few questions about solar power....

Have a car nearby that you can check the 12 volt battery and confirm your meter is working correctly? With digital meters, it is always possible that they have a dead battery. The meter could also be bad, but that is more rare.

If the UPS uses two batteries (or more) batteries--It is possible that they are 6 volt batteries (3 cells each for lead acid batteries). In any case, 6 volt batteries should be reading 6-6.4 volts each (or 12-12.8 volts for 12 volt/6 cell Lead Acid batteries) if they are charged and "resting".

It is difficult to diagnose systems with almost zero details about the hardware.

-Bill

The 4.5 volt battery(?) will not be useful for your UPS project. But you can use it to test your volt meter to see if it reads something around 4.5 volts (assuming the battery is charged).

If your MPPT solar charge controller can read voltage (like the 12.3 VDC), you can simply connect the volt meter and see if it is close to the MPPT reading (usually within 0.1 or 0.2 volts or so). And many MPPT controllers will also display other readings (Array and battery bank voltages and charging currents).

In general, solar+battery power (many times) ends up being more expensive than utility power (in the USA you are looking at $0.15 to $0.50 per kWH) where Watt*Hours are:

Volts * Amps * hours of use = Watt*Hours
Watts * hours of use = Watt*Hours
kWatt*Hours * 1,000 = Watt*Hours (k=kilo or "1,000")

If you need "backup power" (unreliable utility power, etc.), then battery and solar+battery power is certainly useful.

But so is a simple laptop computer (I use a $200 Chromebook laptop computer for virtually all of my day to day stuff). Also a laptop (with battery) tends to be much more energy efficient.

A desktop computer can take 100 to even 300 Watts... Vs a laptop of around 8 Watts (my simple Chromebook) to 100-300 Watts (a larger desktop computer).

Probably start by breaking your task into 2 sections... The first is to connect your solar charge controller to the battery bank and your solar panels to the charge controller, and get the battery charging. During the day, the battery should be at 13.8 to 14.4 VDC or so to be "actively charging". Once the battery has been at 14.x volts for at least a few hours, then it should be "charged".

The second tasks would be to ensure your UPS is working correctly. Ideally you should see around 12.7 VDC (or a bit more) with the battery "resting" (no loads, no charging). And around 12.7 to 11.5 volts "running" your loads.

I am a bit confused about what your battery bank is right now (12 volts @ XXX Amp*Hours?). It is very easy to "over whelm" a battery if your loads are very large relative to the battery--Your loads can pull the battery voltage down and shut down the UPS, or your AC loads will only run for 20 minutes to an hour (most UPS are designed only to run long enough for you to save your work and shut down the computer "safely").

Once you have both tasks functioning, certainly the system should run with solar charging and the inverter running at the same time (again, how long depends on amount of solar vs battery bank capacity vs loads).

Did you figure out how to run the UPS without being plugged into the wall?

I know you already have a UPS--But it may be much easier to start over with a battery+AC inverter+solar charge+solar panels+AC Battery charger (if needed)... It will be much less of a nightmare vs trying to get a UPS based solar system working.

For example, here is a very quick article on how to do this:

https://www.instructables.com/How-to-Build-a-Battery-Backup-Generator/ (missing the MPPT solar in this example)

But before purchasing anything, you really need to measure your loads, define what you need power wise (how many Watts, how many hours per day of battery, how much sun you have (solar does not work in shade) and what it is you are trying for (just sunny weather operation, day time charge/nighttime power, emergency backup power, save money on power bill, etc.).

Note that it is almost cheaper/easier to conserve power vs generate power--So having a very power efficient laptop computer/small computer+monitor will save you more money in the long term.

Then design (on paper) what you want the system to be--And finally, find the hardware that will meet your design goals.

To do this, you will need some basic test tools. A kill-a-watt type meter for measuring (and understanding) your AC loads. And I highly recommend an AC+DC Current Clamp DMM (Digital multi-meter). So you can learn how your system works, and debug it later when something goes wrong. You can get away with just a DMM (or AC current clamp DMM) to save money and get the system going--However, that requires you getting a good paper design together first rather than just going out a purchasing random hardware. That almost always never works out well (and wastes $$$).

Also, find out if you have any electrically inclined friends nearby. Make it a project that you can both work on together.

-Bill

Without looking in detail at the hardware (including power supply, etc.)... It is difficult to guess why you are getting a tingling feeling when touching the metal of the PC (or UPS, or??).

If you are interested, take your AC volt meter and grab one meter lead in your hand (say the black lead), and then touch the other (I.e., red) lead to the places you are getting shocked... More or less, you may measure 24-60 VAC (considered "relatively safe" by electrical codes)... Or you may measure line voltage (120 or 240 VAC--I guess you have 220 VAC @ 60 Hz line voltage, and no standard wired ground outlet).



You can do some experiments... For example, take a small 120/240 Volt filament lamp and wire it between the chassis of the PC and some local ground (cold water pipe, or similar).

If the lamp lights--You have a dangerous situation... If it does not light, take your AC volt meter and measure the voltage across the lamp. Normally, it would be very near zero volts and would indicate that there is not a direct electrical connection/short circuit. You could also take the AC Clamp meter and measure the current in the lamp wiring--While the lamp may not light, it is possible that there can still be dangerous current levels--Basically any AC current that passes through your body that is over ~0.015 Amperes or so, is potentially enough to stop your heart.

The problem is these parasitic AC currents can even be caused by "filter caps" on the AC input (used to reduce electrical noise/interference created by the computer).

Problem is that depending on the "source" of the voltage/current that is causing you a shock --It can range from a "small bite" to an electrocution--depending on exactly what is going one with your equipment and how wet you are (dry skin--pretty high resistance--Wet skin relatively low resistance).

Be careful.

-Bill

When set to volts (and not amps with a "wired" volt meter"), generally they are rated to 600 VAC minimum and are quite safe.

AC and DC each have their own safety issues... Much of these issues depend on the "energy source". Certainly, home 120/220 VAC power is not "touch safe"... But a 12+ VDC battery bank with 100 AH and larger batteries can easily smoke wires and start fires too with a dead short.

-Bill

Simplest, set the meter to measure AC voltage (first test), put red lead in V and black lead in COM, grab the black lead of the meter and touch the red lead to the frame/thing that is giving you a shock and read the voltage.

Same thing, but set to DC Voltage (second test), grab black lead, and touch red lead to thing shocking you.

Most DMMs these days have very high internal resistance when measuring voltage, and you should not get shocked by the meter lead (very high resistance, very low current).

And in any case, you are getting shocked and unharmed, so the volt meter test is a quick a dirty verification of the voltage/current leakage.

Normally, I would not be telling you to grab the black lead with your hand--But we already know that the voltage "leak" from your system is not hazardous (at least so far).

There is the "Lightning Bolt" function on your meter --I am not exactly sure what it does but it appears it can also measure "stray" AC voltage too (as well as for measuring three phase voltage too).

The meter itself is rated for 600 volts isolation on the V and COM leads.

More about your meter:

https://www.petervis.com/meters/dt3266l/dt3266l.html

-Bill