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Photovoltaics, Part 1: Shedding Light on the Basics

The Green Architects explain how to size a PV array and how to calculate your expected savings

Posted on Aug 15 2012 by Christopher Briley

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In order to understand whether a photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. (PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow.) system is appropriate for the project you're working on, you really have to understand the metrics and basics of solar electric systems.

Phil and I sat down, turned on the mic, and did our best to convey the basic concepts and rules of thumb that most green professionals should know. Of course, this episode lays the groundwork for Part 2, in which we will cover the financial implications of a PV system.

The Highlights:

The Showdown: This is a Green Architects' Lounge original cocktail that we think is great for summer. As we mentioned, the only tricky ingredient is blood orange bitters from Stirrings.

Photovoltaics, not solar thermal. We’re generating electricity. What is a PV module, and how does it work? GBAGreenBuildingAdvisor.com senior editor Martin Holladay has a great article on the topic: An Introduction to Photovoltaic Systems.

Solar energy is abundant! Be sure to check the graphic from Perez and Perez in this blog's photos.

You need an inverterDevice for converting direct-current (DC) electricity into the alternating-current (AC) form required for most home uses; necessary if home-generated electricity is to be fed into the electric grid through net-metering arrangements.. It's important to understand that you'll be converting DC electricity to AC and sending it back into the grid (unless you're off the grid, in which case you need batteries).

Net meteringArrangement through which a homeowner who produces electricity using photovoltaics or wind power can sell excess electricity back to the utility company, running the electric meter backwards.. The power company is never going to write you a check, but it will credit your account. This is different from feed-in tariffs (where you get paid to be a mini power company).

How much do they generate? A 1-kW PV array will produce about 1,300 kWh/year on a good solar site here in New England. (This is a good average number for the U.S.)

What does that equate to in dollars? First, you have to find out what your utility company charges you in your area for each kWh you use—see the map as a general guide—and multiply that times 1,300 kWh/year (or substitute a geographically appropriate number). That's how much you will save each year by installing a 1-kW PV system on your roof.

How big a PV array do you need? The EIA says that the average U.S. home consumes 11,496 kWh per year (an average of 958 kWh per month). That would mean you would need an 8.8-kW system to cover 100% of your electricity usage (11,496 kWh/year divided by 1,300 kWh/year = 8.843 kW).

How much space does that mean? A good rule of thumb (that we got from our friends at Revision Energy) is that a solar array rated at 1 kW takes up about 110 square feet (depending on panel efficiency and geographic variables). So the above 8.8-kW system would take up approximately 968 square feet of roof.

Don't forget to check back in later for Part 2, when Phil and I will talk about how affordable PV could be a real game-changer for some parts of the U.S.

Thanks for listening. Cheers.

Official Transcript

Chris: Hey, everybody, welcome to the Green Architects’ Lounge podcast. I’m your host, Chris Briley.

Phil: And I’m your host, Phil Kaplan. Hi, Chris!

Chris: Hi, Phil! How’re you doing?

Phil: I’m a little warm today; it’s summertime.

Chris: Here in Maine, we’re luckier than the rest of the country. Hasn’t the weather been gorgeous?

Phil: It’s been perfect.

Chris: Maine is perfect, isn’t it?

Phil: Well, the thing to remember is: sun is good!

Chris: We’re talking about PV solar.

Phil: That’s right — use that sun, appreciate the sun. Worship it, baby!

Chris: Before we launch ourselves into PV, we’re going to celebrate with cocktails. We’ve become full-fledged advisors on GreenBuildingAdvisor.com. Cheers! You know what that means — we’ll have to tune in more and chime in more.

Phil: We only pretend to care. Now we’ll actually have to do something.

[The guys talk about this episode’s cocktail, The Showdown.]

Phil: Before we get on with talking about solar — and we’ll start with the basics — let’s mention that we’ll end with a song by Beachwood Sparks, a band from L.A., from their album “The Tarnished Gold.” It’s called “Earl Jean.”

Chris: In Part 2, we’ll get into the money, but this episode is the primer. You need to get a handle on how much PV you need and how much output you’re going to get. We’re talking about photovoltaic solar cells. We’re not generating heat; that’s solar thermal, and we’ve covered that before. We’re focusing on generating electricity on site. That's critical for net zero.

Phil: Not solar thermal, not solar hot water — just for electricity. It’s taken us a while to get here, actually, because this is not the low-hanging fruit.

Chris: No, it’s not. It has taken decades of research and pounding the drum and putting solar panels on the White House, and then electing a guy who takes them off the White House…

Phil: This is an important discussion that has evolved over time as incentives arrive and disappear and the cost of energy fluctuates. It’s not a no-brainer.

Chris: I’m going to gloss over this because I’m not a physicist and I don’t want to get it completely wrong, but basically you’ve got silicon cells in a PV panel — two wafer-thin types of silicon right next to each other –– and between them is metal. The sun hits one side of the panel, and some of that energy is turned into heat and some is bounced right back in light, but 20% is turned into free electrons trying to get to the silicon cells on the other side, through the metal — and that’s where we attach our wires to get electricity. And physicists, you can write to us and tell us how we botched this up.

Phil: So, tell me about the 20%. That is our efficiency, correct?

Chris: Yeah. It might not seem like a lot, but try getting electricity from anything else right now without igniting it. Not too shabby, but we can do better. The holy grail is to improve that efficiency.

Phil: Everything we’re talking about compares things right at this moment. Technology will get better. And these things will get cheaper. Keep that in mind as we talk about the ifs and the maybes.

Chris: We’ve got a great slide to share, and it shows the amount of available energy from the sun versus finite sources.

Phil: Coal, petroleum, natural gas…

Chris: Uranium.

Phil: I’m already out of uranium. Big party last weekend. Used it up.

Chris: All those X-rays? Anyway, renewable sources of energy include the massive sun — 23,000 terawatts per year. The sun is an abundant energy source that we need to tap into. When you have solar panels on your roof, you’re generating DC, direct current.

Phil: And we need to convert that to AC.

Chris: Edison championed DC, but Tesla figured out AC. Edison electrocuted an elephant. What a jerk. Anyway, to convert from DC to AC, you need an inverter in your house.

Phil: I’ve seen that inverter. It’s a 2-foot by 1-foot box inside the house.

Chris: If you’re tied to the grid, then when the power goes out, it still goes out — even if you have those beautiful photovoltaic cells up there. Why? Because you have a grid-tied system. You’re converting DC into AC and either using it or putting it back into the grid. If your power company uses net metering, you’re putting it right into the bank.

Phil: Your meter runs backward. You consume a certain amount and produce a certain amount. But why can’t you just make power with your solar system when the electricity goes out? Someone should have resolved that by now. It has to do with safety — if someone is working on the lines when the electricity goes out, and you feed current through the lines with your grid-tied solar system, boom! If you really want to do off-the-grid, you have to rely on batteries.

Chris: Safety is part of the inverter’s job — when the power goes out, the inverter disconnects from the grid completely. Anyway, with net metering, the power company is never going to send you a check for $200 for giving them so many more kilowatts this year. That’s not going to happen.

Phil: But it happens in a lot of other countries, with feed-in tariffs.

Chris: Imagine every house generating its own electricity and feeding a grid that is anemic.

Phil: Let’s think of an easy way to explain a feed-in tariff. Essentially, it’s a long-term contract intended to stimulate renewable energy.

Chris: You can contribute to the power generation in your local community and actually get paid to do it.

Phil: Let’s say electricity costs 15 cents per kilowatt hour. You could sell back electricity from your solar system for 30 cents a kilowatt hour.

Let’s talk some more numbers. Each kilowatt of grid-tied solar panels produces about 1,300 kilowatt hours per year.

Chris: That’s a New England number. In Phoenix, you’d be up to 1,600-plus kilowatt hours.

Phil: That’s what you see on your bill from the electric company — you pay per kilowatt hour.

Chris: Solar panels are talked about in kilowatts — the amount of power they generate from the sun. If the grid-tied solar system on your roof is rated at one kilowatt of power, here it will generate 1,300 kilowatt hours per year.

A great map on PVwatts.com shows you what people pay per kilowatt hour in each state. In Maine, electricity is very expensive; we pay 13.09 cents per kilowatt hour. We don’t burn coal; it’s mostly hydro and oil. In Wyoming, they only pay 6 cents per kilowatt hour. So the PV salesman will have a tough time there. A one kilowatt solar array on my roof here in Maine should yield me the equivalent of $170 worth of electricity. You can figure out your return on investment; all the data is right there.

But, what does the average customer need for electricity? It’s time to get an intuitive sense of what we need to start generating power on the roof of a home.

Phil: As architects, we need to start thinking about this ahead of time. Solar is inevitable. We’re going to need to start generating all of our own energy on our property. How much energy is that, and how big a roof do I need?

Chris: The EIA says 958 kilowatt hours per month.

Phil: So, let’s take 11,500 kilowatt hours per year and divide it by the average of 1,300 kilowatt hours generated by the PV system per year; the result is 8.8 kilowatts. You’d need a 9-kilowatt PV system — and a pretty big roof. Use the rule of thumb of 110 square feet per kilowatt.

Chris: Then I’d need 973 square feet for a PV system — 8.8 times 110.

Phil: Let’s say we’ve got a 32-foot-wide house, an average American home. Divide 973 by 32.

Chris: Thirty feet. That’s a big roof.

Phil: That’s like a 12-in-14 pitch. It’s not going to work. Now, remember, we’re just talking about electricity. If you also use solar hot water, you’re going to need at least 4 more kilowatts of PV.

Chris: As designers, our approach is to reduce, reduce, reduce at the very beginning. If we’re good, we can turn that 8.8 kW into 4.4 kW, We could have a smaller PV system and spend less, and then spend more on the envelope and use more passive solar.

Should we talk about the conversion from kilowatts to BTUs? One kilowatt hour equals 3,412 BTU, but that’s not a straight conversion.

Phil: At some point we will need that conversion, because our energy models use BTU per square foot per year. Watch your units! Chris, how were you in physics in high school? The units always get you.

Chris: OK, that was Part One, the basics. Next, we’ll talk about making it economically feasible.


Tags: , , , , ,

Image Credits:

  1. Richard Perez and Marc Perez, 2009
  2. U.S. Energy Information Administration
1.
Fri, 08/17/2012 - 14:29

How big a PV array do you need?
by Eric Sandeen

Helpful? 0

I'm glad you followed this up with "...to cover 100% of your electricity usage"

Strangely people sometimes get into the notion that if they can't afford to cover all their use, then ... oh well, it seemed like a good idea, too bad.

But there's no real reason you can't just put up 1kW (4 or 5 panels) if that's what's in the budget, or available on the site. There is no mandate that solar must cover all your use. I know you know that, but I often have to point it out to people who ask me about my solar, and then start thinking about their own house. The only downside is a really small install will probably have a higher cost per watt, for various reasons....


2.
Fri, 08/17/2012 - 14:38

Response to Eric Sandeen
by Martin Holladay, GBA Advisor

Helpful? 0

Eric,
I agree with you. That's why I wrote (in my article on the topic), "Although some homeowners size a PV array to meet a specific electrical load, it is far more common to size a PV array to meet a specific budget."


3.
Sun, 08/19/2012 - 16:15

Does size matter?
by Phil Kaplan

Helpful? 0

Eric-
Sometimes our tribe gets hung up on the Net Zero concept, which is pretty motivating. It can feel like if we - really our clients - fall short of the mark, we've failed. I fall into this trap myself occasionally. Now I haven't done any formal investigation on this, but I bet that if we get folks to start with just a little chunk of PV to start, they'll be more motivated to add to it later, incrementally.


4.
Wed, 08/22/2012 - 19:11

square footage of 1 kW of PV
by Fortunat Mueller

Helpful? 0

Did those boneheads at ReVision Energy really tell you that it takes 110 sq ft for a 1 kW system? They don't know what they are talking about.

That number should be more like 70 sq ft for a typical 'standard efficiency' module.


5.
Wed, 08/22/2012 - 19:39

Response to Fortunat Mueller
by Martin Holladay, GBA Advisor

Helpful? 0

Fortunat,
In my article on PV systems, I reported that a 1-kW PV system requires 83 to 100 s.f. of rooftop. Of course, you need to leave a little bit of wiggle room for aluminum frames, aluminum racks, and the occasional obstruction like a plumbing vent pipe.

Different racking systems have different spacing requirements.


6.
Wed, 08/22/2012 - 23:44

I'm going to blame Phil...
by Christopher Briley

Helpful? 0

Fortunat, those boneheads are great people! (tell them we said "Hi". Perhaps we were using an old number or perhaps our numbers got a little jumbled. Since I'm responding before Phil, I'll blame him! Your numbers on this subject trump ours any day. Thanks for your help.

Chris


7.
Mon, 08/27/2012 - 10:52

Missing at least one more Use Case
by David Bainbridge

Helpful? 0

You mentioned only 2 use cases. Either sell your power back to the grid or be "off-grid". You are missing at least one other user case. Some inverters have the capability to sell back to the grid, be off-grid, and act as a UPS.

I have an Outback GTFX3648 inverter with an optional battery bank. This inverter allows me to have any circuit(s) in my house powered by either the battery bank/PV array or the utility power. With a remote controller hanging on a wall I can press a button to switch to either power source. Using this setup easily allows you to start out "small", learn about the system, and continue to add PV capacity over the years as funds allow. When you do acquire more PV panels you can switch more circuits over from the service entrance panel to the inverter panel allowing them to be powered by either source.

This system also acts as a UPS. If I do not have the remote set to use RE power and the grid goes down, the system instantly switches to RE power to keep all the circuits connected to it powered.

Using this setup might be a better setup if you still want power when the grid goes down, want to start small, and/or want to use both grid and RE power without going through the red tape of selling back to the utility company.


8.
Mon, 08/27/2012 - 11:06

Solar hot water statement
by David Bainbridge

Helpful? 0

The statement: "If you also use solar hot water, you’re going to need at least 4 more kilowatts of PV." does not make sense. This statement was made to make installation as simple as possible. You generally should not design a system to use a PV array to heat hot water. A solar thermal panel(s) can heat water at around 80% efficiency compared to less then 20% for PV. So you would need at least 4 times the roof space with PVs to heat hot water compared to using solar thermal panels. Solar thermal panels would also not be affected nearly as much by partial shading/snow cover.


9.
Tue, 08/28/2012 - 09:33

Response to David Bainbridge
by Martin Holladay, GBA Advisor

Helpful? 0

David,
You're right -- Phil's statement is confusing. Presumably, Phil was imagining that the homeowner wants to make hot water with an electric resistance water heater or a heat-pump water heater, not a solar thermal system.

Assuming that's what Phil meant, the reference to 4 kW is somewhat of a head-scratcher. I think that most homeowners with a heat-pump water heater can produce enough electricity to make all their hot water with a 1.2-kW PV system; those with an electric-resistance water heater will probably need a 2.2-kW PV system. To see the math behind this conclusion, read Solar Thermal is Dead.


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