# Electrical DIY solar panels

#### bobcat

I've found some resources online about building your own solar panels. The basic premise I've found is this:
solar cells -> diode -> battery -> inverter -> mocha machine

First off, if anyone has done this before, any general advice would be appreciated. More specifically, I need someone to point me in the direction of how the amount of power required to charge, say, a car battery is quantified. Meaning, if I have a panel that produces 60W at 18V, how do I know how long that will take to charge a 12 volt car battery? What variables are in play? What are the units of measure? etc.

Sorry for being vague, but I'm just getting started with this and barely even know the questions to ask.

#### mgb_phys

Homework Helper
solar cells -> diode -> battery -> inverter -> mocha machine
First off, if anyone has done this before, any general advice would be appreciated.
Stick to expresso, don't ruin coffee by filling it with milk.
More specifically, I need someone to point me in the direction of how the amount of power required to charge, say, a car battery is quantified. Meaning, if I have a panel that produces 60W at 18V, how do I know how long that will take to charge a 12 volt car battery? What variables are in play? What are the units of measure? etc.
Firstly 18V is probably a bit high to charge a car battery without some other control electronics.
Battery capacity is in Amp-hours = how many amps * how many hours.
60W at 18v is 5Amp at 12V, so to charge a 100 AmpHour battery would (in an ideal world) take 20 hours ( = 100A mpHours/5Amps)

#### Pumblechook

A 60 Watt (max) panel is likely to produce 6 Watts on average.

The charging process is far from 100% efficient.

I would estimate it would take at least 250 hours to charge a 100 Ah battery = 10 days on average.....quicker in Summer, longer in Winter.

Solar panels are simply not economic and unlikey to ever pay for themselves.

#### waxman

Pumblechook said,
Solar panels are simply not economic and unlikey to ever pay for themselves.
I beg to differ.
Although solar panels may take years to payback their initial cost, you must consider their "free" energy after that.
Without the investment today, what will you be paying for energy then?
BTW, a 60 watt solar panel will generate 60 watts minimum in full sun.

#### mgb_phys

Homework Helper
BTW, a 60 watt solar panel will generate 60 watts minimum in full sun.
But that's only for 1/3 of the time (8hours in 24) * the proportion of time it rains / snow.
An average of 10% of the peak output of a panel over the year is perhaps a little pessimistic but not much.

It will pay for itself if you have somewhere with no power!
Lighting in a shed/store with solar + battery + low power 12V lighting might be cheaper than running a power cable.
Converting a whole house is a bit trickier, you either need a serious amount of battery power or some expensive kit to sell power back to the grid.

#### waxman

Actually, for the Albany, NY area, only 4.3 hours per day are considered realistic hours of sun averaged over the year.
Regardless, for a typical 4 KW system, It will generate over 500 KWh per month. For 20 years. "Payback period" depends on Congress, and how green your state laws are.
http://energybible.com/solar_energy/calculating_payback.html [Broken]

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#### Redbelly98

Staff Emeritus
Homework Helper
bobcat, welcome to PF

I will offer an alternative project for you to consider: solar heating. The basic idea is that the solar panels are used to heat water, which is circulated into the house where it can supplement whatever heating system is already in place.

I don't know if you're committed to solar electrical cells, or just want to do a project that is "green" and/or will save you money. But a solar heating system should be cheaper to build and may be even more worthwhile than solar electricity (unless you're in Arizona or someplace like that.)

#### bobcat

Thanks for all the responses and the welcome. My goal at this point is to explore these various alternative energy options and perhaps one day use a combination of what works best for me.

#### mgb_phys

Homework Helper
These talks might be interesting

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