We highly recommend the Victron brand charge controllers for all your solar charging needs. Charge controllers generally fall into the "cheap", "industrial", and "super professional" categories. After years of using different systems, here are some takeaways that steer us towards recommending Victron brand controllers for all your solar charging needs, with rare exceptions:

1. Very high quality and durability. The extra cost you pay for these units is well worth it as you are counting on your equipment to work. Charge controllers often run in extreme heat environments and also heat up themselves. It is important to have high quality housings and components inside. Victron uses large generous heat sinks and all their smaller units are fan-less (silent & protected from dirt).
2. Super easy to set up, control, monitor, and adjust. The VictronConnect App is genuinely easy to use to track your daily solar harvest and make adjustments in seconds.
3. Not overly technical: While many sophisticated high end controllers exist on the market, this brand is geared towards the off-grid, mobile, and marine sector - making the units DIY friendly without, for example, requiring purchases of additional equipment just to monitor or configure them.

To size a charge controller for your solar system, begin by adding up the total watts of solar panels you have, facing in the same direction, which are to be connected to one controller.

If you have a 48 volt battery system, take the Amps of the charge controller and multiply by 48 to get the maximum watts that this controller will harvest from the solar array. You can have a larger solar array than the controller's rated power. That is common, usually a good thing, and does not hurt the controller in any way. For example, the Victron 150/70A controller can accept up to 150 volts solar input, and on a 48 volt battery system, provide up to 48*70=3360 Watts to your 48V battery or pure DC heat pump.

Check the PVmax rating of your panels - the maximum voltage they produce. Decide how many panels you will connect in series (adds the voltage), and how many you will connect in parallel (adds the amps). There have to be the same number of panels in series in each set, so if it's an odd or even number of panels can make a difference in your decision making which controller you need. Say you have 5 panels at 42 volts each  - you may want to put them all in series and may need a higher voltage charge controller. But if you had 6, you could have 2 sets of 3 or even 3 sets of 2. So plan out your panels first before settling on your controller.

The combined voltage from your solar array must not exceed the PV Input voltage of the charge controller. The larger charge controllers usually accept larger voltages. Victron makes controllers that accept up to 450 volts. On very cold, sunny days, the voltages produced by your panels can go higher than normal. Especially if you are in a cold climate, be aware of this and allow for a safety margin, otherwise your controller may enter an over-voltage error state on sunny cold winter days.

Higher voltage solar arrays can use narrower cables, but impose increased electrocution hazards. For most users, the typical voltage of solar arrays is between 80 to 120 volts, with larger arrays in the 300 volt range. Higher voltage charge controllers tend to be more expensive, and extra attention must be paid to the installation of disconnect switches and protective conduit for all wiring. The voltage has to be higher than your battery voltage. So if you have a 48 volt battery, you would generally have your panels arranged in such a way that they supply at least 60 volts.

We highly recommend the use of a wire cover box that is is available for all Victron MPPT controllers to protect the PV input ports from pets and nosy fingers. While 24V and 48V batteries pose relatively small risks of electrocution, always consult your local safety code and be sure you know what you are doing (or get/hire help) when installing solar arrays and working with higher voltages. Remember: solar panels produce hazardous voltages and currents, and in you complete the circuit, can kill without running a credit check.