What is Solar Energy?

What’s solar power all about?
There is a lot of solar power to go around. On every square meter of ground, 1000 watts of energy is falling at high noon. How much is that? Well a central air conditioner running full blast takes about 3500 watts. Your roof is about 10 to 20 square meters. So there’s enough power falling on your roof to power your entire house, with lots and lots left over. So if there’s enough power to run your house falling on the roof, how much is that in total around the world? If you add up all the power we use, electricity, gasoline, coal, everything, you get the “total primary energy supply”. In 2007 that was 16 terawatts (trillion watts). The amount of solar energy reaching the ground is 5,000 times that. So, lots.

How can we use that power?
The trick is to turn that power from the sun into something we can use. There’s two basic ways – heat something up or turn it directly into electricity:

Solar heating                                                                                                                                                                                                                               Technically known as “solar thermal”, turning solar power into heat is extremely efficient. In modern vacuum-tube panels, the ones that look like dark blue fluorescent tubes, as much as 85% of the sun’s energy is turned directly into heat. If you have a need for constant hot water, like a pool or a laundromat, solar hot water is one of the most cost-effective solutions you can find. But if you just take a shower in the morning and wash some dishes, most of that power will be unused, heating water that just cools off again.

Solar electricity                                                                                                                                                                                                                                     The other solution is solar electricity, in most cases is generated using “solar cells”, technically known as “photovoltaics” or “PV”. Solar cells are only about 15% efficient at turning sunlight into power, so in general they make about 150 watts of power for every square meter they cover. While this is much less efficient than solar heating systems, what comes out is far more useful. You can still heat water with that power, but you can also run your lights, the furnace, your computer, recharge your phone, and in the future, fill up your car. Even more importantly, if you can’t use that power at a given instant, you can ship it to someone that does over the electric grid we already have. Then they can use it to run their lights, heat their home and fill up their car. You can’t really do that with hot water, there’s no distribution system to sell it into.

How does PV work?
This goes all the way back to a famous paper Albert Einstein wrote in 1905. For a number of years in the late 1800s, scientists noticed that when you shine light on metal, electricity comes out. This was a bit of a mystery, because the effect started the instant you turned on the light, which didn’t make sense if light was a wave. Einstein set the entire theory on its head, and with some simple math demonstrated that it all made perfect sense if light was a particle. This was a groundbreaking suggestion, and it was this paper on “the photoelectric effect” that won him the Nobel Prize. Today we call these particles of light “photons”. As you probably know, you cannot create or destroy energy, you can only move it from place to place. For solar power, we want to take the energy out of the photon -a particle of light – and put it into an electron – a particle of electricity. That part is actually pretty easy, as the 19th century scientists figured out, you just put some metal in sunlight and attach wires. The problem is *keeping* the energy in the electron. In most cases the electron simply returns to the metal, and the energy the photo gave it bleeds out as heat (which is why metal gets hot in the sun). Figuring out how to capture that electron while it still had all that energy took another 40 years. It was the transistor that did it. Transistors use materials known as “semi-conductors”, which allow electrons to flow in one direction and not the other. Major research into their properties started in the 1940s, and this naturally led to use in PV as well. Put a semiconductor in the sunlight and the electrons come out as they will in metal, but now they get trapped because they can’t flow back into the material. To make a solar cell you simply take a thin slice of a semiconductor, normally silicon, and put wires on the front and back. Put this in the sun and the electrons flow up to the top surface where you collect them in the wires. Those are attached to a load, say a light bulb, where they burn off the energy the sun gave them. Then now-spent electrons flow back into the back of the cell, completing the circuit. That’s really all there is to it.

Solar cells? Or solar panels?
Silicon cells, which make up the *vast* majority of PV systems, produce about 0.5 to 0.6 volts under good sunlight. This is about half of a typical flashlight battery, and not enough to be generally useful on its own. This is the reason you don’t normally see a single solar cell, but large numbers of them grouped into panels. When you connect any voltage source together end to end, or “in series”, the voltages add up. Modern panels are normally arranged with six columns of ten cells, for a total of sixty cells. 60 x 0.5 = 30 volts. That’s still not a lot of voltage, your house runs at 120 or 230. How we get from 30 to 240 is a topic we’ll cover below.

Solar panels? Or PV arrays?
Another term you’ll hear in the PV world is the “array”, which is nothing more than a bunch of panels wired together. Since the panels are a bunch of cells wired in series, when you wire panels in series you end up with a sort of “super-panel”, the array.

How are solar panels constructed?
Panels are pretty simple. The front of the panel is a normal sheet of glass, often treated with an anti-reflection coating on the front. The cells are soldered together and then glued onto the back of the glass. A sheet of thin plastic is then glued onto the back of the cells, protecting them while also allowing heat to escape easily (it’s not so easy through the glass on the front). This sandwich is then stiffened with an aluminum frame around the outside. Finally the wiring is attached to a junction box on the back, with wires coming out that can be plugged together with other panels. That’s it – mechanically it’s about the same complexity as a window.

How long do solar panels last?
A very long time. The industry generally suggest that you “degrade” the panels by 0.5% a year. In other words, after 20 years the panel should be making 90% of the power it was making when it was new. Warranties are generally based on a 25-year span, and you get a free panel if its producing less than 80% of its original rating. But this hides an important point. Panels only “degrade” for a short period of time when they’re new. After that it seems that they just keep working, practically forever. Commercial PV production started in the early 1970s in California for navigation buoys and pipeline corrosion protection. Most of those systems have been replaced since then. But the ones that didn’t get replaced are still working at full power today, 40 years later. A more rigorous study has been running since 1982 in Europe. They’ve shown that panels generally don’t degrade slowly, but work fine until there’s some sort of mechanical problem, like a tree falling on it. More typically water leaks into the back of the panel, and clouds up the glass. But even these events are extremely rare. Even if you consider this to be degradation, of a sort, then the measured rate is only 0.23%. So basically the panels should live as long as the house they’re attached to.

What about the [insert new technology] cell (or panel)?
There are a wide variety of “alternative” PV solutions, but they haven’t become major players for a variety of reasons. Existing panels are much stronger than you’d *want*… the problem is the cells are fairly fragile, so you need a plate of glass to stiffen them, and then you need the frame to stiffen that. Ironically, silicon becomes more robust when you get thinner again, which has led to all sorts of “thin film” designs that are at least partially flexible and look more like a normal shingle. Silicon isn’t the only thin film solution, others like CdTe and CIGS have also seen considerable development. To date none of these approaches has managed to make a big mark in the market. That’s not because of any technical issues (in most cases), but due largely to the relentless downward pressure on conventional panel pricing. Through the late 2000s the price of existing designs fell so fast that these alternates never had a chance to get established. Now they are racing to meet lowered price points, only to get there and find that conventional panels are even less expensive. There’s every possibility that one of these thin-film designs will hit the price/performance spot it needs to in order to become mainstream. In the meantime the 60-cell panel looks to dominate the market for years to come.

Does PV make sense?
From an energy generation perspective, PV is probably one of the best sources of untapped power on the planet. It is environmentally benign (it’s mostly glass, silver wiring and aluminum), pays off the energy that went into it in about 2 or 3 of its 40-year lifespan, scales from one panel to millions, and installs in places that are already “used up”, like your roof. This combination of features is shared by no other power source we know.

Does PV make money?
That depends on where you live. PV generates power for anywhere between 15 and 30 cents a kilowatt-hour, depending on the local weather. Electricity on the open market is subject to major daily fluctuations in price, and at its highest it’s about 25 to 35 cents. So on the wholesale side of the market, PV is already pretty attractive (which is why it’s the fastest growing power source in the world). But as a retail consumer, you’re normally isolated from these daily fluctuations by your local power company, who buys in bulk and sells it back to you at a blended rate. Many areas have, or are in the process of, introducing time-of-use rates, which are closer to the “real” rates, and in some of these areas PV is definitely less expensive than buying power from the grid. Good examples are California and Hawaii, and practically anywhere in the “non-developed” world. In other areas PV only makes direct financial sense if your region has some sort of incentive program, or there’s special circumstances. We’ll cover some of these below:

Off-grid systems: Power companies generally make you pay for the power, and a “delivery fee” which covers the cost of keeping the wires in good condition. If you live far from the power mainlines, that last item can get *very* expensive. Running the wire to a new building can cost thousands of dollars. In these cases it may make a lot of sense to install an “off-grid system” with panels and batteries, supplying your own power.

Net metering: this is the most obvious solution for PV – you install the panels on your roof and when the sun is shining the meter starts turning backward. On your bill you pay the “net metered” amount of power you used. That might be a negative number, which means they credit you. The financial benefit depends a lot on how the local power company bills (time-of-use, tiered rates, flat fees, etc) and how you use power during the day. This makes it a little more difficult to predict the cash-flows, which makes it a little harder to explain to the banks. On the upside, installation is a snap, the panels normally plug directly into your existing power panel for almost no extra cost.

FIT and MicroFit programs: “feed-in tariffs” (also known under a number of similar names) pay you for all the power you produce, at a rate that’s deliberately higher than the rate on the grid. For instance, in Ontario the power company will pay you up to 39 cents for power you generate on your roof, and the average rate for buying power is around 15 cents. As you generally produce less power than you consume, overall this tends to come out even in the end. The big advantage to FIT systems is you can easily predict your income, its basically the rate times the number of panels. Banks love predictability, so its relatively easy to find loans. The major downside to this approach is that it normally requires a second power meter, which can be expensive to install.

Can all roofs have PV installed on them?
Some sort of PV system can be installed on pretty much any roof. Facing south is desirable, and the right angle is around 30 degrees for a good chunk of North America and Europe. But those are not hard-and-fast requirements, and there is considerable design flexibility. There’s likely room for at least one or two panels on practically every roof, and mid-sized systems with 8 to 12 panels are rarely a problem.

Can you install solar panels over my shingles?
There’s a solution for practically every roofing surface known – traditional asphalt shingles, tar and gravel, bitumen, cement tile, or metal sheeting. Residential systems on pitched (angles) roofs are normally installed with the panels mounted parallel to the surface, but slightly raised above it. On flat roofs there are a number of solutions for tilting the panels up to a good angle.

Can my roof support the weight of a solar power system?
Modern solar panels and mounting equipment weight about 50 pounds a piece (25 kilos). This adds very little load to your roof. In most cases the system will add less than 4 lbs per square foot to the roof (19.5 kilos per square meter). Most modern homes are designed with 10 lbs/sqft of “just in case” extra capacity. Some older homes may require some structural work to shore them up, but they generally need it to fix sagging anyway.

My roof is older, can I install solar on it?
Unless you need to replace the shingles immediately, go ahead and install. Taking the panels off and shingling around them adds only a few hours to a re-roofing job. But if you roof does need replacement soon, do that at the same time. Simply setting up ladders and safety gear represents a fair amount of the time spent installing a small system. If the roofers are up there and agree to leave their equipment in place, you can save a lot of time and effort installing the PV after they’re done.

How much does a solar array cost?
That depends on the way it’s being installed. Generally you should expect to pay about $2.50 to $3.00 “per watt” for the basic hardware – panels, inverters, some sort of mounting. You should also expect to add about $0.50 a watt for things like wiring and electrical boxes. That means if you buy a system that generates 1000 watts of electricity (at high noon), you should expect to pay about $3,000 to $3,500 for the equipment. But then there’s the installation. The panels themselves are definitely a do-it-yourself type project if you’re handy with basic home repairs. Hiring someone to do it for you might run between $0.50 and $1.00 a watt. And finally there’s the electrical work, and that depends on the system type:

Off-grid: For off-grid systems you’ll likely want batteries, which will run you from about $1,000 to as much as $10,000 depending on how much storage you need. That’s a complex calculation, so you might want to experiment with an online solar battery bank calculator.

Net metering: This is generally the least expensive design to install. In most locations the system is plugged into a circuit breaker in your existing power panel (assuming there’s a free spot) and you’re done. Wiring and inspections might cost $1,000 in total.

FIT programs: Electrically FIT is identical to net-metering in theory, but because you get paid for what you produce *before* you use it, you have to install another meter. That not only costs money for the meter, but there’s all sorts of extra wiring, work by the local power company to turn off power at the street, and various inspections after the fact. Even small FIT systems run anywhere from $2,500 to $7,500 to connect.

How many solar panels do I need?
This is one of the most frequency asked questions. Generally what people are really asking is “how much power do I have to generate in order to power my home”. For most systems the answer is “none” – just hook up to your local power company and you’re done, you probably already have a connection. The exception to this rule is off-grid systems where there is no grid to connect to. In that case the calculation is complex, so please just go to the Off-grid FAQ so you can read it over there. There are a number of online calculators that can help you. Let’s expand on this a bit for systems with a grid connection, FIT or net-metering. In the case of FIT systems you get paid for the power you make, so more panels means more money. Since FIT systems have some up-front costs for metering (see above), the most direct answer to this question is “put up every panel that fits on your roof”. That way you’re generating more power to offset those fixed costs, which makes your financial returns improve. In Ontario, for instance, there’s a limit of 10,000 watts for systems in order to get the best rates, and so the vast majority of systems being installed are a *tiny* bit less than 10,000 watts. For net metering the calculation is a bit more complex, but not that difficult. What you want to do is knock out the power that costs you the most. If you have time-of-day use, look at your bill and see how much power you’re burning during that period. If you have a tiered plan where using more power costs you a higher rate, see how much power you’re buying in that higher price bracket. Now you know how much to install: put up enough panels to offset that amount of power on the average month. Smaller systems, 1,000 to 3,000 watts, are often in the sweet spot.

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