# Light vs power decrease

Will amp capacity or voltage decrease with the amount of light on a solar cell?

Re: Light vs power decrease

For a first approximation...

A solar panel will not output much power at all until there is a minimum amount of light on the panel (say full shade on a sunny day)... Once a minimum amount of light is present, the Voc (Voltage open circuit) and Vmp (Voltage maximum power) are achieved (and very little current is available).

At this point, the power (and current available) is pretty much proportional to the amount of sunlight falling on the panel--I.e., more sunlight, more current. More current, more power where P=I*Vmp

Most solar panels have a graph available that shows voltage and current vs available light. Some also have Power vs Available light drawn on a graph too. Take at look at this SunTech PDF and you can see how it all relates.

Note that Vmp (and other values) do vary some with respect to the amount of light available and panel temperature. MPPT (Maximum Power Point Tracking) type controllers attempt to optimize the P=I*V to get between 0-30% more power from a solar panel vs that with more simple PWM charge controllers (again--amount of increase depends on temperature and other conditions too). MPPT is also used with Grid Tied Inverters too.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Re: Light vs power decrease

Bill

Thanks again for your great response. You answered my question so that I understood. I worked many years in electronics and it was good to see someone use the formulas that I grew up with. Still doing research and putting together the plan.

Looks like I will need about 90 amps at 220v AC to live as I do now. When we go off grid some changes will have to be made, especially regarding heat, now on electric heat(66 amps both systems on).
• Solar Expert Posts: 5,436 ✭✭✭✭
Re: Light vs power decrease

90 amps @ 220 vac= ~20 kw!!!
Generous number 4x panel capacity/load=80kw @\$8w=~\$640,000,, double that for an off grid system,,,call it a cool million.

I guess some changes will have to be made!

Every \$\$ saved on the load side saves ~\$10 on the PV side,,, more if you are off grid. PV for electric heat is VERY inefficient. You are way better off using direct solar heat, or ground source heat pump. Same with solar hot water,,, way cheaper than PV for hot water,,, in fact is your cheapest solar \$\$.

As has been suggested so many times here,,, before you spend a penny on PV solar, consider every aspect of your consumption. Conservation first, followed by conservation, followed by still more conservation. Then solar space heat, solar water heat, and then and only then PV solar.

As as been said, PV solar is sexy, but it is silly to spend thousands of PV solar if we haven't spend the hundreds to reduce the loads to the absolute minimum to make PV solar to make sense.

Good luck,

Tony
Re: Light vs power decrease

You are very welcome OM...

Electric Utility power costs around \$0.10 to \$0.35 per kWhr (depending on where you live and how much you use).

Off-grid power with generator backup, easily costs \$1.00 per kWhr or more (just add up all your costs for the equipment and panels, plus battery replacements every ~5-10yrs, replacement parts--inverters and charge controllers down last forever, and then divide by the number of years of the life of the equipment--typically 20-25 years--times the number of kWhrs per year you will generate):

\$capital costs + maintenance + taxes + parts / (life in years*kWhrs per year generated)

Very quickly you will discover that through conservation and spending on conservation--that you will be much happier.

Also, measuring your actual power use, vs just look at the fuse box and service voltage (and appliance name plate ratings)--will give you a much more accurate handle on your real power needs (probably not as high as you think). Fridge is only on 20 minutes per hour, TV/Lights a few hours a night, well pump--perhaps lots of power for only 10 minutes per day, etc.)

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Re: Light vs power decrease

Tony

I agree with you!! I live on 11 acre and have a lot of hardwood trees that are really large, so I should have a ready supply for heat. Looking at wood heater/cook stoves at this time. This will involve a lot of work and will be messy, but it will be one way to stay warm. I am thinking of using just one of my heat systems that draws about 23 amps @220v as measured at the main panel, along with wood heat. Still looking at other ways of heating. I don't like the wood pellet heat systems because you have to keep purchasing pellets.
Have you any experience with the wood boilers that produce steam?
Re: Light vs power decrease

Bill

I have a Sperry clamp on amp meter and have measured all my high power users at the panel. Had a couple of surprises.

It looks like with 6 solar panels and 6 100 ah battery's I can run most of my house with the exception of the big AC and all the electric heat. Using power management--turning off most of the large users of power and turning only one at a time will allow me to live close to what I do today. I know my power bill will go down, but I really want to get as independent as possible, knowing that a pay back is not in the cards. If the economy keeps going down hill being independent will have its on pay back.
• Solar Expert Posts: 5,436 ✭✭✭✭
Re: Light vs power decrease

OM,

Your 23 amps if heat is still ~5kw.

As I suggested before, using PV solar for resistance electric heat in any fashion is crazy.

I am not a big fan of wood fired boilers. I do think they have a place, but most applications I have seen have led to big air pollution problems because the fire chokes down much of the time. Many jurisdictions have banned or severely limited their use. Also, the parasitic energy requirements for circ pumps is another limiting issue.

Personally, if you have 11 acres of wood, even if you had to hire someone to cut an split your wood, I would consider a good EPA rated wood stove,, or even a couple. This is one of my favorites,,,http://www.pacificenergy.net/product_spectrum.php not the best looking, but burns very clean, very efficient. The old calculation is that a good wood lot should produce ~1 cord of wood per acre per year. With modern Selkirk/metalbestos chimney systems a good stove can be installed almost any where. Wood cookstoves tend not to be a very good idea. The make lousy heat stoves, despite claims to the contrary, and they heat up the house too much in the summer. If you really want a wood cookstove that also heats reasonably well : http://stovesandmoreonline.com/StoveBakersChoice.asp Amish made, works pretty well from what I have heard. Or try this one,,,if you've got big bucks:http://www.lehmans.com/jump.jsp?itemType=PRODUCT&itemID=716 Beautiful stove, but lots of money, and not a very good heater,,I've had one. For the cost, unless you live in a very small house (which if you are using 90amps of power I would guess not!) I would put in a good wood stove, and then a gas range. If you are looking to go off grid, do not consider a gas range that has any sort of hot surface ignition system. These hot surface ignitions can draw ~1200 watt or more, as much as a toaster, every time the oven turns on,,,many times during the roasting of a turkey for example. Standing pilot stoves can be had that work quite well.http://www.brownstoveworksinc.com/gas.htm

On the other hand, I would also consider high efficient propane space heat, such as a Rinnai. These come in many sizes and are very efficient, and will heat a space very quickly, very economically. http://www.rinnai.us/

Finally, as Bill has suggested, to go off grid to attempt to mitigate the effects of the current economy is crazy. You WILL spend about double per KWH than a grid tied system. If you want to ensure your financial security IMHO you would be WAY better off, (after doing all the conservation you can) to put twice as much into your PV panels rather than a battery system. It would be a much better/faster/bigger payoff.

If you are worried about the loss of the grid, for emergency power, a cheap generator is WAY cheaper than a battery/solar system.

Tony