Category: Solar Panels

Mismatched panels can be put together in the same string, but there are some rules to follow to keep the system safe and efficient.

In a series string, you need your panels to be the same technology, similar output, and a similar open circuit Voltage. The limiting factor with series panels is the current – if you have a low current, high voltage panel in series with a high Voltage, low current panel, it will limit your output current and potentially overheat the low current panel.

For parallel strings, your panels must be the same technology, your parallel strings must be as close as possible to the same open circuit Voltage (5% out is considered the safe limit), the tilt angle must be within 5%. This is if they are ‘paralleled’ on the roof, without a breaker in between or power conditioning equipment such as micro – inverters.

If you have multiple parallel strings going through breakers, they still need to be the same technology and within 5% Voc, but they may be facing different directions / at different angles.

Are these 12V panels?

In industry, panels that are under 25V open circuit are commonly referred to as ‘12 Volt panels’, which leads people to think that if they have a 12V battery, they need a ‘12 V panel’ (which is actually 24V). Not the case.

There are two basic types of solar controllers – There’s a whole other tech talk on them but it boils down to:

PWM: Can halve the Voltage coming from the solar panel (so can bring 24V down to 12V). Not very efficient, not very good for your batteries in the long run as they tend to let through high frequency Voltage ‘spikes’.

MPPT: Can handle much higher Voltages (Most ‘12V’ MPPT’s take between 45 and 100V max. input). More efficient. Often comes with 3 – stage charging, takes excellent care of your batteries.

This means – With an MPPT, any panel you like can be a 12V panel. Generally speaking, getting a secondhand commercial grade house panel and an MPPT controller is going to cost about the same amount of money as a brand new 12V panel with a PWM (there are good 12V panels and there are also extremely bad ones), and will take better care of your batteries.

What’s the difference between monocrystalline and polycrystalline panels?

Record–breaking efficiency belongs to the monocrystalline panels, but generally speaking a 200W monocrystalline will be the same size as a 200W polycrystalline. Monocrystalline panels generally do better in the heat, polycrystalline panels genrally do a bit better in diffuse light conditions (i.e. partial shade)

What’s the best flexible panel

There are three main types of solar panel cell. Monocrystalline, polycrystalline, and amorphous.

Monocrystalline and polycrystalline are made from silicon wafers. Whilst a few brands like Sunpower have made more flexible cells, all mono-and-polycrystalline cells are brittle. A mono or a poly flexible panel is simply not going to last very long. You’ll get longer if they are framed or if they are mounted well, but you will be lucky to get 5 years from a good one and they often don’t last 2.

Amorphous flexible panels exist. Amorphous solar cells are very flexible and durable, and amorphous flexible panels are very expensive.

When you have two or more panels, there are a number of ways to wire them up.

We are often asked – ‘What’s better? Putting our panels in series, or in parallel?’

(tl;dr at the end)

Series

Putting panels in series is just like putting battery cells in series – You plug the positive terminal of one panel into the negative terminal of the other. This adds the voltages together (so two 50V panels in series will give you 100V), and the Current from the two panels stays the same. Remember – Power is current multiplied by Voltage, so whether you put them in series or parallel, you are still getting the total power of Panel A + Panel B. There is a common misconception that more amps = more power, but this is not the case if it comes with a reduced Voltage.

Pros: In order for your solar controller to start charging, the Voltage from the panels must be higher than the Voltage of the battery system (In the Victron MPPT’s, at least 5V higher, but it varies from unit to unit). This shouldn’t be a problem with a 50V panel and a 12V battery, but on a cloudy day, early in the morning, and late in the evening, a higher Voltage is going to put more juice into your batteries, even if the total power of the panels is the same. (i.e. 400W of solar at 100V will give you more charge than 400W of solar at 50V, on a day when conditions are cloudy)

Cons: Series panels are very vulnerable to partial shading. Within the panel, your cells are also in series, so if you shade a single cell, it blocks some of the current going through the whole circuit, and the panel also starts to heat up which reduces efficiency. If you have two panels in series, it blocks the current going through both, so shading a single cell on one panel can dramatically reduce the output of the whole string.

(Note: there are various design features used in panels to mitigate the effects of partial shading such as bypass diodes and multi – busbar technology – so take this as a general statement)

Parallel

To put two panels in parallel, you put the two positives together, and the two negatives together. The most common way to do this is to use an MC4 double or triple branch adaptor (pictured), though the same behaviour applies if they are hard wired on the roof or if parallel strings are combined at a busbar closer to the solar controller.

Pros: If you partially shade one panel, it only reduces the output of that panel. Generally, partial shade won’t significantly decrease the output Voltage (depending on severity), so the shaded panel will continue to operate at reduced output and the unshaded panel will continue to operate at full capacity.

Cons: Under uniform low light conditions (cloudy days, early in the morning etc), your output Voltage will be reduced, which will limit charging.

Summary (Tl;dr):

No shade: Panels should go in series, as long as they stay 20% under the rated maximum voltage of the controller. Partial shade: Put them in parallel, as long as the output Voltage is going to be higher than your battery Voltage.