Solar voltage info needed

[Artic Warrior]

Hi All,

Think im missing something here.

Here goes,,,I understand volts/amps/watts etc,,,,

A 30 volt panel is larger in dimension for the same amperage as a 12 volt panel ? because the way they are wired ?

I understand why the voltage is higher on a house system as it means thinner wires are needed.

I have a 50 amp, 75 volt Mppt controller, but think I need to be buying 12 (17 open) volt panels rather than 30 (36 open)volt panels

Cheers folks

 

[Artic Warrior]

I think the 250 watt house panel is 48 volt

 

[VicS]

Hi All,

Think im missing something here.

Here goes,,,I understand volts/amps/watts etc,,,,

A 30 volt panel is larger in dimension for the same amperage as a 12 volt panel ? because the way they are wired ?

I understand why the voltage is higher on a house system as it means thinner wires are needed.

I have a 50 amp, 75 volt Mppt controller, but think I need to be buying 12 (17 open) volt panels rather than 30 (36 open)volt panels

Cheers folks

A 30 volt panel is larger in dimension for the same amperage as a 12 volt panel ? because it contains more cells ... 2½ times as many.
One the same power (watts) would be similar in size.

"I have a 50 amp, 75 volt Mppt controller, but think I need to be buying 12 (17 open) volt panels rather than 30 (36 open)volt panels" Can you explain your thinking on this

I think the 250 watt house panel is 48 volt

???????

 

[charles_reed]

As we have no information as to the type of specification of your MPPT controller it would be foolish to advise you in detail.

First, as regards panel cell-numbers, this might help you: http://www.kyocerasolar.com/residential-solutions/solar-panels/current-products.htm

To get the actual output voltage of each panel in your possession you need to have the specification sheet - but counting the number of cells will give you a rough idea.

My MPPT controller accepts any dc voltage up to 72 and converts it for a nominal 12v system.

The actual voltage produced by a PV panel varies according to insolation and can be from 16v from a 36-cell system (nominal 12v) to 45volts (nominal 48v) for an 80-cell one. Even larger arrays are available for industrial use with a nominal output of 72v.

I'm afraid your wattage nomenclature is of no help as it's the # of cells that determines voltage output and the quality and size of cell determining current produced.

 

[vyv_cox]

A slight thread drift on the output voltage of panels, if I may. Working in labs for much of my career, we were not allowed to carry out electrical work on AC of 230 volts or DC of over 50 volts, presumably because there is some hazard associated with this level. So is a panel with an output voltage of 72 volts hazardous? And if so how much? Death? Nasty burns?

In the past when DC power was provided to domestic premises I believe that people who touched live wires found themselves unable to let go, as with AC that throws the wire away immediately. Is this true of panels?

 

[jdc]

If all cells are evenly illuminated, to a first approximation it's only the total area (which with fixed cell size equates to number of individual cells) which matters, not how you wire them up. To a second order then the inverter is a little more efficient and wiring can be thinner if the input Voltage is high,. This, ie uniform illumination, is the condition which applies to most domestic rooftop installations. so Voltages like 72V used to be common.

However on a boat it's quite unlikely that there is no shadow anywhere on any panel. Under these circumstances the output current of the affected cells falls. Were all cells to be wired in parallel then that would scarcely matter, but then the Voltage output would only be about 0.3 to 0.4V which is inconvenient. So cells are actually wired in series; which has the good effect of boosting the Voltage, but the catastrophically bad effect of limiting the output current to that of the weakest cell; hence if 35 (say) cells were fully illuminated and producing 1 Amp say, and one in shadow and producing only 0.1A, the series combination of all 36 would produce only 0.1A! (The excess power being dissipated as heat).

To get around this, in part, arrangements of diodes around groups of cells, often 4 or 5, or switches to by pass bad cells, are usual. This changes the optimum output Voltage as a function of uniformity of illumination, so an MPPT converter is required. However it's still better to have cells in parallel if you can, and there is even a trend in this direction for domestic installations, using so called micro-inverters (ie an inverter for each 12V panel rather than one for panels in series at a higher Voltage).

What this boils down to for a yacht is that you are best (= maximise power into the batteries for a given total area of panels) to have several smaller, lower Voltage, panels and wire their outputs to separate (or a multiple input) MPPT controller(s). It's not the cheapest arrangement unfortunately.

 

[noelex]

A slight thread drift on the output voltage of panels, if I may. Working in labs for much of my career, we were not allowed to carry out electrical work on AC of 230 volts or DC of over 50 volts, presumably because there is some hazard associated with this level. So is a panel with an output voltage of 72 volts hazardous? And if so how much? Death? Nasty burns?

I think this a good point. Voltages over about 50v DC can be lethal. I lot of people are wiring their solar arrays in series with high voltages. The wiring is often not well protected against chafe and typically led down the solar panel arch.

We are in a wet salty environment coming out from a swim and touching a live stainless steel arch could be serious.

I must admit there seem very few practical instances of problems and I am anti the "safety police", but it is easy to wire panels so the voltages are below the lethal level (say 24v, or 36v) and I think this is wise.