Can the sun power my building?

May 18, 2009 11:49 AM

Is solar power feasible yet for a multi-unit dwelling?

I live in a 16-unit building in Brooklyn, 4 units per floor. I'm guessing that the square footage of the roof is in the ~2000 - ~2500 sqft. range.

Using current technology, is it possible to install enough solar panels to power the building? If not, is technology heading toward a place where this will become feasible in 5 years? 10?

If it is feasible, what's the back-of-the-envelope estimate on cost? Has anyone done any kind of analysis to determine what kind of cost/revenue structures something like this would entail?

posted by mkultra to technology (11 comments total)

The economics depend a lot on whether the utility company will allow reverse metering, which could have the electric meter running backwards during the day when home usage drops with people out at work, school, etc. My quick googling doesn't say one way or another whether ConEd supports it.
posted by exogenous at 12:05 PM on May 18

Sorry for missing not including this in my first comment, but other big factors are government tax or other incentives. A friend had his rooftop installation in Maryland almost completely paid for by federal and local (state or county?) government credits, plus he enjoys reverse metering.
posted by exogenous at 12:07 PM on May 18

I used to work at a company that did analyses for installing solar panels, in addition to other energy saving technologies. The rule of thumb for solar panel power capacity is about 100-150 watts per 100 sq ft, which means if you were to fill your roof with panels you could generate between 2000 to 3750 watts, or 2 to 3.75 kW.

However, that is rated capacity, not the actual output of the panels. The actual output would be significantly less, depending on the orientation of your roof and the weather conditions. A southward facing roof is best, since it faces the sun. In San Diego, we expect to get the equivalent of 1600 hours of full sunlight every year on a panel. Given that you're in New York, you might get around 1000 hours or so, which would be an annual energy output of 2000 to 3750 kWh. If electricity costs $0.20/kWh, that's about $400 to $750. Also, keep in mind that solar panels degrade in efficiency over time, at a rate of about 1% per year.

2000 watts might be enough to power one unit, but not the whole building, unfortunately.

Our cost rule of thumb for a solar system in San Diego, before utility incentives and tax rebates, is $8800/kW of installed capacity. Your system could cost anywhere between $17,600 and $33,000, installed, depending on labor costs.

Given these numbers, I wouldn't recommend putting set of panels on your roof unless you can get some large utility and government incentives to do so. It's tough to say when it would become financially feasible to install solar PV; every other day someone reports about some new development that will make PV super cheap and efficient, but they're always 2 years away from commercial use. Of course, we heard that 2 years ago, and 2 years before that, and 2 years before that...

Another possibility is to look into solar hot water heating for your roof. The materials aren't as expensive and you don't have as many efficiency losses, and you can store the heat for later use much more easily than you can store electrical energy from solar PV.
posted by zompus at 12:19 PM on May 18 [1 favorite]

Well, annual daily insolation in NYC is 4 kWh/square meter. So say that you had a 2,500 sqft roof, that's 232 sqm. Coverage won't be 100%, maybe 80% is a good guess. That would be 185 sqm active, which would be 742 kWh/day. However, photovoltaics aren't 100% efficient. I think 20% efficiency would be pretty good for a commercial panel, so that's 148 kWh/day or ~54,000/year.

Average per-capita retail electricity consumption in New York state is ~7,000 kWh/year. That means, at best a rooftop array could serve ~7 people, and it is probably even less than that, because my calculations don't include the efficiency of power inverters, etc. If the panels were 100% efficient, which they won't ever be, you might be able to support 35 people, which might work with a little over 2 people per unit on average.

So, really not feasible any time soon, and that's without any cost consideration.
posted by Good Brain at 12:25 PM on May 18

Running an entire 16-unit building on solar power? Not happening. There are several factors. First thing first is that you need to determine your load requirement. How many watts does the building use up per day?

The average insolation (the amount of sunlight that the earth recieves per day) is 850w/m^2, You say you have about 215m^2 on your rooftop? That will net you 182,750 watts IF you cover the whole roof with solar panels. Which brings me to

Efficiency. The world record for efficiency in solar panels is around 40%. And those are the types used on satellites. The kind you could go down to lowes and get will most likely be anywhere from 10%-20%. So that really puts you at a little more than 18,275 watts. It just isn't feasible.

Price is another thing. For a standard cabin in the woods looking to run a fridge, microwave, etc.. It might run you upwards of $100,000. While I only have experience with off-grid (battery bank) systems an on grid system might be a bit cheaper, but not by much. It typically takes 25-30 years for these things to start "paying for themselves" so to speak.

NYSERTA gives you a 40% reduction if you do the installation with one of their certified people. So, that's that.

--

Now, the other thing you could do if you would still like to incorporate solar energy into your building is to utilize Solar Thermal technology for your domestic hot water or heating. This is a lot more efficient than solar photovoltaic and is also cheaper to install.
posted by saxamo at 12:30 PM on May 18

From http://www.solar-electric.com/solar_system_costs.htm

If you use 700 KWH, that is 23.3 KWH per day.

Divide that by the number of full sun hours you get per day on a yearly average. Multiply it by 1.15. That will give you a pretty close estimate of how many watts of solar panel you need. So if you get 5 hours per day, divide 23.3 by 5 - that gives you 4.66 KW, or 4,666 watts. Multiply that by 1.15, which gives you 5,360 watts of solar panel needed.

Average installed cost of solar electric if you do it yourself is around $7 per watt, or $9 if you have it installed by a licensed contractor. So that system will cost you around $37,500.

Current panel output is around 10-12 watts per sq ft. With 2500sq ft, thats 450 sq ft required per unit, so you could power 5 units and would cost you $150,000 to build out.
posted by wongcorgi at 12:35 PM on May 18

More on the Good Brain's comment. Average solar cell efficiency is about 15% according to wikipedia, so you're even worse off. And he's right, you run into losses of at least 40% more after inversion, etc.

Like zompus says, look into solar hot water heating. It becomes more practical the more people that live in the building (up to a certain point), since efficiency actually goes up the more people are using it. Otherwise that heated water goes to waste.
posted by JauntyFedora at 12:36 PM on May 18

While you're not going to power the whole building, it's possible you could easily power the public power of the building, such as hallway lights, elevator, hot water, heat, etc. with solar.

How easy that is depends on how the building is wired. How much benefit it will be for the owner depends on how many services the building provides to tenants.

Virtually no one throws up solar panels and switches off the grid. Most buildings that get close will take dramatic steps to cut consumption and increase efficiency, and are built from scratch with this in mind. Add on to this 16 units full of people making their own decisions on electricity usage and it becomes very very difficult. (Compared to an office building where it's easier to set policy and therefor control usage for everyone.)
posted by Ookseer at 4:52 PM on May 18

I'd echo the recommendations that you do solar hot water first, to cut down or eliminate your use of fuel (probably nat. gas) to heat domestic hot water.

I'd also point out that all the above calculations make the reasonable assumption that you aren't doing much to reduce consumption. Changing habits and technologies could probably save up to 50% of the electricity consumption of those units at a fraction of the cost of putting on the solar panels. Start there.
posted by meinvt at 7:14 PM on May 18

Thanks for the info, everyone. It looks like solar heated water is the path to pursue at this point. Is there somewhere that breaks down a "here's what you need to know first" list for this kind of project? I'm mostly interested in what kind of questions I should be asking a contractor.
posted by mkultra at 9:16 AM on May 19

Informational links:

Info page from my old company.

A Department of Energy guide to solar hot water, including what to ask a contractor.

DOE's solar heating site.

I didn't mention it before, but what might also be feasible for you is solar space heating. I always forget to mention that because there's not much use for heating down where I live, but it might help to save heating costs where you are. DOE's solar heating site mentions that as well.
posted by zompus at 11:31 AM on May 19