Calculating Your Annual Solar Energy Output

[caption id="attachment_685" align="alignleft" width="300" caption="Solar Map"]  Solar Map


The first thing to do when trying to decide if  Solar PV or Thermal will work for you is to never listen to what politicians say.  Do your own research!  The map at left is a good start to gauge how well a solar system will perform on your house.

If you live in the south-west, then your solar resources are pretty great.  The south-east looks pretty good (FL is currently the site of the largest Solar PV array installed by FPL) and the north-east is ok.  Surprisingly, one of the states with the highest incentives to do solar is New Jersey!  So just because you may not live in AZ doesn't mean solar wont work for you.

After getting a rough idea from the map above on your State's suitability for Solar you should go to NREL PVWatts to get some numbers.  Once on the site click on, "The PV Watt Photovoltaic Solar System Performance Calculator" and then  "PVWatts Version 1 Calculator".  Alternatively, if you live in a city in the U.S. that does not come up in the version 1 calculator, you can use the version 2 calculator to select your location by zip code.

PVWatts is developed by the National Renewable Energy Laboratory (NREL), the same people who did the map above, so they obviously have some great tools.  The online Calculator they have produced is meant only for grid-tied solar systems; basically systems that put the energy they produce on the electricity grid and not in a battery bank.

When you get to the Calculator there are two main things you will need to enter:  the size of the system you are interested in (which will be 3 - 5 kW for most residential systems) and the DC to AC derate factor.  This derate factor encompasses the efficiency of the panels and inverters, losses due to connections and wiring, any shade present, the age of the system, etc.  Basically, the derate factor encompasses any inefficiencies in the system because no energy producing system is 100% efficient (2nd law of thermodynamics).

The standard derate factor PVWatts fills in is .77.  Since some panels are more efficient than others this derate factor will change depending on the panel you decide to use for your house.  For instance, Sunpower, which currenlty has the highest efficieny panels on the market suggests using .833 as the derate factor.

Once you hit calculate the most important value you get is in the bottom right hand corner.  This is found under the column "AC Energy (kWh)" and you want the yearly value.  Once you have this value, you have to multiply it by the amount that your utility will pay you for solar energy under the net-metering agreement between you and the utility.  The Calculator asks you for what you pay for electricity, but this is not always the rate your utility will pay for your Solar power!

For instance, I pay almost 10 cents/kWh for electricity from my utility, Georgia Power, but they will credit me 17.7 cents/kWh for any solar power a system I install produces!  They do this because they use it for their "Green Energy" program.  And utilities usually don't cut you a check, they just credit your bill for the amount you use.  You can see the results of a calculation I ran below.

After completing these steps, you will have an excellent estimate of annual amount of money your solar panels will generate for you!

PVWatts Calc Output

PVWatts Calc Output

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I took a look at Verdegía's site with great interest. At first, it seemed too good to be true. My final evaluation is... IT'S DEFINITELY NOT TRUE. It may be some kind of hoax, or a scam fishing for unaware investors, or even some April 1st joke. I am an Electronic Engineer, working in one the world's largest energy companies, Petrobras. For the last 6 yrs I've been dealing with Renewable Energy, so I consider myself to be pretty well informed on the latest developments on this area. After careful reading of Verdegia's site, I can say their proposal it totally impossible to accomplish. No pictures at all of an ACTUAL equipment. Lots of nonsense information, wrong assumptions on their calculations. Unrealistic figures for their "device" output. To make things very short, just consider their own figures for the so-called "Solar Engine": HOW CAN A 2sqm SOLAR COLLECTOR PRODUCE AN AVERAGE OUTPUT (24x7 they say!) OF 39 kW of ELECTRICAL POWER? From their own figures 4.5kWh/sqm/day X 2sqm = 9kwh/day = 9kWh/24h = 0.375kW average - and this is the RAW SOLAR ENERGY COLLECTED at the solar panel. This would be further reduced, due to the efficiency of their "turbine"... my final estimate would be something like 0.1kW for the whole apparatus. Quite far from their claim of "39kW" fact, 400 TIMES smaller
RD, isn't Petrobras a legal monopoly oil company based out of Rio? It seems your company is the fossil fuel competitor.
I'm an engineer and, having just had a solar panel stringset installed I was interested in how the vendors produced their Annual Energy Generation [AEG] calculation. I've made use of the and sites to garner some basic data, and from a common-sense perpective, assumed that the general calculation might be: (Number of Panels)*(Panel Power Pmax)*(Inverter efficiency)*(Insolation factor derived from Lat&Long coords)*(angle of panels factor)... but this isn't giving me rational output yet... any comments? PS. The comments from Rubin Diehl appear to be generally correct IMHO. Scott G's response didn't address the calculation....
Sounds a lot like the freon engine i designed years ago kept the prototype for many years.Never found anyone who wanted to invest money without stealing my invention.
It’s time to move away from large expensive solar modules and into the next generation of high performance renewable energy generators - The Solar Engine Systems from Verdegía boast 39kW/h and operate 24/7. That´s enough energy for 12 or more homes! And, they only take up the space of a single 200W PV panel. Visit:
Do you work for Verdegía or represent them? Want to get in contact with the company but can't get a response.
I believe they have sold or allocated all their units until 2012... no need to guess why.
We just did a Request for Quotations for a client in Arizona. Most of the bids we received used the PVWatts calculator to generate their annual energy production. Some even provided the screen shot of the inputs and the results they generated. I asked one of them why they use this calculator. They were happy with the results because the calculator uses a conservative derate factor, and most of the solar energy developers can guarantee the amount produced from PVWatts. Version 2 of the calculator allows for very specific siting instead of general areas within a state.
Mate, that website shows a very interesting little engine, but i can only guess at the cost of it. I've heard about a company that is looking at plastics as a meduim for solar panels instead of the expensive silicon. But at the moment they are only getting approx. 5% efficiency out of them, whereas the best silicon panels are now up around 25%. Plastics would bring the price of solar panels down considerably but clearly a lot of work is needed to get the efficiency up. I'm sure it won't be long. I wonder how much a typical solar engine costs? David

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