On Tue, Dec 14, 2004 at 12:52:47PM -0600, djgoku wrote:
Reading on some solar panels that are out there I found these since my computer is using around 300 watts with monitor. I'm kinda confused on what Approx. Watt-Hrs / Day = 1200 - 1500. I know you just can't run your computer off these panels but you would need a battery of some sort for the panels to charge. But does that mean that it charges 1200 -1500 watts/ a day so you would need a couple 700 watt batties or? Most car batteries are like 300-700 watts no? not crank just the storage part if that is what it is called.
From my read of the "my laptop ate my TV" thread, I know there
are people on this list who have more experience doing these kinds of calculations and can probably explain it better, but here goes:
A watt is a unit of power. Power is a measure of energy transferred per unit time (electricity being just one way of delivering power) watt-hours are a (common, but non-SI) measure of energy. Energy is something that gets stored, power isn't.
The SI unit of energy is the Joule (J). A watt is a rate of energy transfer equal to one Joule per second. So, a watt-hour is 3600 joules (1 joule/second * 60 seconds/minute * 60 minutes/hour * 1 hour).
When speaking of batteries, watts would be relevant in the sense of how fast you could charge or discharge the battery, but not so much in terms of what its storage capacity would be. A common (but again, non-SI) unit of battery capacity is amp-hours. Amps are similar to watts in that they are a unit used to express the rate of transfer of something. In this case, though, instead of it being the transfer of energy, it's the transfer of charge. The SI unit of charge is the Coulomb, which is the charge carried by one mole (~6.02E23, in other words, a whole lot) of electrons. The flow of one coulomb (C) per second is an amp. So, one amp-hour would be a charge of 3600 coulombs.
Now, the one missing piece here is how to tie the amount of charge to the amount of energy. This is where potential, expressed in terms of volts, comes in. One volt is one joule per coulomb. So, every amp from a 12 volt lead-acid (ie, car) battery translates into:
1 amp = 1 C/s * 12 J/C = 12 J/s = 12 watts
A typical home circuit, let's say a 15 amp circuit at 120 volts, provides:
15 amp = 15 C/s * 120 J/C = 1800 watts
and so on and so forth.
Aside from blue-skying about solar power possibilities and trying to figure out battery capacities, getting a handle on the arithmetic and factor-analysis involved is useful for sizing power supplies, among other things.
I was just shopping at Radio Shack for some DC power supplies for some hand-held consumer electronics devices we have (a TV and a digital camera--there seems to be no substitute for actually trying out all those little tips) and we ran through some rough calculations to make sure that the 800mA power supply I was looking at had a transformer big enough to supply the TV (which, I figured, needed a bit over 500mA). The specs given for the two devices, the power supply and the TV, were not expressed in the same terms, exactly, so we had to do this kind of conversion.
Beyond this, there are other fun things you can do, like digging up the factor for converting between British thermal units and joules, and, with a couple of utility bills, figuring out just how much more expensive it would be to heat your place with electricity instead of gas. (Or, to turn that on its head, you can figure out how much your computer hardware habit is contributing towards your heating needs in the winter, and what kind of cooling burden they place on your AC in the summer, since all the electricty consumed gets dissipated pretty much as heat, all but for that bit of monitor and LED glow that escapes the house through the windows).
One can also use this sort of thing to help understand the appeal/practicality of heat+electricity co-generation, fuel cells, hybrid vehicle powertrains, etc etc.
Good times for geek minds.