I found this thread to be rather interesting in a number of ways:
OP had a simple requirement. Use the batteries with the inverter from 3pm-9pm to power around 1500 watts of load.
At 9pm then switch the inverter to pass through. In other words use the grid to power the 1500 watts of load.
At 10pm then charge the batteries while powering the 1500 watts of load.
That sounds simple enough to do. The OP received an inverter from SS that failed to do that.
I was very impressed that SS would spend the amount of time to try to make it work. I am not at all impressed with their methods though. Changing the firmware to try to jury rig a inverter into doing what it should have done out of the box to me is just plain not acceptable. It can lead to multiple problems. It implies that every unit has a different firmware and then when a field engineer upgrades it then it no longer works.
My thought is that if you have a defective unit then it needs to be replaced. I have two thought patterns either the OP had a defective unit or the inverter has a design flaw. He provided plenty of evidence that the unit was defective. From what he described the unit would do whatever it felt like doing. It would start to sell power back to the grid which presents multiple problems. Who wants to get involved with the Power companies and have them threaten to turn off your power because you have a defective inverter? So to me this was a major issue. The fact that the inverter did not have a sense of time was also disturbing. If you program your alarm to go off at 6 am, then is it okay to have your alarm go off at 9:21 am?
The OP received an inverter that did not do what it was suppose to do which we really do not know if it was just a defective unit or not. This is where SS really dropped the ball. They could have setup a unit and then programmed it to do as the OP was trying to do. I can see the frustration from the OP because who wants to remove a 100 lb inverter from the wall and then package it up again?
The only thing that really bothers me is that we have a lot of assumptions without any real facts.
1- Is this a design flaw or is it a defective inverter? If the OP would have returned the inverter and then got another one then we would know whether it is a design flaw of a defective inverter. Now I am just left wondering.
2- Would I purchase from SS after reading about this? Probably but I am not really sure because it seems that their support team is overwhelmed and there is not a clear policy on how to handle issues that come up. It is great that they have a USA office and you actually do get support. I just have a problem with changing the firmware for a defective unit. I think a more appropriate response would be for the Engineers at the company to simply program a inverter to run as the OP needed to have which is rather simple in my opinion. Some of the responses from SS were totally inappropriate. I could sense a great deal of frustration from both parties.
What would be nice since
@Will Prowse has a similar unit and wants to do a video, If Will could disconnect his Solar coming into the inverter and then program his inverter to run a 1500 watt load from 3pm-9pm on batteries. Then at 9:01pm switch the inverter to pass through and then at 10pm then run the load and also charge the batteries. If Will Prowse does this then we will all know that this inverter operates properly and the OP just got unlucky.