So like many newbies on here I am planning on throwing up some 600W+ panels on my roof.

I came across this video in my research

From around 2 mins in he says it is about the Watts per square inche. 

He says the panel with the higher Watt rating is just gonna be bigger and more expensive and that panels that exceed 440 W are typically commercial panels.

This video makes it sound like 400W is what you should target.

I wanted to get as few large panels as possible but this did make me think.

How do you guys appraoching sizing?

Starting to think calculating the Watts per square inche and compare that with the cost

Personal 2c. Pay more attention first to matching the voltage and current specs of a reputable brand of panel optimally to the electrical input requirements of the MPPT. Then to choosing a Wattage of panel that divides close to a whole number of times into the input Wattage limit of the inverter. Then to checking the Rand cost per W.

For size considerations, first check the length and width of the panels relative to the available roof space for the string(s).

I guess that W/Area is useful to minimize the total space used by the panels, but unless you have a shortage of roof space, I'd go for the biggest sensible Wattage panel to reduce the total number of panels, junctions and fittings.

This is all talking around the issues, from here it would be useful to evaluate a concrete example of a system you are considering. Key message is do not consider the panel size without considering the specific inverter's requirements.

8 hours ago, lavaland said:

How do you guys appraoching sizing?

I went the route of bang for buck, this is 30months ago, but ended up with 280W JA Solar panels, because the cost/W was the best (most cost  effective) ratio.

Obviously you have to consider the panels current, just under 9A, which tied in nicely with the Sunsynk 5kW 11A, at the time, I think, or was it even 9A limit per MPPT and initial idea of 9S string would have kept me well safe from a Voltage perspective as well.

Yeah, from experience you can go with higher output panels but you need the budget and space... It usually works out cheaper to get lower rating panels and they take up less space, most of setups I deal with have 455W panels at the largest and there are one or two with larger panels but that usually for industrial complexes etc. 

Your most important factor is the Voc (Open Circuit Voltage), you're better off using that as your guide. Getting panels that when multiplied in series exceed the voltage rating of the MPPT is going to cause trouble. You want a panel that when connecting 2 or 3 in series the Voc lands below the MPPT max and as close to the recommended MPPT voltage as possible. You can always add some parallel strings to increase current and power output, just make sure you're using the Voc instead of the Vcc (Closed Circuit Voltage).

I went with 12 x JASolar 540W panels (two strings of 6) because they fit the available roof space better. I couldn't get three rows of 460W panels and two rows of larger 540W panels in landscape orientation both maximized the area covered as well as reduced the cost of the mounting kit. The string voltages are only 240V so it may not be in the max efficiency range of the Sunsynk 8KW MPPT controllers but I really don't feel that it is such a big issue. Weather plays a far bigger role than MPPT efficiency curves. Just use what fits your roof layout and stay within the electrical specs.

If a manufacturer came along with a 30% efficient panel then sure, that would be a game changer but at the moment solar PV panel effiencies are so close to each other it makes little sense to pick the biggest panels one can buy simply because it sounds better.

Edited January 7, 20232 yr by Surge