1 hour ago, Denns said:

Basically P = V^2/R. R is just rated wattage/rated voltage. Your method just merges the whole calc.

You are spot on. That's in line with relatively and as you point out 270 to 290V. This is less than 10% change. The current on the other hand will move between about 3A to close to maximum which can be around 14A depending on the panel and Irradiance.

Read it off the specs provided by the manufacturer. Current on left scale based on Irradiance and voltage at bottom scale. Not my own way but how a panel behaves as per manufacturer's specs.

This curve also shows the main disadvantage of not using a MPPT. If we add the characteristic curve of a resistor (V - I * R) (black line), then the operating point is where the two lines cross:

So, as irradiance goes down, both current and voltage drop, and you lose power to the load at a squared rate, while available power from the panels only goes down linearly. So you lose a lot of available power at anything less than optimal conditions. (Power = V * I, and the maximum power the panel can produce for a given irradiance is just after the point where the current starts dropping.)

Things are even worse with a PTC element - as it heats up, its resistance increases and you get something like:

And you lose a massive amount of available power under good irradiance (although it does become more efficient at lower light conditions).

So it does work without a MPPT, but you don't utilize the available energy from the panel very well.