solar PV ABC's

There’s a lot of jargon out there. Let’s clear things up with some definitions.
First off, you have to call it “Solar PV” because there are other kinds of solar power, like solar thermal and CSP. Generally, these other technologies work by using the sunlight’s heat to make steam that powers a turbine. Solar PV, on the other hand, makes electric current out of sunlight more directly, using the photovoltaic effect… which I’ll describe in a future post.
Within Solar PV, there’s a variety of technologies as well. What distinguishes one technology from the next is its semiconductor of choice. The semiconductor is the material that actually “converts” the sunlight into electric current.
When you imagine a “solar panel,” what you’re most likely picturing is more precisely a crystalline silicon PV module. Crystalline silicon (or c-Si) is today’s most common semiconductor. It’s fairly cheap; in fact, it’s the second most common element within the earth’s crust. A few years ago, module manufacturers bought much of their silicon in the form of byproducts of the electronics industry, but by now that supply has been hugely outgrown.
Back to the solar panel you’re imagining. If you look at the module’s glossy, blue-gray surface, you’ll see the cells. They’re the little checkered quadrants, a few centimeters wide. The cells are the basic units of the PV process. It takes many cells held together by some hardware and a frame to make a solar module, and many modules mounted together make what’s called an array. So an array is what you have mounted to the roof of a house. Add the racking system, the inverter, the cabling, and possibly the battery and its accessories, and you have a complete PV system.
Back to the cells, the little quadrants on the surface. If you look closely at different arrays, you’ll notice that some cells appear to have rounded edges, while some have flush, squared ones. The rounded cells have most likely been made with monocrystalline silicon, and the squared ones with polycrystalline silicon. It has to do with how they’re manufactured. Bearing that in mind, here is a list of the 3 most common varieties of solar PV technologies.

Monocrystalline silicon cells are generally the highest-efficiency, the most expensive, the longest-lasting, and the most waste-intensive to manufacture. The round part you see within the cell is the wafer, which has been sliced like salami from a cylindrical ingot of silicon. The ingot is the original “crystal” that has been “pulled” from a pool of molten silicon. The molecules that comprise the ingot are aligned in a pristine lattice pattern [3]. You might think of monocrystalline silicon as single malt whisky — unadulterated and drawn from the same barrel.
The cells you see with squared edges are probably polycrystalline silicon. These cells are made by pouring molten silicon into square molds. They are more affordable and somewhat less efficient than monocrystalline… but this doesn’t mean they’re of lower quality. Lower efficiency just means they generate less electricity per unit area than mono c-Si. So unless space is severely limited, polycrystalline is probably the economically wisest option for you.
If you’re looking for something a little more cutting-edge, then you might check out thin film. Instead of crystalline silicon, thin film uses any one of a variety of semiconductors, most commonly CdTe, CIS/CIGS, and amorphous silicon. To manufacture thin film solar cells, the semiconductor of choice is mounted (or better yet sprayed, sputtered, glazed, vapor-distilled, inked, or printed) onto a substrate of choice. This means you can have flexible PV laminates that conform to any curve, or translucent PV glass. Because it uses much less material than c-Si, thin film can be less expensive to produce, but it’s also less efficient (per unit area). Traditionally it’s ideal for building-integrated arrays, or for installations where design is an issue and limited space isn’t. Keep an eye on it, too, as its still very much an emerging technology.

These 3 technologies make up the majority of solar PV installations available on the market. Which to choose is up to you.
Actually, it might be less up to you than you think. In many ways, the right system for you has already been determined by your site’s potential, your energy consumption habits, and local availability. In other words, there’s already a PV system out there with your name on it… it just takes a little investigation on your part to find out what fits your needs.

ready?

Okay, let’s take a step back. You’re interested in installing a PV array because you want to save energy and money, and eventually to earn some ROI in energy savings and through the FIT. You aim to accomplish this primarily by generating power for your own household, rather than by exporting it, because the average retail price for electricity in the UK is about 3x the rate of what you get through the export tariff for PV. So it’s economically unwise export when there’s laundry to be done.
Provided that these are your aims, you’d do well to have already boosted your home’s energy efficiency by simpler means, like draft-proofing and insulation. In fact, you’re obligated to do so, as your home will need to have an EPC rating of D or better (PV installation included, fortunately) in order to receive the premium generation tariff. More details available here.
If you haven’t yet taken care of that stuff, then congratulations — you can save more energy than you thought :). If you have, then you’re ready to check out solar. So let’s begin by separating issues from non-issues. Here’s a short list:

orientation: You want your array tilted towards the equator, usually along with the tilt of your roof. Some variation to the east or west is okay, and even full-on east- or west-facing arrays, while not ideal, can still be viable. Facing away from the equator is a no-go. If that’s your only option, consider a ground-mounted array instead. Note: solar_trackers have been proven to boost efficiency by up to 40%, but consider the potential for more required maintenance (moving parts!), and the additional cost vs. the cost of a few extra panels.
space: If maximizing available space is important, then cell efficiency matters. If you’re tight on space, consider monocrystalline silicon. If you’ve got lots of room, then consider a lightweight thin film installation.
open skies: your array should be totally shade-free for at least six hours of the day. Even a little bit of shade on one module can dampen the output of the entire array — for most technologies, anyway. I’ll elaborate on this in a forthcoming post.
retrofit vs. building-integrated: If you’re building a new roof, then consider integrating your modules into the roofing material. You’ll likely lose 5-7% efficiency due to restricted cooling [6], as there’s no gap for airflow, but what makes building integration desirable is aesthetics and unity of design. Still, roof-integrated modules during a new construction should cost less than a retrofitted array after the fact. If, on the other hand, you’re planning to fit a crystalline silicon array onto a pre-existing roof, then the installer may need to penetrate your roof in order to install the racking system. This will involve removing and replacing roofing material, or drilling right through it. Be assured that the MCS guidelines for waterproofing and flashing are rigorous.
grid-connected vs. off-grid: If you’re not grid-connected, then you won’t be able to export your excess electricity to the grid (although you’ll still be eligible for the generation tariff!). It would be wise for you to investigate battery and charge controller options.

And just in case you’re worried about things you don’t need to be, here are some non-issues:

compatibility: They make racks to fit roofs of all shapes, sizes, and materials. The only potential issue here is the strength of your roof. Your installer is obligated to inspect your roof for robustness before any installation commences, and racks are rigorously tested for wind- and snow-loading, although the dead weight is the most important factor. In most cases, a weak roof can be strengthened as part of the installation.
permissions: If you’re in the UK, you’ll be happy to learn that most pitched-roof solar arrays are considered permitted development, so you don’t need to apply for planning permission unless you live in a listed building or a special property. In any case, it’s your (MCS-certified) installer’s duty to be up-to-date on licensing and permissions.
maintenance: Compared to other renewables, solar is relatively maintenance-free. Still, consult your installer for recommended maintenance procedures… a good hosing may be in order now and then to clear up dust or bird droppings.

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One of the great things about solar PV is that you can have an array as big or as small as you please. That said, the price of most home installations ranges from £3 to £30 thousand or more, with size being the determining factor. Perhaps surprisingly, the modules themselves will probably account for less than half of the bill. You’ll be happy to know that the industry currently has its sights set on bringing the soft costs down.
So then, here’s a basic checklist:

modules: 15-40% of total cost. Have module prices hit rock bottom?
racking: generally accounts for 5% to 10% of an installed system’s materials cost and 20% to 40% of labor cost [1]. Ground-mounted arrays generally use less expensive racking, but you run into other problems (shading, space). Flat roof installations are easier to rack as well, though you’ll probably need planning permission (except for in Scotland).
inverter: 8-10% of total initial cost, though you should plan on replacing it after 10 – 15 years. Recent innovation has brought us the built-in micro-inverter, a technology that enables each cell to have its own tiny inverter, and thus to export its own AC current.
cables & hardware
labor: could be up to 40% of your total initial cost, especially if any roofing needs to be strengthened before the installation begins.
fees: your DNO may charge you to conduct a study to test the impact of your newly installed system on your local grid. Find out more here.
storage (off-grid): If you’re not connected to the mains, then you should look into storing the extra power you generate. Along with a a battery, you’ll want a charge controller as well. This monitors the trickle of charge-up current so that once your battery is full, the current doesn’t reverse its direction and damage the load.

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Now for the good part.
Let’s say you go for a 4.8 kWp installation, something on the order of 20 panels. This is a little larger than average, but let’s say you use more energy than average as well, a robust 5 thousand kilowatt hours per year. Your house has a good EPC rating, and you live in Manchester. You pay 14 p/kWh for electricity, which adds up to £700 per year.
Right. So when the installation is said and done, you’re out £9,600. (This is assuming an installed price of £2 per Watt-peak, but you should be able to find a better deal!) The important thing is, you start earning on day 1: 13.99 p per kilowatt-hour under the generation tariff (for the next 20 years), 4.5 p per kW-hour for electricity exported, and then, most importantly, the energy you don’t have to buy from the grid.
Based on insolation data for Manchester — and if your site is good — this installation should generate over 3.5 MW hours (3500 kW hours) in one year. (An idealized way to imagine 3500 kWh is to imagine 1000 hours of sunshine that, once it hits your array, is 3.5 kilowatts strong. So for 1000 hours, your array generates 3.5 kW, which is 73% of its “peak” capacity of 4.8 kWp.) Expect production in the summer months to be some 3 to 5 times greater than in the winter months [6]. This means you might be exporting in the summer, but probably buying power from the grid during the wintertime.
During the month of May, for example, your installation could be humming along at near full capacity for almost half the day long, and by the end of the month it might have generated 500 kilowatt hours. Even though your usage for May was probably less than 500 kWh — say, 400 kWh — you still had to buy 90 of this 400 kW from the grid in order to keep your lights on and watch some television after supper, when your array wasn’t generating. So of the 500 kWh generated that month, you only used, say, 310 kWh yourself, meaning you exported the remaining 190 kWh. Your total for May, then, is as follows: £43.40 from the 310 kWh that you didn’t have to buy from the grid, plus £69.95 for your 500 kWh from the generation tariff, plus £8.55 from the export tariff (the 190 kWh you sold to the grid), minus £12.60 for the 90 kWh you had to buy to keep your lights on after dinner. Congrats, you’ve netted £108.30 for May!
During December, on the other hand, your installation might put out only 100 kW hours. You’ll have to turn your lights on earlier because it gets darker earlier, and you probably won’t be exporting anything. Assuming that your December usage is 400 kWh, then you’ve saved about £14 by not having to buy from the grid and earned an additional £14 from the Generation Tariff. Not much, but it will add up.
With an installation like this, expect a return of somewhere between £600 and £900 per year.
Luckily, maintenance will be minimal. Rain should keep the modules clean, though speak with your installer about recommended upkeep. Most solar modules have warranties of 20 years or more, and you can expect an installation to last over 25 years — and by then, who knows what your upgrade options will be. A caveat: you will probably need to replace your inverter at some point during this period. Count that as £1k against your net profits.
In the final calculation, this larger-then-average rooftop installment on a consumption-heavy Manchester home pays for itself in 11 – 17 years. After that, the £600 – £900 generated per year is shear gravy.
You can find lots of other similar estimates on the web, as well as more personalized ones. It’s not unheard of for a solar PV installation in the UK to pay for itself in 7 years or less. If you really want to get started, check out our search engine.

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