We’ve talked about the difference between an MPPT controller and a PWM controller, and briefly about sizing your controllers.
MPPT’s use a transformer to step down the Voltage from your solar panel to the Voltage of your battery, usually with an efficiency of over 95%.
The calculation for this is simple. If you have a 200W panel and a 12V battery, divide 200 by 12 to get 16.66 Amps. Conversely, if you have a 24V battery and a 20A controller, multiply 20 x 24 and you get 480W, so that is the size of a solar panel that will give you the full 20 Amps (discounting inefficiency).
Aside from that, you need to make sure the solar panel Voltage is lower than the rated MPPT voltage (i.e. if you controller is only rated for 25V, don’t put a 45V panel on it!)
This is the reason we prefer certain brands of solar controller. House panels tend to be higher quality and higher Voltage, (and lower price), so we prefer an MPPT that will take a house panel.
So this is where it gets more complicated (but I promise – it’s a good thing).
Some MPPT controllers can be ‘overstacked’, meaning you can put some excess power on them. For example, if you have a 12V battery and a 10A controller, that’s 120W of power (or closer to 130 if you account for inefficiency).
Take the Enerdrive, which is a combined DC-DC and MPPT controller with a charge current of 40A at 12V. That should be a maximum of 480W of solar (40 x 12) but on the side of the box it says can take up to 1250W.
Victron MPPT’s are even more complex. Whilst each controller has a ‘nominal’ power on the side of the box (which is already 20% over the controllers rated charge current) if you go into the specs you will find a maximum input voltage and a maximum short circuit input current. Input voltage is based on the open circuit voltage of your panels (not the maximum power) and current is based on the short circuit current. Provided you stay under both of those values, you can often put more than double the power you would otherwise.
What’s the advantage of this?
Panels are cheap (especially secondhand panels. Many of our used panels come out around 30 cents per watt).
Controllers are expensive.
Due to the low cost of panels and relatively high cost of batteries, it’s often desirable to oversize your solar array for those cloudy, rainy days when your solar output is only at 10% – 20% capacity.
By oversizing your array relative to your controller, you are guaranteed the rated output of that controller earlier in the day, and a higher output on cloudy days. Of course there are scenarios where you want the max. output from the panels, such as mobile setups where space is tight, or applications where you use far more power on Sunny days (air conditioning, irrigation etc), but knowing you can overstack a controller can bring down the overall cost of a system without affecting your performance.
A final note on voltage.
That open circuit voltage on the back of the panel is based on the panel being at 25 degrees celsius.
Under colder conditions on a clear day, that Voltage goes up higher than what the panel is rated for.
We had one week last year where three separate people brought in MPPT’s that had died due to overvoltage. All of them were being used with panels that were theoretically 10% under the rated input Voltage. So we recommend staying 20% under the rated Voltage on an MPPT, just to be safe.
