Um. So, I'm not in the UK, I'm on the other side of the pond. But my system was installed back in 2008, which also makes it 12 years old. And there's Stuff.
So, back in the day, one had a choice between amorphous silicon (has this mottled blue color), which was cheaper than the alternative, crystalline silicon (solid blue, no mottled color, more efficient so one needed fewer panels for the same power. Further, the power variation was more severe (over temp, daylight angle, and all that) with the amorphous silicon than with the crystalline.
Next: The inverters. At the time in the U.S., batteries were pretty much of the lead-acid type so, unless one really needed them, one didn't install them. Given that that was the case, strings of solar panels in series and/or parallel (say, 9 panels in a string, put in parallel with another 9 panels in a string) were wired up and applied directly to an inverter. When the sun was down, 0 volts. When the sun was just over the horizon, maybe 100V; when it was noon, 240V, and so on. So, wildly varying voltage, the inverter would wildly vary the current drawn from the panels, always hunting for the point where the product of V*I was maximized. So the power output would very definitely vary with time of day, cloud cover, and so on. And throw into that mix the temperature, because (natch) the panels tend to be a bit more efficient when they're cold, and so on.
Now, lets talk about losses. Say one has that example of 9 panels in series, paralleled with 9 other panels in series. Say that the sun shineth at some angle and temperature. Those panels have manufacturing variations: Some will have higher voltages, some will have lower voltages, and the variation will be at random because, well, manufacturing. This means that the nominal voltage on one batch of 9 panels won't be the same on the other batch of 9 panels - so the higher guy will kind of back-drive, but not completely, the lower batch. Result: One will not get the max power out of the panels on the lower-voltage string. Which leads to losses. It's not unusual to get 5% to 10% less power out of the panels on one's roof than one might think by summing up the power available by multiplying the manufacturer's spec'd power times the number of panels.
It gets worse. Remember that inverter? If one designs an inverter that has a wide-ranging input voltage/current, it's not going to be quite as efficient as an inverter that has, say, a fixed input voltage.
Finally: Just because some panel with some illumination has a particular voltage and current where it maximizes its output power, the panel next in line might very well have different voltages and currents that maximize
its output power - which might be very close, or different, than the first panel.
So, in my case, I happen to have a nominal maximum power of all the panels on the roof of some 9.02 kW. The inverters (I happen to have two) have a maximum output power of 7.8 kW - and they very rarely hit that power level. It's all about the losses/manufacturing variations. That's a 13.5% loss at max power, and we're not even talking about what happens at 1/2 or 1/4 illumination, temperature, and all that jazz.
However, Technology Has Changed, And For the Better.
First, let's talk about how string/panel technology has changed.
- Typically, each individual panel has upon it a DC-DC converter that takes the voltage and current from the panel and converts it to a different voltage and current on the output. These converters are typically highly efficient, in the 95% to 98% range. Further, the panels in series can communicate with each other, for the purpose of setting the output voltage. You'll see why in a minute.
- Next: Wire up a bunch of these panels in series. The outputs are in series. Because they're in series, they all have the same current. But! They talk to each other. The total voltage across the string is set to a fixed value, typically 300V.
- Let's say we have 10 panels in series. So, each panel's DC-DC converter output would be, nominally, 30V. But, say that one panel is doing well today: It increases its voltage compared to the rest a bit, so it's V*I = P = Pmax for that panel, at that time. The others, who aren't doing as well as our well-doing one, reduce their voltage a bit. But, given Variations, we end up with an enforced 300V across all of them, with different voltages, but the same current across each panel, with the result that each panel is independently operating at its maximum power given the conditions.
Breather time. So, say that these DC-DC converters have, say, 3% inefficiency. But that
more than makes up for the lack of losses because of manufacturing-variation-mis-matched panels!
Further, since all these strings of, say, five strings of 10 panels each are in parallel, the overall voltage is
still 300V, with each panel doing its personal level best.
Finally: We have a nifty, fixed, 300V on the Entire Array. Feed that to an inverter that:
- Is optimized for 300V operation. Yea! Less losses.
- And here's the tricky bit: Has a 300V battery built in.
- And talks to all the DC-DC converters out there.
Charging/discharging the battery works well, it's on the solar panel side of things, pretty much, so one doesn't have to go crazy (like with my 12V lead-acid battery example) upconverting or downconverting the voltage into the batteries. The inverter side of things converts from 300 VDC to city power..
The electronics is cheaper.
The panels run at the full efficiency that they're capable of, illumination, temperature, and all.
There's losses in the DC-DC converters, but they're much less than the 13% to 15% I was citing for my system.
With greater efficiency in the panels (crystalline vs. amorphous) one needs fewer panels to get the same amount of energy.
With greater efficiency in the electronics, one needs fewer panels to get the same amount of energy.
OK, fine. But how does this apply to you?
1. You don't have any of those nifty DC-DC converters. Um. Oops.
2. Don't know how this will affect installing a different inverter.
3. Don't know how this will affect installing a battery.
The World, Somewhere, probably has an inverter/battery system that can work with your panel system, but it might take some searching and comparing this-with-that to find one that meets what you want.
Not saying it can't be done, mind you, but, yes, The Technology Has Changed.
