I got a great email the other day:
“One solar installer has told me that it is best for the inverter to be as close as possible to the panels and NOT to the meter box as your checklist indicates. Is he crazy?”
Short answer. Yes – he’s crazy.
As close as possible to the panels? Whatcha gonna do put it on the roof? That would be insane!
I get a lot of stick for this advice in one of my solar installation videos:
The argument against this advice is basically that the voltage drop is proportional to the length of the wire and that the voltage drop between the panels and the inverter is (allegedly) more critical than the voltage drop between the inverter and the meter. Therefore you should minimise the length of wire from the panels to the inverter at the expense of the length of wire from the inverter to the grid/meter.
I’m not sure where this argument comes from. The truth is that in most grid connect systems you have a higher voltage coming out of the panels than out of the inverter. The voltage out of the inverter is 240AC and the voltage from the panels is usually 300 to 600V DC. To cut a long story short this means that to minimize voltage drops in the solar system as a whole you should make the distance from the inverter to the meter as short as possible to minimize transmission losses, even if this means the distance from the panels to the inverter is a lot longer.
But even if your installer disagrees with the laws of physics you should insist that your inverter is as close to your meter as possible. This is because, if you want to be sure that your solar system will operate reliably, then the voltage drop between the inverter and your grid connection needs to be less than 1%.
Why? To ensure that the inverter has enough excess voltage to be able to push its energy onto the grid. If you have a long run of cable, then the voltage from the grid may fall outside of the inverter’s AC operating window, and it will shut down. That would be a bummer.
Of course the best way to minimise voltage drops in your solar system – which will maximise your power output and maximise the return on investment – is to use cables that are as thick as practicably possible. The thicker the cable, the lower the voltage drop across the cable.
Typical wire thickness (measured in cross sectional area – millimetres squared) for the DC cables that run from the panels to the inverter is 1.5mm², 2.5mm², 4mm², and 6mm². If you want to minimise losses insist on at least 4mm². Believe me some of the large and super cheap solar installation companies want to save every last cent on every install and will try to get away with the 1.5mm² wire!
As for the AC cable, i.e. from the inverter to the meter, don’t go smaller than 6mm². And if you have to place your inverter more than a few metres from the meter then go thick enough to keep the losses in that wire less than 1%.
My advice is to stipulate to your installer (in writing) that the losses in the wires (voltage drops) should be less that 2% overall for the system as a whole. After all electricity is expensive and wire is cheap!