OK, so a quick clarification on the topic of kilowatts vs kilowatt-hrs
(and just a bit on solar stuff, too)...
If you look at a copy of your last electric bill, what is the unit of
electricity that the utility is charging you for? Kilowatt-hrs (kWh),
right?
So you can think of the kilowatt-hr (kWh) as a volume measurement of
electrons - like a big bucket full. (It is actually an /*energy
*/measurement).
A kilowatt (kW), on the other hand, is - amazingly enough - a *rate*
measurement. Like the flow rate in your hose as you spray your
(organic, of course) tomatoes - how much is flowing out the end at any
one moment. (This is actually a /*power */measurement).
And when you measure a rate over time (eg, kW x hrs), you get a volume -
hence the reason that your utility charges you for kWhs, not kWs.
Lastly, on those pole-mounted PV panels, they are rated in watts (or
1/1000ths of a kilowatt). Hence, this is a power rating, not an energy
rating (it is not a volume measurement).
To get volume, you take the watt rating of the panel (or panels), and
multiply it by the time that the panels will be in the sun per day.
This varies from place-to-place, month-to-month, day-to-day, etc. (New
York State, by the way, averages 4-5 hours of sun per day, over the
course of the year).
So those "comparing a home's electrical use per day to the solar panels'
combined output" discussions are a bit, uh, out of sorts.
Some helpful links:
The wonderful Michael Bluejay page that discusses the whole "watts &
watt-hrs" complication (and electricity costs in general):
http://michaelbluejay.com/electricity/cost.html
A common solar panel, and its electrical specs, including the wattage
rating:
http://www.evergreensolar.com/app/en/products/item/739
The "magic" NREL database of US Solar Radiation (Insolation):
http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/atlas/
Hope this helps,
Mike
Shawn Reeves wrote:
Abstract: The author finds that electricity should cost more, we
should use less, and it should be cleaner...good thing those all go
together in most any PV project. Also, it's a good thing the author
took some time to remove the snarkier remarks from the early drafts.
Andy, 20090804: "I think their math is a bit fuzzy. Powering 80,000
homes on 80MW would
only give each home 1kW and I think typical homes use more like 4-6kW."
Right:
NYS residents use 2600 kWh per capita per annum. ( 2005 number,
http://apps1.eere.energy.gov/states/residential.cfm/state=NY ) That's
0.30 kW (there were 8760 h in 2005). So, at 10% capacity factor, we
would need 3.0kW of PV installed per capita (not saying we should be
dependent solely on PV, just using the number to see how many homes it
can handle). 80,000 homes, having between 2 and 3 people, thus would
need between 480MW and 720MW. Likewise, 80MW of PV would produce 8MW
on average, so supply 27,000 New Yorkers, about 10,000 homes. Jon: the
average American uses a lot more electricity than the average New
Yorker, although I guess the city skews the numbers quite a bit; NYS
uses less electricity per capita than 48 other states! Give me fifty
cents and I'll crunch the real numbers from NYISO to figure the Finger
Lakes average. I think the difference between Jon's approach and
Andy's was the capacity factor, but PSE&G (NJ), as reported in the
Daily and Sunday Review, is overoptimistic by a factor of 3.
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