What's the best way to describe the 'rating' of a solar power system?

There is a lot of confusion about Watts(power), kilowatt-hours(energy), and the size of the system in square feet or acres(area).

We also get questions about hours per day, and the rate or value of the electricity produced. That gets into the physical lifetime of the system, and the economic lifetime of the investment.

There are all good & valid points. What's the best way to express them so the most people are comfortable comparing system and can make the best decision?

Tags: compare, cost, economics, invest, lifetime

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Permalink Reply by Paul on September 12, 2008 at 10:09am: Hey Gordon,

I guess $8/Watt was the show special price... ;^); ▶ Reply to This

Permalink Reply by rose marie zamora on September 2, 2008 at 9:20pm: Hi. I am thinking of getting a solar system. This consultant suggested a 6.8 system. I wouldnt want to get a bigger size cuz at the end of each month, you get energy credits in the form of your meter going backwards, right ? then at the end of the year, if you have lots of energy, the elec co. will not give you credits anymore, it will be forfeited. Is this true in your area? So, I am just curious, how big of a system did you get ? My electric consumption was 1400 last month ( it was hot here and had the a/c running.)
thanks ... I really would like to know which one to get.

marie; ▶ Reply to This

Permalink Reply by Max H Herr on September 13, 2008 at 2:09pm: You're describing a grid-tied, net-metering interconnection agreement. It's not so much a system of "credits" as much as a simple determination of how much electricity you've produced minus how much you've used, based on the difference between each meter read (still done monthly, not annually, in most areas).

This is the only form of interconnection currently available in California and most other states. It's true that if you generate more than you use over the course of the year, that the utility gets the benefit of your production at no cost to them. However, most residential systems are not sized to overproduce electricity, and most only meet 75-80% of one's total need.

The federal government is considering a Feed-In Tariff, which would require the utility to pay for your excess generation. Don't expect this legislation to pass anytime soon, as the utilities are greasing the halls of Congress with lots of graft intended to prevent it from ever happening.

As for the proper size system, just do business with a reputable contractor with lots of prior experience and excellent references. They'll do the math to size your system correctly (see also Paul's excellent math example above, pretty easy to figure out on your own). If you want to meet or exceed 100% of your need, go for it and forget about whether you get any $$ from your utility company or not. It will simply increase your total cost.

A better solution might be to install the 80% system, watch how it performs for a few months, and perhaps choose to add a few more panels at a later date. Couple your 80% system with energy-saving changes like LED lighting and other Energy-star appliances might get your consumption "down" close to the 80% level. (For example, my new kitchen lighting with 9 Cree-LLF LR6C LED recessed lights uses less energy than one 100 watt incandescent light, and each light provides over 650 lumens (about the same output of one 26 watt compact fluorescent lamp. The illumination is nearly shadowless!)

Just make the decision to go solar now. The longer you wait to act, the less benefit you'll obtain. Solar anything (PV or solar-thermal/CSP) is the right thing to do for our planet today and forever.; ▶ Reply to This

Permalink Reply by Paul Kopper on September 3, 2008 at 4:50pm: Another way to look at it is that on the "average" (based on the particular PV system's effeciency) is that 100 sq. feet of PV will equate to .5 Kw of power, or 200 square feet ("2 squares") will give you 1Kw. This will vary in CA, or MI because of the predominate weather and which product you are using for your effeciency rating. About 4 squares should power a well insulated home in CA, while it will take 6 in MI.

P. Kopper Builder/Developer Kuyper Investment; ▶ Reply to This

Permalink Reply by Max H Herr on September 13, 2008 at 2:44pm: Your "average" math overgeneralizes the ability to produce PV electricity. My rooftop system footprint of about 240 sq ft produces 3.87 KW. It consists of 18 SunPower 215 W panels -- highest rated output available at the time I purchased them in 2007 -- their newest 230 W panels would increase the capacity by 6.5% to 4.14 KW, more than double your "average".

As to how many "squares" of PV it takes to "power a well insulated home," it's going to depend more on how much energy is used in that home. Here in SoCal, tract homes range in size from 1500 to 6500 square feet, and custom homes can exceed 10,000 square feet. No 6,500 sq ft home can be powered with a 2 KW ("4 squares") system as you calculate, and few 1500 sq ft homes could either, no matter how well insulated.

Also the difference in efficiency between "CA and MI" has some more to do with latitude than weather. It's harder to orient panels efficiently the farther north one gets. At 42.71 N, a rooftop in Lansing MI would have to be built to nearly a 11:12 pitch in order to flush mount solar panels to a 42 degree angle. A typical 6:12 Michigan roof would only achieve about a 60% orientation to the sun.

True, with more days of sunlight, southern CA probably beats almost any area north of 34 or 35 degrees for light capacity, but PV solar panels produce energy whenever there is light (even on cloudy/rainy days, and some panels even more efficiently under those circumstances than under full sun). Still, proper orientation to the earth's equator will maximize PV output, regardless of generalized weather patterns.; ▶ Reply to This

Permalink Reply by Paul on September 24, 2009 at 1:41pm: Ah-ha! I've found a solar retainer that doesn't actually rate their system, the only claim what it will do. In this case it will filter the pool. Net cost $5,000 -- Cost after rebates... TBD. That's the way to avoid the sticky problems of all those numbers! :^P

The Light is Green!; ▶ Reply to This