L1 to L2 voltage is ~1v coming into SolarEdge inverter

Welcome to the forum Kurtjx,

Here is a good article on 120/240 VAC split phase power (what we typically have in homes in North America):

https://en.wikipedia.org/wiki/Split-phase_electric_power

Note: The only way to reliably diagnose this issue is to remove the AC cover from your GT Inverter and to remove the cover from your Main AC breaker panel. This exposes dangerous electrical voltage and current. A slip of a hand, ring finger, screwdriver, cover, etc. can electrocute you or start a fire. Only do this if you are trained/experienced with working on home panel wiring.

The first question would be has there been any rewiring at your home or has the utility power been changed (assuming your voltage readings are correct)?

Do you have any 240 VAC appliances in your home such as AC, Heating, electric stove, electric drier, water heater? Are they working correctly? These larger appliances tend to use 240 VAC power...

Do you feel comfortable/safe removing the cover from your main electrical panel so you can access the wiring/breakers/etc.?

If not, you probably need to have an electrician come by and check your panel.

More or less, what I would check:

1. Pull main panel cover
2. Check AC power in L1/L2/N... As you understand (correctly). L1 to L2 should read 240 VAC, and L1 to Neutral and L2 to Neutral should read 120 VAC.
3. You should have a 2 pole circuit breaker that has L1 and L2 going to your AC inverter (should also have a green wire ground and mayu or may not have a White Neutral too).
4. Check the L1 and L2 voltages at the breaker. Should read 240 VAC
5. Check the L1 to ground--Should read 120 VAC (neutral and green wire safety ground/electrical panel metal box are all tied together inside the main panel/bus bars).
6. Check the L2 to ground--Should read 120 VAC 

There are several things that could have happened....

1. L1 and L2 are not wired to your home correctly... You could have (for example) two L1 connections from the pole transformer. All 120 VAC power/appliances work correctly. No 240 VAC appliances work (probably not what is wrong).
2. Look for signs of water/rust/corrosion inside the AC panel (and your AC box for the GT inverter)... Sometimes water/rain follows wiring into an electrical box and corrodes an electrical connection. Look for corrosion and/or browning/blackened connections at binding screws, etc.
3. Assuming your L1/L2/N connections are correct to your home, and any other 240 VAC appliances are working correctly, the assumption is you have a failed electrical connection (loose screw, corrosion) between the 240 VAC 2 pole breaker and your GT inverter AC power box (most likely). Or possibly a failed AC breaker (L1 from bus bar to breaker terminal is "good" and L2 bus bar to breaker terminal has failed open).

What could have happened... L2 (just an example) electrical connection has failed from breaker panel to AC box of GT inverter... And L1 is "powering" L1 terminal of GT inverter, and you have no L2 connection... And the L2 terminal voltage is just leakage current from the L1 terminal... That way your meter reads the 120 L1 voltage, and the L1 to L2 leakage current via the inverter AC wiring as 120 VAC too... But when you measure L2 to Neutral (or to safety ground), you see 120 VAC, but L1 to L2 shows zero volts (because L2 is "powered" by L1 thru inverter).

The 120/240 VAC wiring is very simple:


The transformer outside your home provides the 120/240 "phases".

Loose/corroded electrical connections can cause "strange" voltage readings. Small animals can chew through wiring, etc...

A relatively common sign of a bad electrical connection somewhere can be seen in a home when a large electrical load is turned on (washing machine, plug in electric heater, etc.) and some lights "go dim" in the home, and other lights "go bright". That is a typical example of a corroded/failing Neutral connection.

I have had that happen in two separate homes I have owned. Water from the "weather head" (where AC power comes into the home)--Just a few drops over many years or decades) corroded a neutral terminal (one home behind the AC meter socket, another inside the main breaker panel)... Water following wiring into an electrical connection is a common issue). Dressing the wiring entering the weather head so that all wiring went "up" into the weather head stopped water from going into the weather head and further problems.

Your main breaker panel may look something like this inside (this is a sub panel, but similar to typical main panel wiring):


If you hare not "comfortable" working in areas like this--Time to call an electrician.

-Bill "be very careful" B.