How many solar panels can I wire in series on a 150 V MPPT charge controller?

Four in series is not acceptable. The datasheet Voc is measured at a cell temperature of 25 °C, and voltage rises as the cells get colder. At -10 °C the cells are 35 °C below the reference, so Voc rises by 35 × 0.27 = 9.45 percent: 37.2 × 1.0945 = 40.7 V per panel.

Four panels then produce 162.9 V, above the 150 V limit, and three produce 122.1 V. The maximum PV voltage is an absolute rating of the input stage; exceeding it can destroy the controller, and it happens on a cold, bright morning with the battery full, when the array is open-circuit. The answer for this controller is three panels per string.

Use the lowest temperature the site can reasonably reach, not a typical winter value, and assume the cells are at air temperature, since at sunrise they have not warmed up yet. Many installers also leave a few percent of headroom below the limit. With three panels you are at 122 V of 150 V, so you have margin down to far colder conditions.

If the panel datasheet gives the coefficient in mV per °C instead of percent, the calculation is the same idea: Voc_cold = Voc + coefficient × (T - 25). Some controller makers publish a string sizing tool; its answer should match this hand calculation.

There is a lower limit too, set by the hot end. Most MPPT charge controllers are step-down converters, so the array's operating voltage has to stay above the battery voltage by a margin stated in the manual, often a few volts, for charging to start and continue. A 24 V bank is charged at up to about 29 V.

Panel voltage falls with heat. On a summer afternoon the cells may be at 65 °C, and with a typical power-point voltage coefficient near -0.35 percent per °C, Vmp drops by about 14 percent to roughly 26.7 V. One panel alone would be below the charging voltage, so the minimum string is two panels (about 53 V hot) and the maximum is three.

Then check current and power. Three 400 W panels are 1200 W. Into a 24 V bank charging at around 27 V that is about 44 A, inside the 60 A output rating. Two strings of three (2400 W) would want about 89 A, more than the controller can deliver. Most MPPT controllers simply limit their output to the rated current in that case, but that is permitted only if the maker's stated maximum array power and maximum short-circuit input current are respected, so look those up before adding the second string.

On a 48 V bank the same controller could handle about twice the array power at the same 60 A, which is one of the practical arguments for a higher battery voltage.

Wiring notes for strings like this. Only put identical panels in a series string, and give parallel strings the same number of panels so their voltages match. With two strings in parallel, string fuses are generally not required, but with three or more each string should have its own fuse or breaker rated as the panel datasheet specifies, because a fault in one string can be fed by all the others.

Treat the array as live whenever there is daylight. 120 V DC is a serious shock and arc hazard, and DC arcs do not extinguish themselves the way AC arcs tend to. Use a DC-rated isolator between array and controller, connectors of the same make and type, and never unplug connectors under load. Connect the battery to the controller before the array, as most manuals instruct.