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Comment by 00joe | Hacker News Reader

In Tampa FL with optimal tilt and no shade a panel rated at 1 peak watt would generate 5.3 watts a day X 30.4 days a month * 4500w panels = 725 Kw per month

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Thanks. For optimal tilt that implies a sun tracking system, correct? Do most installations track? Do the sun trackers use much energy to rotate all the panels? Are they reliable, or do they add maintenance costs. The system costs we are talking about here do not include trackers or installation of trackers, correct? How much would that add.

Florida gets a lot of sun. I agree with you that a tracking array of new panels in Florida gives those numbers.

For people in other parts of the country and without trackers I find this site that has monthly solar radiation numbers to be useful for estimating output.

http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/

Looking around I see that a 4500W system, no installation, no tracking, runs about $20,000.

http://www.infinigi.com/4500w-solar-grid-tie-system-20-sanyo...

$20000 isn't a good price. Check here to see more competitive pricing:

http://www.sunelec.com/comparisons_of_grid_tie_systems_price...

Optimal tilt just refers to the tilt the panels are installed at, no tracking.

No tracking system. Here are some average numbers of hours of peak energy a day for a fixed system Portland Or 4.0, Chicago 4.4, Tampa 5.3, San Francisco 5.4

Total hardware costs are currently about $1.75 watt for panels, inverter and hardware. Fed Tax rebate is 30%.

Do these installations just pick the best single angle and stick to it all year? It seems like you must be able to get a reasonable benefit by adjusting your panels say, once a month.

Yield from that isn't very big, and far outweighed by the cost of making the whole installation moveable. If you're going to put everything on a moveable frame anyway, it's better to have it servo-controlled so that it's at least adjusted continually.

Thanks. Ok, your peak equivalent hours number for Portland OR using a fixed position is 4, implying an average of 365 * 4 * 4.5k = 6570kWh/year, correct?

The federal DOE site calculator for the same 4.5kWh fixed tilt plant gives 4580 kWh/yr for Portland (http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/US/...), ranging from 142 kWh/mo in December to 578kWh/mo in July. What accounts for the discrepancy between the two calculations?

A good bit of the discrepancy is that the calculator assumes that the DC to AC conversion is only 77% efficient (see "DC to AC Derate Factor" in the calculator), while the naive calculation assumes 100% efficiency of conversion, right?

fed tax rebate turns into a tax credit at the end of this year, btw! Move fast!

Your units are off there. Watts are a measure of power (joules per second), not energy (joules).

Pretty sure GP meant watt-hours.

It took me some thinking to decode it as well. He has ratings numbers per watt-peak of the rating. Watt peak is a normalized output rating for a panel under some conditions and doesn't actually mean it's producing that in a given condition. He has numbers that a brand new 1 watt-peak panel gives 5.3 watts a day of power in a certain location with a certain configuration. He has a 4500Watt peak installation. So he multiplies the 4500 watt peak by the watts per day per watt-peak. The watt-peaks cancel out, giving watts per day. Multiplying further by days per month gives the average watts per month (average per year) for the 4500Wp installation in the given location. Basically, it's correct, but one has to understand that the W, typically called Watts, in the panel rating is not Watts, but a normalized Watt-peak rating of the panel. It is worth noting that solar salesmen will often intentionally confuse the two, leading homeowners to believe that their new installation has much higher capacity than it actually does.

It's not a matter of decoding-- it's wrong. Watts are a rate of energy usage. It's like saying that my car can drive 60 MPH per day. Doesn't make any sense.

(I do think you're right about the confusion and exploitation of the difference between peak power and average power.)

My car gets 60 miles per hour per day, when I am pushing it. At that acceleration I can do a quarter mile in just under 27 minutes.

I think you mean 5.3 Watt-hours (Wh), 4500 W, and 725 kWh?

http://www.energylens.com/articles/kw-and-kwh

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