#### Comment Preferences

• ##### Efectively 0%..(0+ / 0-)

I can make some really, really rough guesses...If we assume that 0% of US rooftops are used, we can calculate out rough square footage, and thus roof area:

150 million person labor pool in the US needs 500 square feet to live and, say a cubicle (100 square feet) to work.

ninety billion square feet right there...cut in half to account for north-facing roofs, shade, etc. etc. Cut in half again, just because I've forgotten things, and I'm pulling numbers out of my whatever...

So that gives roughly 2 billion square meters of roof space at 200 watts/meter (per diary), that gives a peak theoretical performance of 400,000 megawatts.

Sandia (Dept. of Energy) estimates that the US is tooled to supply roughly 1000 watts per person. (300,000 MW)

Ergo...the US full of solar panel roofs would generate all our electricity...At least during the day.

...Those are some sadly rough calculations.

It is curious to see the periodical disuse and perishing of means and machinery, which were introduced with loud laudation a few years or centuries before. -RWE

[ Parent ]

• ##### If your numbers are even vaguely correct(0+ / 0-)

That would be most of our electricity, since we use most during the day anyway.

Those who labour in the earth are the chosen people of God. - Thomas Jefferson

[ Parent ]

• ##### Actually, come to think of it...(4+ / 0-)
Recommended by:
Odysseus, Timaeus, mikolo, wondering if

If we all covered our rooftops in solar, wouldn't we worsen our waste heat problem seriously?  If the panels are only 20% efficient, doesn't that mean 80% essentially bakes off as heat?  Isn't that essentially the opposite of the idea to coat all rooftops in white paint to up the albedo and send all that light back into space to cool us down?

Those who labour in the earth are the chosen people of God. - Thomas Jefferson

[ Parent ]

• ##### yup(4+ / 0-)
Recommended by:
Odysseus, Timaeus, mikolo, Ezekial 23 20

although some buildings can cool the PV with water, and then use the hot water either as is or with additional heating.

A little help can be obtained through the use of thin film infrared reflecting coatings, which reflect about 1/3 of the sunlight back out before it gets to the PV cells.

A similar issue comes up for desert based solar.  The albedo of the desert is around 0.3 to 0.4, so even with 20% conversion and shipping the power away you end up absorbing more sunlight than before.

• ##### Panels ideally are located off the roof itself(0+ / 0-)

with air circulating under them and between the back of the panel and the (still white?) roof. The effect is one of shading.

Liberalism is trust of the people tempered by prudence. Conservatism is distrust of the people tempered by fear. ~William E. Gladstone, 1866

[ Parent ]

• ##### Not talking about heating the roof.(1+ / 0-)
Recommended by:
wondering if

Sun hits dark colored pv, some converted to electricity, rest turned into heat that's then trapped inside our lovely bunch o greenhouse gases.

Those who labour in the earth are the chosen people of God. - Thomas Jefferson

[ Parent ]

• ##### Most residential - and almost all commercial -(0+ / 0-)

roofing is already black. Why shouldn't 20% conversion of incident energy decrease the heat load, rather than increase it, since the albedo's already close to zip.

• ##### Not anymore. Commercial roofs(0+ / 0-)

at least in California, need to meet title 24 energy conservation requirements (cool roof)

• ##### think the diary made an error(0+ / 0-)

high end commercial PV runs around 20% efficiency, lower cost PV is more like 8 to 15 percent.

But the power ratings are for peak output, full sunlight nearly perpendicular to the PV surface. The actual average output is more like 20% of the peak, a 100 watt output PV module panel will generate 100 * 0.2 * 24 = 480 watt-hours per day, roughly a kWh for a square meter 20% efficient panel.

So 2 x 10^9 m^2 times 0.5 to 1 kWh is 1 to 2 TWh.  U.S. electric power consumption in 2008 was 3,855 TWh, or more than 10.5 TWh/day.  And that's just current electricity consumption, you'd need to factor in the increase for electric powered vehicles (about 1.5 to 2 TWh) and other replacements of burning fuel.  Of course you'd also need to factor in reduced demand as a result of conservation and efficiency increase, and which point you'll need a grant to actually do a meaningful analysis.

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