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# Solar Panel: Question About Home Solar Panel Systems? (10/13/2011)

Online stores selling solar photo-voltaic generation kits gives specs indicating the power generation capability of the system. For example, Solar World Grid-Tie Solar Electric System with 245W Panels & PV Powered PVP2000 Inverter, 1.2 to 2.4 kW. This seems to indicate that the system can generate 1.2 to 2.4 kW. Is that per day? Per month? I’m trying to calculate the return on investment, but can’t because I don’t know how much power a system such as this will generate in a month.

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Daniel C October 13, 2011 at 1:35 am

Did the thought cross your mind that the absolute best source for an answer to this would be Solar World?

Ed October 13, 2011 at 1:40 am

That would have to be in that instant of time or they would say kilo watt hours or KWH for short. But being the sun doesn’t shine at the same intensity over any given hour you couldn’t say a solar panel rated at 2.4KW would produce 2.4KWH of electric in a given hour. The power produced would have to vary as clouds passed over head. The power would also be reduced on days of heavy overcast or rain/ snow.

roderick_young October 13, 2011 at 2:10 am

As Ed said, that 1.2 kW is an instantaneous rating in bright sun.

The way to do this right is to consult the maps here http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/atlas/ to find the number of equivalent sun-hours your location gets per day. Select, Average, Annual, Flat plate tilted south at latitude. A map will come up. For northern California, it shows 5 equivalent sun hours per day, for example. If the system is 1.2 kW, then 1.2 x 5 = 6.0 kWh of energy the system will generate per day, on average. That takes into account cloudy days, short winter days, long summer days, everything. The 2.4 kW system would give double that, obviously. But that’s an example, based on a specific location. Phoenix would do better, Seattle would do worse.

J. October 13, 2011 at 2:26 am

There are several items here.

1) the panel produces 245 Watts of energy that can either be used directly to power items or charging batteries up to the output of the 245 Watt panel. Loads in excess of that amount would be fed via a battery up to the amount of charge on the battery or the capacity of the inverter.
2)The size of the battery determines how much and how long you have power and the size of that battery determines how effectively you can charge it in a day.
3)the inverter is the device that is converting that Direct Current from the panel and stored energy in the battery to Alternating current for unmodified AC appliances, entertainment, etc. The size of the inverter determines how much current you can draw from it at a maximum of 1.2 Kw continuous to 2.4Kw peak intermittent, such as starting an electric motor. The rating is in effect instantaneous ratings.

To determine what size panels you need, you first need to know what your current usage is per day in Kilowatt Hours.

To figure out how long it will take to pay for itself, take your current Kilowatt Hour charge on your electric bill, divide that number into the total cost of the system, multiply it by 4.1 and that gives you the hours you need that system producing energy in terms of Kilowatt hours, so however long the weather and daylight permit you to reach that amount of time should be considered your break even point of cost recovery.

In short- that system might be enough to eliminate the need to plug in an RV to an AC source, or for a small cabin or home. with maybe 1 kilowatt hour to 2 kilowatt hours of usage each day assuming a lot of sunny days. Assuming heat is not electric and hot water is not electric. and that you are not powering a well for irrigation.

If you are looking to go off grid, in one fell swoop, 245 Watts is not enough for an average family without some serious changes to usage patterns. 2000 Watts of panel “might” be adequate for some, but people still need to change usage patterns.

If you are looking to have a system installed, you can get some idea of some components and vendors through http://www.builditsolar.com and http://www.homepower.com The latter’s magazines tend to read more like a sales brochure than a magazine.

If you plan on doing the work yourself- be sure to get it inspected for a multitude of reasons. You can get some additional pieces through http://www.northerntool.com From panels, to inverters, to controllers, etc. Just look in their alternative energy section.

If you happen to have aboutt 30,000 dollars to drop on the installation and you plan to live their more than 10 years, talk to your power company and they can set you up completely.