Where I'm at, grid power is super cheap at night, sometimes down to zero cents/kWh, but often much less than 2 cents. During the day I run an air conditioner off a PV/battery setup, but I don't have enough battery to run more than about three hours after sunset. I switch the AC over to grid around midnight and run it until sunrise, but the batteries are depleted as low as I want them to go from running after sunset, so I have to wait until around noon to switch the AC back over to PV.
What I was considering was building a simple rectifier to turn my grid 120VAC into 120VDC, plus some large capacitors to reduce ripple, and connect that to the PV inputs on my KID. I'd use a diode to prevent backfeeding the PV panels, and then theoretically the KID should be able to use that to charge the batteries. They're AGM, in a 2x2 array to give me 24V FWIW. This would all be controlled by a simple timer that switched on at midnight and shut off at around 6am.
One question would be, do I need some sort of current limiting? Another is, how crazy does this sound? I looked at quality 24VDC chargers that would connect directly to the battery array, but they're several hundred dollars and though I'm doing this mostly on principle, I'd still like to actually offset the cost via reduced electric bills.
Generally not advised.
Not only would the grid be too stiff, especially when the KID is trying to sweep (MPPT Mode) your 120 DC will be closer to 200 VDC. Others have done it but it is really not a good idea.
HTH Doug
There are inverter/chargers but I suspect that doesn't align well with doing on the cheap.