Solar Panel Wattage Differences

Oswegatchie · Jul 7, 2017

We want to put a 260 watt single solar panel on the roof of our truck camper and add 2-100 watt renogy portable panels on the ground. I know that convention says the panel wattages have to be within about 10% of each other. If we use a controller that is specific for the 260 watt unit and a separate controller on each of the portables (or 1 controller for the combined portable panels), can we have very disparate wattages? basically is the wattage rule dependent on the controller or the panels?

John61CT · Jul 7, 2017

Actually it's voltage that should match especially MPPT.

But in any case it is best to have a controller per panel anyway, reason people don't is cost.

But with a top SC like Victron 's MPPT line, at the low end 75/15 (75V panels max, produces up to 15A) going for under $100 delivered, you don't have any worries, and optimize for partial shading as well.

Oswegatchie · Jul 8, 2017

John61CT said:
Actually it's voltage that should match especially MPPT.

But in any case it is best to have a controller per panel anyway, reason people don't is cost.

But with a top SC like Victron 's MPPT line, at the low end 75/15 (75V panels max, produces up to 15A) going for under $100 delivered, you don't have any worries, and optimize for partial shading as well.

Does that mean if we choose the right controller we can have widely different wattage and voltage panels or it is best to have each panel with it's own controller. I know that the renogy units have their own controllers for only a slight amount more? Are the voltages typically much different from a 260 watt stationary panel vs. a 100 watt Renogy type portable?

Trebor English · Jul 8, 2017

Two panels that start off the same are totally different when one is in the sun and the other is in the shade.

If all are 36 cell "12 volt" panels with a 20 volt open circuit voltage you can connect them in parallel to one controller. If they have the same current you can connect them in series with an mppt controller. One in the shade can stop the entire charging from that series string. Bypass diodes help.

You can use multiple controllers to feed one battery. Then it doesn't matter what the panels are. You can use one controller for the 260 and another for the pair of 100s. My controller is a cheap PWM that was $20 a year ago. Now they are closer to $10. For $10 you can use one per panel. There is no need for panels to have same length wires or identical controllers or even identical programming of the controllers.

Portable panels will have much longer wires. The wire size matters. Two panels in parallel produce twice the current, twice the loss due to wire resistance. In series with MPPT the voltage doubles, current doesn't.

John61CT · Jul 8, 2017

Oswegatchie said:
Does that mean if we choose the right controller we can have widely different wattage and voltage panels or it is best to have each panel with it's own controller. I know that the renogy units have their own controllers for only a slight amount more? Are the voltages typically much different from a 260 watt stationary panel vs. a 100 watt Renogy type portable?

The Renogy controllers aren't great, just cheap for kit buyers. The Victrons are better value, even if they were more expensive.

I would buy panels to suit the SC rather than vv. But also you should maximize the space available.

Ideal is each panel gets its own controller.

If multiple panels per controller, e.g. to get Volts up so MPPT is efficient, then they should match.

And yes voltages can be all over the place.

Oswegatchie · Jul 8, 2017

John61CT said:
The Renogy controllers aren't great, just cheap for kit buyers. The Victrons are better value, even if they were more expensive.

I would buy panels to suit the SC rather than vv. But also you should maximize the space available.

Ideal is each panel gets its own controller.

If multiple panels per controller, e.g. to get Volts up so MPPT is efficient, then they should match.

And yes voltages can be all over the place.

I hate sounding dumb, but can you tell me if SC stands for solar controller? And vv is?