Electrical – how do solaredge power optimizers work? Can I use them to charge battery pack directly

• It can be done

• It will probably NOT save you money.
  PV power is still not really competitive with grid power on a level energy field in many cases. Subsidies and similar may sway this somewhat, but maybe not enough.

What Bobbi says. Plus:

You could feed the panel into the 12V directly via a simple regulator that only supplied energy when the PSU power-good signal was asserted, showing that the supply was active, and the regulator could be arranged to never cause voltage to rise above 12.1V or some other limit.

You'd need to be sure that when PV input was applied that the PSU did not backfeed in unexpected manners. A diode in the PV feed stops backfeed to the PV but stopping psu backfeed may or may not be as simple as adding a diode in the PSU's 12V output to the GPU.

If you are doing this "from scratch" you want to determine if it is economic compared to grid power. If you receive local subsidies and feed in tariffs (FITS) it may be. Otherwise maybe not. And it may depend what you cost your time at.

At an average of 3 full sunshine hours per day over a year at a typical location you get about 3 units x 365 = 1095 hrs/year or say 1000 units of electricity per year from 1 kW of panels. So if electricity costs N cents per unit effective cost including any FITs subsidies etc then this will save you N x 1000 cents a year or 10 x N dollars. So if your electricity costs 20 cents /unit you save 20 x 10 = $200/year in power costs. Say $300 outside.

A very optimistic cost for installed panels* would be $2/Watt so 1 kW of panels would cost $2000 installed so would cost $2000/$300 =~7 years to break even if your money is worth 0% if invested. That's at 30c/unit and $2/Watt installed and 0% on your money. Adjust as required. * I can land panels here ex China for about $1.30/Watt at my door. Then add installation costs, mounting hardware, cabling, any control equipment, any regulatory costs, and my time at $1/hour. Add ongoing cleaning and possibly costs for hail or other damage long term.

If you feed direct DC then your panels will provide 12v/18V ~= 66% of rated output at full power unless you use an MPPT or other downconverter at extra cost. A decent grid tie inverter should beat that even allowing for the 18VDC - 240 VAC and 240VAC-12V1DC conversion BUT adds extra cost.

All up, unless you are subsidised hand over fist or are off-grid, chances are that using your own PV is not worthwhile compared to using grid power.

If you can use night-rate power for part of your grid energy this would be even more true. If night rate power is much cheaper than peak load power then bitcoin mining at night may prove more economic while taking longer.

Connect a relay to the solar panel. When there is enough voltage, the relay will flip and power will be provided by the panel.

As said above, regulate the voltage to 5V before it enters RPi but it seems that you are already doing that.