FAQs

Electrical

Strings

The cartoon above describes how panels are clipped together in 'series'..which simply means a positive to negative connection of the MC4 plugs on the back of every panel. As each new panel is connected the voltage of the 'string' increases.

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If you take the time to look at the datasheet for any solar panel you will see that there are some electrical and temperature characteristics, with the electrical having two sections...STC and NOCT or sometimes NMOT. STC means 'ideal lab conditions' which happen, almost never, and NOCT means 'more normal' but only for pretty excellent solar conditions too. In reality, for most of the year and on any particular day, neither is an accurate reflection of what the voltages and current generated by the panel will be. They are both over-stated.

When the sun rise first strikes the panels it may not be enough voltage yet to fire up the inverter so, there is no 'load' and there is an 'open circuit' condition. You can often see your inverter start up and then close down again several times in the very early morning (and late afternoon), but then the irradiation from the sun slowly rising increases and there is enough voltage to start the inverter, the 'load' to complete the circuit is present and current now flows, and the inverter starts. Once it's going, the inverter usually needs a lower voltage to maintain operation.

The current flowing as the day begins is very low, but the voltage (typically 25V to 27V per panel) remains remarkably constant from dawn until dusk. As power (Watts) is Voltage x Current, the low current, steadily rising and steady voltage, means you get what's called a bell curve of production with power production (maximum watts) produced between 11am and 2pm.

A typical string of 14 x 275W panels will produce a combined voltage of 350V all day long, and a maximum current of 9 amps, so at the peak of production the theoretical 3.85kW of panels really only gets to 3.15kW of power. If the panels are a bit dirty, or a few years old, it will be even less produced.

That's why all solar companies recommend 'oversizing' a system (more panel power than inverter capacity).

When we design a solar installation, we work out maximum and minimum string lengths, making sure that there are always enough panels on the string to start up the inverter, even when conditions are poor, and not too many that we exceed the maximum input voltage and current of the inverter. We use long established, worldwide accredited formulas to do these calculations. Sounds good?

Actually no. The formulas don't really work correctly in reality, or at least not in Perth.

The formula for working out the maximum string voltage uses two important parameters...

1.The Open Circuit Voltage of the panel under Standard Test Conditions.

2.The Temperature Coefficient (Voc) of the panel.

The problem with the formula is the first one. Standard Test Conditions.

It assumes that there will be solar irradiation of 1000W/m2 at the coldest time of day.

The reality is that on the coldest morning in Perth, a bright and sunny 3.6 deg C the beam solar irradiation was 644W/m2.

In fact on that cold sunny day (for Perth), the irradiation never went above 702W/m2 all day long.

So, back to the formula...

If we take a regular Canadian Solar 275W panel with Voc of 38V at Standard Test Conditions and a Temp coefficient of -0.31% then the formula says that the maximum open circuit voltage of the panel on that 3.6 deg morning is 40.5V.

The reality is that with 35% lower solar irradiation than the formula dictates, it was 35.3V (accurately measured).

So if trying to stay under the 500V limit of an LG Chem battery, the formula says 500/40.52 = 12.34 panels max.

The reality was = 500/35.3V = 14.16 panels

So the reality is, rounding down, 14 panels on a string compared to 12 for the formula.

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