Q & A


What is a grid-tie solar system?

If the solar cell EROIE (Energy Return on Investment Energy) is truly greater than 1, and the demand is greater than the supply, why aren't there solar cell powered factories churning out more & more solar cells?

How much energy is in a gallon of gasoline? What would it really take to live purely on solar energy?

What are the reasons to convince others that solar electric is cleaner energy than other renewable energy such as small hydro power, geothermal, biomass?


Questions with Answers

Question: "What is a grid-tie solar system?"


An electricity generating array of solar cells, connected to the normal power lines by a special power inverter. The electrical power lines connecting most homes, business, and power-plants are referred to as "the grid", or the electrical "power grid". In many ways, this type of system is a great compromise for most home-owners. You generate the amount of energy your home uses in an entire day with solar cells. During the daylight hours, when you can generate electricity, you feed any excess back to the utility (to the grid). At night, when you can't generate electricity, you obtain the power you need from the grid. Since the investment of running power lines to most homes as already been made, homes already have a relatively cheap way of obtaining electricity. By generating the power when you can (during daylight hours), it coincides with the peak demand of electricity. This, in turn, helps meet the growing demand for electricity, so the grid operator (i.e. the power utility), doesn't need to build new power generation plants. This also means you don't need to pay for energy storage (e.g. batteries) that would be required to run your home when you can't generate the power you need from solar cells, which reduces the cost to install & maintain solar cell electricity generation. The key to a grid-tie installation is the power inverter which synchronizes itself to the frequency of the AC (Alternating Current) power used on the grid, and can "push" or "feed" power into the grid, when the solar cells are generating more power than the house needs. These inverters are specially designed for this type of use, and connecting to the power grid requires other safety equipment, so only a licensed electrical contractor or power utility agent should be involved with their installation.

Here is the Wikipedia entry on the Power Grid (Electrical Power Transmission)
Here is the Wikipedia entry on the Electrical Inverter

Question: "If the solar cell EROIE (Energy Return on Investment Energy) is truly greater than 1, and the demand is greater than the supply, why aren't there solar cell powered factories churning out more & more solar cells?"


The short answer is the economic factors involved. With demand high, funding such a factory is probitively expensive. Then there is the cost of real-estate, the current energy cost for other forms of energy, the fact that you can only run the factory during the day, and a host of other issues. Here are some good links that address this issue:

Jeff Vail addresses this in a piece on his website (Jeff Vail is an energy intelligence analyst for the U.S. Department of the Interior) Energy Payback from Photovoltaics: Problems in Calculation
Here is the Wikipedia entry on Photovoltaics
Here is the Wikipedia entry on EROIE

Question: "How much energy is in a gallon of gasoline? What would it really take to live purely on solar energy?"


IMPORTANT NOTE: For quite a long time, there was a large math error at the beginning of this when converting 1 gallon of gasoline to kilowatt-hours, which made the scenario much more costly than it should have been. Thanks to a viewer who brought this to our attention. Also by request, a few other scenarios are now included. Finally, we have now added links for references at various places.

1 gallon of gas is rated at about 115,000 BTUs (as high as 124,000, as low as 108,000 - the actual value depends on many factors - to keep things conservative, a round / lower value will be used in these calculations, and I believe the 124,000 is an older value before various MBE/Ethanol reformulations became common)
U.S. Environmental Protection Agency data
U.S. Department of Energy / EIA - Energy Calculator
About.com various fuels & energy
Wikipedia.org GGE (Gasoline Gallon Equivalent)
U.S. Dept. of Energy - Fuel Table (PDF)

btus * 0.00029287 = kw-hr
So 1 gallon of gas = 33.7 kw-hrs

The average home uses about 20-30 kw-hr per day

[The original scenario]
Here is my personal energy usage and what it would take to do in pure solar.

I do about 20,000 miles a year - at 20mpg, that's 1000 gallons a year

1000/365 = 2.75 gallons/day

2.75 gallons x 33.7 kw-hr = 92.7 ~ 100 kw-hrs day

Giving home / work weighted electrical consumption to give me my personal usage, I figure about 25 kw-hrs / day for me personally

So I am 100+25 or 125 kw-hrs per day.

(Note the order of magnitude difference - hurting a few tons of steel down the road takes a lot of energy...)

Here's some solar #s (I am going to use "m2" for sq meter)

A rule-of-thumb puts about 1000 watts per m2 of incident solar radiation. With efficiency issues, etc. you get 100 W / m2 of "obtainable" energy. The rule-of-thumb in the solar industry is 5 hours per day, so 1 m2 gets about 500W-hrs per day, or 0.5 kw-hr.

10 m2 yields 5 kw-hr
100 m2 yields 50 kw-hr

So to get my 100+25 or 125 kw-hrs per day, I need 250 m2. So lets add another 20% for good measure, meaning I would need 300 m2, or 17 m x 17 m, or roughly 60 ft x 60ft, which works out to be approx. 3600 sq. ft., which is the size of fairly large house, or roughly 2 times the size of typical house.

Looking at a few catalogs, you can figure about $500 (March 2011) per m2 (was $400(!) in April 2004, $700 in July 2008 - supply and demand are out of sync, as most electronic products tend to get less costly over time) , so 250 m2 in solar cells would cost $125,000.00 - this doesn't count the cost of real estate, setup, storage, etc...

[So where did the cost per m2 figure come from? - looked at some solar module reseller links on our links page, picked a panel (that was available and had detailed specs), looked at the cost & did the math. Note that this is a rough estimate - undoubtedly the price could be lower, especially if buying the quantity required to fulfill this scenario…]

If that sounds like a lot, well, be thankful you live in a time where nature has concentrated all that energy in a readily available resource…

Also note, that since writing this scenario many years ago, this no longer applies to me - I am now much closer to the following average scenario…

[Average scenario]
Here is an "average" scenario:

Same scenario as above, but using 11,000 miles per year, and figure the person trying to do this would have a fuel-efficient vehicle, so use 30mpg.

U.S. Department of Energy - vehicle miles traveled studies
www.fueleconomy.gov, random pick

Then use 12 kw-hr per person as an average electric power usage - this is based on 920 kw-hrs per residence per month/30 (days in a month), divided by average number of people in a residence of 2.61.
920 kw-hrs/month at U.S. Dept. of Energy, Electricity Basic Data
2.61 people per household at Fact Finder, U.s. Census Bureau

So now, the math at 11,000 per year at 30 mpg, or 367 gallons a year
367/365 ~= 1 gallon/day

1 gallon x 33.7 kw-hr = 33.7 kw-hrs day

33.7 kw-hrs day plus 12 kw-hrs day ~= 46 kw-hrs

46 kw-hrs per day plus 20% for some breathing room, gives us 55.2 kw-hrs

So we need 110 m2 solar panels, at $500 (March 2011) per m2, we are at $55,000. This is 1200 square feet, or about the size of a small house.

[Plug-in Hybrid scenario]

Here is the "average" scenario with a plug-in hybrid car available, and the owner does 60 miles per day (theoretically never needing gasoline):

0.26 kW·h/mi from Plug-in Hybrid info at wikipedia.org
So 60 miles times 0.26 kw-hr/mile, gives us 15.6 kw-hr
So this brings our total daily usage to 12 kw-hr plus 15.6 kw-hr ~= 28 kw-hr.

28 kw-hr plus 20% extra, gives us 33.6 kw-hrs per day required.

so we need 67.2 m2 solar panels, at $500 (March 2011) per m2, or $33,600. This works out to 723 square feet.

Final notes - the cost of PV (Photo-Voltaic) solar cells will eventually drop, as more manufacturing capacity comes on-line, new technologies become available, and supply and demand reaches an equilibrium. It is relatively short-term oddity that prices have risen over the past several years (2008). To become cost competitive with existing energy costs, some in the industry are using the $1/watt as a long-term goal for solar electric. If that were the case, the final plug-in hybrid scenario would cost only $6720 for an individual to be energy self-sufficient!

Question: "What are the reasons to convince others that solar electric is cleaner energy than other renewable energy such as small hydro power, geothermal, biomass?"


Solar electric energy utilizes a wireless connection to a nuclear fusion reactor located at a safe distance, and harnesses usable electrical power. In actual use, it is very clean. If mounted on already existing structures (e.g. a roof) it adds electrical power generation to the functions of the roof. The only real negatives are the costs involved. But these are not minor issues. There is a significant amount of energy required to manufacture the solar panels, create the metals used to mount them, the fuel to transport them to the installation site, the labor involved in mounting them, and all the other technology involved (wire, power inverter, etc.).

Most other useful energy production is tied to solar energy, but indirectly. For example, hydro-power captures the potential energy of water that is evaporated into the atmosphere by the sun. Capturing this energy is considered clean, but there is the environmental impact of damming rivers, the land covered by the resulting lakes, the impact of connecting power transmission lines over distance, etc. Fossil fuels such as oil & coal also trace their energy content back to solar energy, just over a very long period of time - technically these types of fuel ARE renewable, just not in human lifetime terms. Biomass typically requires some sort of food as fuel, and this relates back to plants that obtain their energy from the sun.

Only geothermal isn't directly or indirectly related to solar energy, since it captures heat energy that is inherent in the earth. To easily access this energy, however, requires access to this heat. Some places in the world have this at or near the surface, but most places do not (otherwise our planet probably wouldn't be as hospitable to life as we know it).

So what is the "best" choice for clean energy? Taking human nature as a constant, you can rest assured that the cheapest, most readily available energy source that meets people's needs will be the one that is chosen. Due to different environmental, geographical, economical, and political forces, the choice for one group of people will most likely be different than another group. What is exciting is that changes are in motion on many different fronts, and that there are many choices.

The fact that fossil fuels are so readily used (not so "clean" energy) is indicative of how much energy is stored in these particular forms (coal/crude oil). The fact that drilling or mining for these materials, processing them, transporting them, and including all the costs involved with these activities still result in a relatively cheap fuel illustrates how energy rich these materials are. Some would argue it would be foolish to ignore these fuels while they are still readily available. Others would argue the carbon footprint (environmental impact) of these fuels should be factored into their price. The real concern is how quickly the party will come to an end once these energy rich fuels become scarce, and how society will react if there is not a readily available replacement.

Solar electric is a triumph of the human race's understanding of technology in that we can take a rock (silicon) and harness usable electricity, which can be used to meet our energy demands. However, this does not necessarily mean it is the best, cleanest, or only source of energy that should be considered. Note that the "Law of energy conservation" means in practical terms that all we are doing is converting energy from one form to another. Each form and approach to harnessing energy turns into an engineering problem - How can we obtain the most energy, with the least environmental impact, for the lowest cost? As stated above (and luckily), we have many options available. If you want to convince other people of any particular solution, provide the most energy, with the least impact, for the lowest cost - the convincing part will take care of itself.


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