Sizing a solar charge controller

[Tenedos]

Planning an upgrade of solar panels of the newly purchased boat. Currently have a 30A PWM controller, I would like to replace it with a Victron MPPT. Trying to decide on sizing.

In the immediate term, I am planning to put 400W of solar panels, with plans to upgrade to 800W in September when we redo our bimini and dodger. Things may stay there but there is a low possibility to add another 100W-200W with flexible panels, bringing the total to 900W or 1,000W. Though staying at 800W is the likely outcome.

So assuming 800W, how does the sizing work? I have a 12V system so for 800W, it is 800W/12v ~= 67A. Do I really need a 70A MPPT (which are very expensive)? In reality, I rarely get 50% of the theoretical capacity so wondering if putting a 50A MPPT would make sense. I also don't want to risk damaging the controller if it happens to be a perfectly bright and shiny day (or we sail down to Mexico one day).

What do people normally do?

[SunPoweredYachts]

With a new MPPT charge controller you will likely get up to 20-25% more power than a PWM charge controller would give you. This is because an MPPT will lower the voltage from your panels to charge the 12V battery battery bank, take the difference and turn it into 'extra' amps whereas a PWM just lowers the voltage and that's it.

Be sure to check the specs on your solar panels and make sure when multiplied it doesn't exceed the input specs for the charge controller.

Maybe think about splitting up the solar panels and having two smaller MPPT charge controllers rather than one large one. It's often less expensive, gives you a degree of redundancy and will get slightly better performance from the panels.

[Emoyeni]

Nice. That’s a lot of panels.

[Seafarer7]

Originally Posted by Emoyeni

Nice. That’s a lot of panels.

Oversized solar arrays (ie wattage) shouldn’t be a problem for MPPT charge controllers so long as you don’t exceed the open-circuit voltage spec of the controller (typically 100v or 150v dc). If your think about it, most MPPT controllers experience “over sizing” in their absorption and float charge phases: they simply ignore the excess power by raising the input voltage (up to the maximum V-oc when zero watts is required).

There are actually some economic and energy-yield benefits to mildly over sizing: greater energy yield at a lower cost. Above a certain point returns diminish: Grossly oversized arrays are a waste - economically & energy wise.

[Tenedos]

Originally Posted by Seafarer7

Oversized solar arrays (ie wattage) shouldn’t be a problem for MPPT charge controllers so long as you don’t exceed the open-circuit voltage spec of the controller (typically 100v or 150v dc). If your think about it, most MPPT controllers experience “over sizing” in their absorption and float charge phases: they simply ignore the excess power by raising the input voltage (up to the maximum V-oc when zero watts is required).

There are actually some economic and energy-yield benefits to mildly over sizing: greater energy yield at a lower cost. Above a certain point returns diminish: Grossly oversized arrays are a waste - economically & energy wise.

Super helpful, thank you.

One thing I am trying to understand is in which condition I could exceed the open-circuit voltage spec of the controller (say 100V). I made my decision to connect all solar panels in parallel. Panels I am planning to use have open circuit voltage of 24.1V. So in parallel, that really is 24.1V. Is this only a consideration when you connect a bunch of them in series?

[smac999]

don't try to buy one controller for all panels. the best is one per panel. so a bunch of 15a ones is better then a 70a. and is the same price. or put them all in pairs with 20 or 30a controllers.

this means as you add panels you just buy another controller. instead of trying to guess at beginning.

[smac999]

Originally Posted by Tenedos

Super helpful, thank you.

One thing I am trying to understand is in which condition I could exceed the open-circuit voltage spec of the controller (say 100V). I made my decision to connect all solar panels in parallel. Panels I am planning to use have open circuit voltage of 24.1V. So in parallel, that really is 24.1V. Is this only a consideration when you connect a bunch of them in series?

yes if you connected 4 in series you'd be at 100v

[Tenedos]

Successfully upgraded solar panels today and installed a Victron 100/50 MPPT controller. Already produced much better results in the 3 hours after installation compared to the previous setup!

[noelex 77]

Keep a watch on the temperature of the controller. A 50A controller is going to be working very hard with a 800w input, and the installation does not not have much space above the heatsink of the controller (Victron specify a minimum of 10cm). If the internal temperature of the controller reaches 60°C, with the latest software the unit will self regulate so may also experience a reduction in the 50A capacity.

It would be worth adding some ventilation holes in the top of cabinet above the controller, if you have not already done so. If there is a lid or cover to the box shown, perhaps it is practical to permanently remove this. If not, add lots of ventilation in the lid. These steps will also help the inverter.

[Tenedos]

Originally Posted by noelex 77

Keep a watch on the temperature of the controller. A 50A controller is going to be working very hard with a 800w input, and the installation does not not have much space above the heatsink of the controller (Victron specify a minimum of 10cm). If the internal temperature of the controller reaches 60°C, with the latest software the unit will self regulate so may also experience a reduction in the 50A capacity.

It would be worth adding some ventilation holes in the top of cabinet above the controller, if you have not already done so. If there is a lid or cover to the box shown, perhaps it is practical to permanently remove this. If not, add lots of ventilation in the lid. These steps will also help the inverter.

Thanks noelex, good advice!