Calculations

[Bryan]

Many people on this site sound like experienced professionals and know the formulas by heart. Is there anywhere I can find a simple description of the necessary components and the formulas needed to begin understanding how the solar electrical system works?

Thanks,

Bryan

[Sunking]

Here is your first formula and definitions you need to know.

W = Watts is the rate at which POWER is consumed or generated at a given moment in time. Example a 100 watt light bulb consumes 100 watts.
H = Time in hours
WH = Watts x Hours is a measure of ENERGY or amount of work done. For example a 100 watt light bulb turned on for 10 hours uses 100 watts x 10 hours = 1000 watt hours or 1 Kwh
W = WH / H
H = WH / W
K = Kilo or x 1000. So if I say 3.0 Kwh I am expressing 3000 watt hours
M = Mega or x 1,000,000. So if I say 3.0 Mwh I am expressing 3,000,000 watt hours = 3000 Kwh

Electric utilities charge you for watt hours consumed at some rate like $0.10 Kwh. So if in a given month you use 1000 Kwh your bill is $0.10 x 1000 Kwh = $100.

It is all easy 5th grade math anyone can master. That of it as your hourly wage. If you make $10 per hour how much money do you make? Well that question cannot be answered because we do not know how many hours you worked. So think of Watts as your hourly rate of pay. How much money you make is like Watt Hours and there has to be a time element (hour) involved.

So if you have a 1000 watt solar panel system and you receive 5 Sun Hours each day in theory the solar system will generate 1000 watts x 5 hours = 5000 watt hours or 5 Kwh. There is a catch though, we have not taken system efficiency into account at this point so the real answer is less than 5000 Kwh.

Using the above example as a problem to solve if you have a solar system that generates 5 Kwh per day and you receive 5 Sun Hours in a day, what is the size of the solar system? Well we know for fact Watt Hours = Watts x Hours, and we know how many watt hours and how may hours we have then all we have to do is factor out the time element to find the watts. If WH = W x H then we know for fact that W = WH / H = 5000 wh / 5 h = 1000 watts.

[Bryan]

Awesome! Thank you. That makes sense. Now I need to absorb it. I am starting off with a very small camping system designed to run a fridge and my computers. I will get the details and apply the calculations. I will probably be back here soon.

Bryan

['NuffSaid]

You should know the Solar Constant, which is 1 kw per sq. meter (about 11 sq. ft) at the earth's surface, so a 1 sq. meter solar panel at 16% efficiency (facing the noon sun?) will give you 0.16 x 1000w = 160w.
You can also look up "insolation" for your city on the Net.

What I have not easily found is how many consecutive cloudy days you can expect at a given location to some level of confidence, say, 50%, 95%, or 99%, so that you can size your storage capacity, if you want your storage to supply your needs at a high level of reliability.

Unfortunately, compressor/motor loads like a fridge can give you some problems, as can the switch-mode power supplies in computers.
Sometimes investing in a "soft start kit" for the fridge is worth it, if you can find one.

[Bryan]

The freezer I am looking to run draws 1.35 amps per hour from a 12 volt power source according to the manufacturers specs. Another user says it draws an initial 5.36 amps and then settles down to a constant 4.86 amps within about 30 seconds. That is a wide discrepancy in the numbers, so I expect the actual number to be somewhere in the middle. Of course the compressor turns off when it reaches the desired temperature, so that will effect the daily draw. How would I find the current that my unit draws and what do you think I will need to run it while camping in the California desert?

[Sunking]

Bryan this sis not going to be very feasible. When you say camping are you talking RV type or hiking.

Forget Amp Hours for now, it is meaningless and not as imprtant as watt hours consumed in a 24 hour period you have to deal with.

[Bryan]

I take my truck into the desert and camp, often with no power. I have a secondary 12 volt deep cycle wet cell that I will be mounting in the engine bay. It will charge while the the engine is running but I want it to be charged by solar at other times. I was parked for 10 days last month. However, most of my trips are weekends.