers, if you consume less energy from them, they will speed up due to the speed droop of the prime movers. Victron emulates this.It will be difficult to use other means of control, such as modbus or api's to reduce the output, as this is typically subject to ramping and it also isn't very fast orrealtime.You can combine PV with generators, I suspect the 1:1 rule will turn into a 1:3 rule (for 1 VA of pv, 3 VA of generator) mostly also due to harmonics.You cannot combine PV with inverter/generators or with other 2-quadrant inverters. Eg: Two ideal sources do not share load.

author: mrnams
author_rank: Newbie
timestamp: April 18, 2025, 05:47:48 pm
content: Thank you all for spending time to reply my post. I feel i should buy hybrid inverter than playing with these.