Can I add more panels

Hi all I have a 4215bn controller and currently 2x260w Canadian solar panels what is the absolute max panels the controller can take as I’m struggling to run a 300w water heater for my greenhouse don’t mind disconnecting the extra in summer if need be just need the boost in winter
thanks all

Mike - Hard to know why the heater isn't working without knowing the voltages.

BUT ... this detail I found suggests you could have a total of 6 of those panels.

This 40 amp MPPT Solar Panel Charger works with 600W Solar Panel on 12V Battery System and 1200W on 24V Battery. Max Solar Panel 1560W, multiple load mode allow you set the load on/off in different situation.  

Your panels are generating 520W at noon, So I dont see why it cant power a 300W load at noon. Obviously at midnight the system is 'dead'.
With an effectively 4hrs runtime/day for solar panels. Your heater is going though, 300W*4Hr=1200WHr.
Pt = (4.2 × L × T ) ÷ 3600, where L is liters/quarts and T is temperature difference
Pt = (4.2 × 40 × (50-10)) ÷ 3600, or Pt = 1.867KWHr
So I think for 1.2KWHr you should be able to raise 20Liters/5gallon of water by 50C.
But maybe you are trying to raise the temp of 1000L/250gallon if so I think the temp will only go up by 1 degree.

Pt = (4.2 × L × T ) ÷ 3600
Pt *3600 = 4.2 x L x T
1.2 * 3600 = 4.2 * 1000L * 1.03C
4320 = 4200 * 1.03C
4320/4200 = 1.03C
If you let me know how much water you are heating I can tell you the expected temperature increase, assuming there is no heat lost while you are 'charging up' the water with heat.

S Bengi wrote:Your panels are generating 520W at noon, So I dont see why it cant power a 300W load at noon.

No, that statement is most likely incorrect.  Their 260W rating is determined in a controlled temperature test chamber under artificial sunlight scaled at 1000W/square meter.  Voltage goes down with increasing heat, and panels sitting in the noon sun will NOT be at the 25oC test temperature.  And, the real sun can't be counted on to be producing 1000W/M.  For people that work in the real-world with solar, derating the panels to 80% is typical.  Less than perpendicular orientation, and it's even less.  I'd say if at any point in the day if the panels passed the 400W mark, I'd say that was outstanding.