Solar panel technology has made incredible strides in recent years, significantly boosting rooftop energy production. Solar panel manufacturers have achieved this mostly by optimising their panels to push out more current than ever before.
But this progress has given rise to a challenging situation: the world of solar inverters is struggling to keep pace. Finding inverters capable of effectively managing the increased current output of modern solar panel arrays has become increasingly difficult.
Read on, and I’ll delve into the challenges posed by high DC string currents and discuss the urgency for inverter manufacturers to catch up on current specifications or push back on panel makers to turn their ever-increasing wattage into more voltage.
This post is technical. If you are reading this as a lay person – you may want to skim the more technical stuff and just get a feel for how much can go wrong in solar system design and why a good installer matters – especially with modern, high-power solar panels.
The Challenge of High DC String Currents
Making solar inverters handle higher currents is difficult and expensive because heat increases with the square of the current: P=I²R.
Fronius inverters, with their active temperature control fan, blow off heat, run cooler, yield more power, and have a higher current-handling capacity. In contrast, a random competitor like SoFar addresses heat issues by incorporating a large alloy heat sink and attaching a warning sticker about the unit being hot to the touch!
So, while I think it would be helpful for panel makers to revert to higher voltage, lower current solar panels, at the end of the day, it’s the solar inverter manufacturers who are going to have to catch up if we’re to avoid problems with panels that are currently on the table.
Many Inverters Now Can’t Handle A Parallel Array
Many roof designs require two strings of solar panels connected in parallel. When you parallel two strings, you double the current from the solar panel array.
Right now, unless you specify a commercial-sized 3-phase inverter in the 15-30 kW range, few domestic inverters can effectively handle the current from parallel arrays.
This leaves designers hamstrung when they try to get big systems built using high-current panels. If you want a maximum-effort power system, you must buy two inverters.
As the solar industry progresses, inverter manufacturers must rise to the challenge.
Hurrah For Enphase IQ8
When you’re comparing 360VA IQ8ac at 14 amps, and 366VA IQ7a at 10.2 amps, the similar VA (wattage) ratings are shown up as a bit misleading and that has caught out some less than observant installers.
REC Panels Allow More Design Flexibility
As we outlined recently, REC has now released the latest iteration of the Alpha Pure range. The new RS variant is a whacking 470-watt panel (with an enormous 1205 mm width), but the best part is the voltage. REC has made their new model with 88 cells instead of 54, allowing for better shade tolerance and more voltage.
Solar panels like this tip the scale back into your favour because if we take an average 6.6 kW system, the panels can be more flexibly deployed on a challenging roof or simply run at higher efficiency if the space allows a simple layout.
Take the high current/low voltage Jinko Tiger Neo JKM470N-60HL4 with the following specifications:
- 42 VOC = with de-rating applied and a 600 VDC ceiling, the maximum string length is 13 panels, not 14 for a whole 6.6 kW system.
- 35 VMP = working string voltage for seven panels is only 245 V at maximum power.
- 13.4 amps at maximum power means even if you could find enough voltage, parallel panels would clip badly on most inverters.
Even on an ideal roof, this low voltage might cost you a 2% yield for the entire system life. So, while you have the latest 22% efficient panels with less than ideal voltage, you might as well have some 3-year-old 350 W units with the right characteristics.
Whereas the REC Alpha Pure-RX 470 W panel has a high voltage/low current specification:
- 65 VOC with de-rating would allow for 8-panel strings.
- 55 VMP means the string voltage for six remaining panels in a 6.6 kW array is still a healthy 387 V
- 8.49 amps at maximum power simply means less waste heat.
Even on a complex roof, a string of 4 can provide a passable 221 V, while paralleling two strings of 5 at less than 20 amps can result in 227 V. All of a sudden, it’s feasible to make a much more painful roof work for you despite dealing with increments of panel size that are nearly half a kilowatt each.
Australian Standards Madness
Commercial installations are allowed to operate at up to 1000 VDC, and with yet another quirk of unharmonised Australian Standards, so too can a domestic system, but only if it’s not grid-connected.
We’ve covered it before in other posts, but silly Australian regulations have been addressed somewhat by some forward-thinking solar panel maker Winaico. Increasing the number of strings in parallel becomes a potential solution for larger systems, because they can be built with fewer things to go wrong.
Winaico’s answer is to embrace high current and an even higher 30 amp fuse rating for their panels. However, this begs the question: Why would a current-limited source like a PV panel, which only delivers 13.8 amps under short circuit conditions, require such a high fuse rating?
Understanding Fault Protection
The answer lies in the design of DC string configurations. Bear with me for some electrical abstraction.
With panels connected in parallel, it is vital to account for potential faults. If one string becomes short-circuited, it must be able to handle the energy flow from all the adjacent strings being funnelled into the fault. Herein lies the complexity. According to regulations, when there are more than two parallel strings, additional protective measures are required to prevent overwhelming the system.
Suppose a system consists of three parallel strings, and one develops a fault. In that case, the remaining two strings can supply a combined current of 26.32 amps, exceeding conventional panels’ standard 20-amp fuse rating.
To mitigate the risk of fire, installing fuses on every string is obligatory, which increases components, labour, and overall complexity. This, in turn, raises the chance of failures as fuses degrade over time due to thermal cycling. Consequently, compromised fuses may blow unexpectedly, resulting in lost energy yield and additional service calls.
The Invisible Problem
Parallel strings blowing fuses can go unnoticed, and the warning bells don’t ring loudly. Solar Analytics might pick it up for you. A savvy customer interested in the monitoring may also have a seat-of-the-pants suspicion.
Still, it could take years before anyone realises the lack of energy production from the system. They’re also difficult to diagnose, for example, in old SolarShop jobs, which feature hidden fuses secured under panels with specialised 5-pointed screws.
It’s an insidious problem. I have seen firsthand where energy bills continue to accrue because the system owners don’t realise half the $35,000 – 10 kW system isn’t working, but the green light is winking on the inverter, so nobody is aware. That instance was the first time I sold third-party monitoring for solar after they’d lost months and months of yield.
So What Is the Upshot?
When qualified electricians need to spend a couple of grand and a weeks training to get an actual solar design endorsement, there are a lot of industry veterans who rail against the notion that unqualified salespeople can design and specify solar without really understanding what they’re doing.
The vast bulk of solar power systems are configured by a computer program, which largely takes human error out of the equation. Sometimes compromises have to be made to make things fit; other times, there just isn’t enough space, but from my perspective, it’s even worse to see the occasional solar industry professional, those with experience who should know better, make fundamental errors in panel and inverter compatibility.
I’ve had end users who can find and use online configuration tools to prove they’ve been sold a pup, and those are awkward conversations to have with an installer when you ask how these errors were made. In many cases, I think it’s just an oversight brought on by the timely stock availability at the right price.
Sometimes it’s just laziness. The best advice I can offer is to drill your installer. Don’t treat them like idiots, but show you’re interested, and if they’re truly across the brief, they can explain the details.
And finally, beware of the sales company that sells you one thing and substitutes it for another on the install day; it’s a sure sign they’re shonks.