Your Questions About Solar Panel Calculator

Mandy asks…

Solar power, number of panels needed?

I visited a site that has a solar panel calculator to calculate the number of panels I need to power the house. It said I would need enough to produce about 16,000 watts. I don't understand this as the inverter I plan to use would only require 12 Volts DC. If I send 16,000 watts from all those panels into the charge controller, it would be reduced to 12 Volts DC before going to the inverter. It seems to me that if the inverter requires only 12 volts DC, I could use just enough panels to keep the batteries charged and feed the inverter it's 12 volts DC.

As i understand it, the solar panels don't power the house, the inverter does. So if I get a 5000 watt inverter and feed it the required 12 volts won't I get 5000 watts out of it. My question in a nut shell is: Why do I need so many panels to power a 12 volt inverter and keep the batteries charged?
It just seems strange that a 12 volt battery bank can run the inverter but it takes 16,000 watts to charge the batteries. Even if your panels are providing 100 million watts, the charge controller will allow only around 12 volts to pass through.
For example:
A Kyocera Solar Panel model # KD210GX-LP
produces 210 watts,
7.90 amps, and 26.6 volts.

26.6 volts is twice as much that is needed for a 12 volt inverter. So it seems to me that this one panel would power the inverter.

ssadmin answers:

You seem to be confusing volts and watts, so let's review some definitions:

A ‘volt' is a unit electrical “pressure”. It has its uses but it's a minor player here. In this context, it should be used to see that the parts all match up. And a 26.6 volt panel doesn't sound like a match to a 12 volt inverter! As you probably well know, you don't try to power a 6 volt anything with a 12v battery. You can damage it.

Make sure the inverter can handle the voltage of the panels. Buy the panels first. See the wiki link on inverters.

A ‘watt' is a unit of _power_. A computer might take 70 watts. A hair dryer typically consumes 1500 watts. If you run a hair dryer for 2 hours straight, you've used a certain amount of _energy_ and, for our purposes, we can use the term ‘kilo watt hours'; the hair dryer would have used 3 kWh (2 hrs x 1500 watts / 1000).

The starting point of your calculation should be your electric bill which'll define how much energy you use per month; the value will be in kWh. I'm starting you here because 16 kW (kilowatts, not kWh) is a lot and I'm suspicious that there's been an error along the way.

Suppose your electric bill says your household used 720 kWh in a peak month. Divide that by (roughly) 30 days and 24 hours/day (720 hours) to find that your average power is 1000 watts (you can see why I chose 720 kWh). So you have to add in some fudge factors to account for overcast days and solar collection inefficiencies. Don't go crazy with the overcast days thing (unless you live on the Oregon coast); I'll return to this topic later. The ‘solar collection inefficiencies' result, usually, from the panels not being pointed at the sun all the time – and the sunlight gets weaker as the sun nears the horizon (extinction coefficients…). The solar industry says that, for my latitude of 38 degrees, a stationary solar panel will produce energy as though it got direct sunlight for 4.3 hours. So our 1st fudge factor is 24/4.3 or 5.5; the panels would have to produce 5.5 kW. But wait, there's more…

Everytime you store or retrieve energy from batteries, you lose about 10%. It's a corollary of the “no free lunch” rule. So multiply the 5.5 by 1 / .8 = 6.9 kW. And your inverter might dump 10% of its efforts into heat. 6.9 kW / .9 = 7.7 kW.

7700 watts / 210 w = 37 panels. That's a bunch but this midpoint calculation is just to get an idea of ‘near worst case'. So, still ignoring the overcast day issue, we got to about 1/2 of the 16kW. But my starting point of 720 kWh wasn't wimpy. The house that came from has a hot tub, 3 computers, air conditioning in a hot climate etc.

You have options:
1. If you have the land etc, why not have the panels track the sun? You can improve greatly on the 4.3 hour value.
2. Consider only doing part of your house's appliances, the really necessary ones: refrigerator etc. If air conditioning is a major energy sink, look into alternatives (smart vents that let hot air out, for instance).
3. Ebay frequently sells panels for about $2/watt. Pay an expert to make sure you're getting a high quality set of panels. $2/watt x 7700 watts is $15400 and you still need mounting, an inverter, etc. Still way shy of $125000. You could pay the expert $800 and still make out like a bandit.
4. The amount of money considered here is substantial enough for you to consider other sources of energy savings, such as double paned windows, thicker insulation, modern and more efficient air conditioner, refrigerator etc.

Regarding the overcast days, you don't have to just multiply by 2 or whatever. You can make your own “energy plan” such that you gleefully use all solar energy when it's available but use the grid to supplant it when it's overcast. Much of the time, you'll likely have an excess of energy which you can sell to the power company.

In the above, I _presume_ you're not building a house in Timbuktu where there is no power company. Obviously some of your options change for such a locale (but you'd sure have lotsa sun!).

I've also assumed in the above that the solar panels are being kept clean. Another reason to keep them off your roof.

Jenny asks…

will calculator recharge its battery if I switch it from lithium to li-ion battery?

my calculator do have solar panel and was used to have non rechargeable battery on it, so will it recharge itself if I use li-ion type?

ssadmin answers:

No it will not the size of that panel is only powerful enough to power the calcatulator or even to just increase batt life by supplementing batt power. So in no. Also they don't make li ion in that size

Nancy asks…

Making a Laptop solar panel? Possible?

so i kno that they make solar panels for laptops but they are big, it would be cool just to have something as big as the screen so it could fit on the back of the laptop, so cant u make one out of the panels on calculators? or are those a different kind of solar panel? would it be able to produce enough wats? remember it would be a crap load connected together enough to cover the front side of the laptop?

think itd work? or no?

ssadmin answers:

Can be done, but won't be cheap or light. Do remember that calculators don't use anywhere nearly as much power as a computer

Maria asks…

Can a small fridge be powered 24/7 by being covered in solar panels?

You know the panels you see in a calculator or a solar charging device for a mobile phone, if you covered a fridge as might be used by a single person, as well as some other technical jiggery pokery could it work, be a commercial product?

ssadmin answers:

This sounds impractical, considering the way normal refrigerators are opened and closed many times a day.

There are excellent insulating materials today, and it is possible to build a refrigerator-sized box that will keep ice solid for a month with no energy input at all, in shirtsleeve weather. All one would need to do is hook up a small cooling unit to such a box, and eventually it would reach refrigerator temperatures. That cooling unit could be powered by solar panels, and only a few hours a day. The problem is, once someone opened the door, it would take a long, long time to cool down again.

Joseph asks…

Making a model Solar Panel?

For a science project we have to make a model solar panel that can actually convert light into electricity to light a small light bulb. Any easy way without having to buy professional equipment and stuff? Could we use a pre-made one from a calculator and hook that up? Would that work? Thanks!

ssadmin answers:

Making solar panels is easy. Here's a link to a DIY of solar cells. 10 easy steps

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