Solar panels. Series, parallel or individually wired?

[madabouttheboat]

I'm in early stages of planning a solar installation that will probably comprise two (or maybe three) panels. I know there are a few on here with good knowledge of such things. I have googled it, but found i'm going round in circles.

What is the perceived wisdom here? Are they best wired in series or parallel (through a single controller), or even just wired individually through their own controller to the same battery bank?

If joining together, the advice i've found online seems to fall down on parallel, where there might be some shading such as from a mast, boom and rigging.

What does the forum think?

 

[ghostlymoron2]

Parallel is simplest although series theoretically gives lower voltage drops.

 

[ithet]

I thought series was best if using a MPPT controller?

 

[Jules W]

I thought individual mppt controllers ...

 

[PaulRainbow]

For most modest boat installations, series if no shading, otherwise parallel.

 

[Sandy]

What wattage are you planning?

I have 2 * 100 watts with a nominal output of 5.32 amps each (10.64 total) going to a Victron SmartSolar MPPT 75/15. One on the port rail and the other on the starboard. They are joined before arriving at the MPPT controller.

Planning to have an additional portable panel that I can add to the system if needed.

 

[Baggywrinkle]

I had 300W on my last boat. 3 x 100W panels and one Victron Smart MPPT. On our summer holiday in the Adriatic I did a week with them in series and a week with them in parallel. The energy produced was the same for both weeks ...

Installation looked like this ...



When the panels are in series, the voltage is higher and the current lower ... so there is not so much loss in the cables, and the MPPT reaches its' start threshold slightly earlier in the morning and carries on slightly later in the evening.

When in parallel, the voltage is lower and the current higher, so slightly thicker cables to counteract cable loss, and the MPPT reaches its' start threshold slightly later in the morning and stops slightly earlier in the evening.

Shading of one panel has less effect when in parallel, but half-cut cells mean panels can compensate better for shading as (hopefully) only half of the panel drops out and the MPPT re-calibrates its power point and generates more from panels in series compared to series arrays where the cells that are not half-cut.

 

[Sea Change]

IMHO the best way is to have one MPPT per panel and generously sized cable to minimise voltage drop.
This gives you the best protection against both failures and shading.
A number of smaller MPPTs can actually be cheaper than one big one anyway.
It's also worth looking for panels with a high nominal voltage, not the standard 12v (~17v open circuit) ones.

 

[lustyd]

IMHO the best way is to have one MPPT per panel and generously sized cable to minimise voltage drop.

With smaller panels this is not good as you need 18V for the MPPT so if a panel is half shaded and a bypass diode kicks in then that whole panel is dead. In series that would not be the case. I agree with the sentiment and would probably put an MPPT in for every two panels and run them in series.

With parallel setups you’ll need to fuse the panels if you have more than (I think!) two in parallel. Not difficult but it’s another component and potentially more connections.

For smaller setups of a couple of panels series seems a no brainer now that panels all include bypass diodes.

 

[Baggywrinkle]

IMHO the best way is to have one MPPT per panel and generously sized cable to minimise voltage drop.
This gives you the best protection against both failures and shading.
A number of smaller MPPTs can actually be cheaper than one big one anyway.
It's also worth looking for panels with a high nominal voltage, not the standard 12v (~17v open circuit) ones.

On my current boat I'm going for a mix, 4 Panels in 2 strings of 2 in series, each string connected to an MPPT .. reason also being failure tolerance.

If a panel fails it can be bypassed in a series array, if it's in parallel, then it might go un-noticed, but the array will function fine, just minus the capacity of the failed panel.

If an MPPT fails and the others aren't sized to take multiple panels, then you lose a panels worth of generating power and have a serviceable panel sitting generating nothing.

In the real world, this is a moot point on 12V systems, as there are plenty of single panels that will reach the MPPT start-up voltage required for 12V, so not worth worrying about IMO.

It looks different as you increase battery voltage. My current boat will shortly be getting 48V Lithium batteries .... which means whatever panels are connected, the array needs to deliver 60V or above for the MPPT to achieve start-up and run voltages - right up to a panel temp of 70°C+. This means less choice on MPPT sizing and suitable panels - and in my case series is the only way I can achieve a reliable 60V+.

Each of the MPPTs I intend to fit will also take all 4 panels as an array of 2 series strings in parallel .... so if an MPPT fails, I connect everything to the remaining one and keep my yield the same.

If a panel fails, the voltage on one of the MPPTs will be too low to charge, so I have a wasted panel and only 50% capacity remaining. The MPPTs are also sized to take 3 panels in series, so I can re-configure to one string of 3, and still use the capacity of all remaining good panels - 75% - with one dead MPPT.

The system will function until I get replacement parts.

Finally, probability of failure is interesting. With every MPPT added, the probability of experiencing a failure increases - like buying multiple lottery tickets. I personally think the sweet spot lies somewhere between one MPPT per panel, and all panels on one MPPT - with the optimal configuration depending on panel position, shading, available space, battery voltage, probability of failure (avoid unnecessary complexity and high parts count), and budget. Every extra wire and component adds another point of failure (or confusion when it comes to fault finding or reconfiguration).

 

[lustyd]

If a panel fails I can bypass it if it is part of a series array, if it's in parallel, then it might go un-noticed but the array will function fine, just at half capacity.

With bypass diodes you might very well not notice in series either though so this would be the same either way.

In the real world, this is a moot point on 12V systems, as there are plenty of single panels that will reach the MPPT start-up voltage required for 12V, so not worth worrying about.

Again, bypass diodes are in almost every panel meaning a small shade will halve the output voltage of a panel and take it below the required voltage.

 

[Baggywrinkle]

With bypass diodes you might very well not notice in series either though so this would be the same either way.

Again, bypass diodes are in almost every panel meaning a small shade will halve the output voltage of a panel and take it below the required voltage.

In series strings, you notice a failure developing in the MPPT statistics (Victron) .... the recorded max-voltage per day starts to have days where the voltage doesn't achieve it's normal maximum, this doesn't happen due to shading, as when the shade disappears, the voltage records a proper max. I noticed a panel on the way out this way about a month before it failed completely.

As the max and min current isn't recorded, you can't check parallel strings in the stats this way ... it only shows in the yield, but that is also weather and shade dependent - the daily max voltage isn't.

Half cut cell panels behave differently in shade to standard full cell panels.

 

[William_H]

I would urge OP to look at solar PV panels made for massive domestic installations. The numbers made and sold around the world mean that you get the best panels cheapest. They do tend to be quite large in dimensions. I think the biggest a man can carry and usually are 40 v or more open circuit. OP might find one of these will deliver around 350 watts basically a series panel with one MPPT controller to 12v battery. Google Search (2 oz dollars to pound)
What are really cheap here are second hand lower efficiency panels. 40v 200w great if you have lots of space. ol'will

 

[noelex]

On a yacht, in terms of performance or the total yield from the solar panels, individual controllers are generally the best, followed by parallel and finally series connection, which is usually the worst performing.

Unfortunately, the difficulty and cost of installation is generally the reverse of this list, with series being the easiest and least expensive, followed by parallel connection, leaving individual controllers as the most difficult and usually the most expensive.

 

[Sea Change]

I would urge OP to look at solar PV panels made for massive domestic installations. The numbers made and sold around the world mean that you get the best panels cheapest. They do tend to be quite large in dimensions. I think the biggest a man can carry and usually are 40 v or more open circuit. OP might find one of these will deliver around 350 watts basically a series panel with one MPPT controller to 12v battery. Google Search (2 oz dollars to pound)
What are really cheap here are second hand lower efficiency panels. 40v 200w great if you have lots of space. ol'will

If you have the space, this is the way to go.
I've just fitted three 450w bifacial panels. Nominal 32v, each goes to its own MPPT.
In the UK this size of panel can be had for about £60.

 

[madabouttheboat]

Thanks all. I'm not sure that I would have space for a domestic style panel having looked at the size of them, but I will have a measure up.

 

[madabouttheboat]

On a yacht, in terms of performance or the total yield from the solar panels, individual controllers are generally the best, followed by parallel and finally series connection, which is usually the worst performing.

A follow up question. If I have individual controllers for two panels, is there any issue with having two individual MPPT outlets inputting to the same battery bank?

 

[noelex]

A follow up question. If I have individual controllers for two panels, is there any issue with having two individual MPPT outlets inputting to the same battery bank?

Rarely, minor issues such as consecutive absorption times may occur, but these are typically avoided if the charging parameters are set appropriately.

The Victron smart solar controllers excel in this area, as they enable multiple controllers to synchronise over Bluetooth.

 

[Sandy]

Thanks all. I'm not sure that I would have space for a domestic style panel having looked at the size of them, but I will have a measure up.

You may want to look at 100W Rigid Mono High Density Solar Panel Kit - PV Logic as they are a bit narrower than the standard panel at 1150 x 460 x 35mm rather than the more usual width of 670 mm. They are a bit more expensive that the £60 quoted above.

I went down this route as they sit vertically between the deck and the top of the pushpit when I am not onboard.

 

[PaulRainbow]

A follow up question. If I have individual controllers for two panels, is there any issue with having two individual MPPT outlets inputting to the same battery bank?

For a couple of panels using separate controllers is a waste of money. The difference in yield will be miniscule, or non-existent.

If they are wired in series they just become a bigger panel, as far as the controller is concerned. The voltage doubles, so voltage drop is less, allowing lighter, cheaper, easier to route cabling. The voltage required to start the controller is reached sooner and they will work better in low light, where the voltage of a single panel might not be enough for the controller to work with. Only downside is, if one gets shaded it will affect the overall yield.

Wiring in parallel loses the advantages of series connection (higher voltage etc), but if one panel suffers some shading the other is unaffected.