What you are looking for is the 'amp-hours' rating on the battery.
Car batteries have this kind of rating but it might not appear on
the case because they aren't deep cycle and therefore not really
well suited for this kind of application. Most car batteries are
30 to 40 amp-hours. Some of the larger ones go as high as 60 to 90
amp-hours. The older they are, and the more they have been used, the
less their capacity.
Multiply amp-hours by volts to get watt-hours. Car batteries are 12 volts.
(Actually, they are more than 12 volts, but this works for this purpose.)
To figure out how many watt-hours you will need you multiply watts by
hours for your loads. You don't say how much your computer draws so
I'll use 200 watts as an example.
TV is 80 watts x 18 hours = 1440 watt-hours
PC is 200 watts x 18 hours = 3600 watt-hours
Total = 5040 watt-hours. Divide by 1000 to get kilowatt-hours or kWh.
This is 5.04 kWh a day.
If you want to run these devices for one day off a 12 volt battery
you simply divide watt-hours by volts to get amp-hours.
For example, 5040 watt-hours / 12 volts = 420 amp-hours.
You would need batteries that add up to 420 amp-hours.
It's a little more complicated than this if you really want a good
number. For instance, did you measure the wattage used by your TV
and PC or did you just read the UL sticker on the back? The UL
sticker is a worst case number, or maximum. Most devices use less
power than is shown on these stickers.
How efficient is your inverter? If it is 90% efficient then it means
10% of the energy is lost going from DC to AC. If it is 50% efficient
then it means 50% of the energy is lost. If you need 350 watts AC
and your inverter is 60% efficient then you will need (350/.60=) 583
watts DC.
You also typically will not want to run your batteries flat as it
will damage them. Most folks suggest 80% depth of discharge max.
That means you only want to take 80% of the energy out of that
battery. If you figured you needed 420 amp-hours then the battery
would have to be (420/.80=) 525 amp-hours in order to have 20% left.
Then, of course, you will need to multiply by the number of days
you want to be able to run off batteries before you recharge them.
You can wire batteries in parallel (plus to plus, minus to minus)
to increase their total amp-hours but this works best when all the
batteries are the same type and age.
If you need 525 amp-hours and you are using 40 amp-hour batteries
then you will need (525/40=) 13 of them. They make deep cycle
batteries all the way up to 1100 amp-hours if you want to spend
the money.
My suggestion would be that it's sometimes cheaper to reduce your
power consumption first. For instance, a typical laptop computer
can use as little as 20 watts instead of 200 watts for a desktop.
With the addition of a TV tuner it can double as a TV. Even if you
needed a stand alone TV you could buy an LCD version that would use
less than 80 watts. These devices would also be able to run directly
off your battery (12 volts) so you wouldn't need an inverter and
the energy it would lose.
Say you use a laptop & LCD TV and they draw 40 watts together.
40 watts x 18 hours = 720 watt-hours
720 watt-hours / 12 volts = 60 amp-hours
60 amp-hours / .80 depth-of-discharge = 75 amp-hours.
Some of the larger car batteries are over 75 amp-hours and a
typical deep cycle battery like an L16 is over 300 amp-hours.
A couple of L16's (they are 6 volts so you would need pairs of
them wired in series) could run this combination for 4 days.
You may also want to check out these pages.
http://www.homepower.com/education/comp_battery.cfm
http://www.homepower.com
http://www.trojan-battery.com/default.htm
Anthony
This gets into "PV system sizing". There are various websites that
will help you out as well as any good PV salesperson. It's been
covered in this newsgroup numerous times so a good google search
should find many references.
The general idea is to work backwards from what you want to power.
In your case, a fridge, some lights, etc. All of these vary, but
for example my fridge uses 1.3 kWh/day, some 14W CF lights running
a few hours a day, say .25 kWh/day, everything else .1 kWh/day
for a total of 1.65 kWh/day. Batteries are around 80% efficient
and I'll want 5 days of storage in case of bad weather and I'll
only want to discharge them a max of 80% so I'll need something
like (1.65kWh /.8 /.8 * 5) 13 kWh. Let's say it's a 12V system
for convenience that means (13,000 Wh / 12V) 1083 Ah. A typical
6V golf cart battery runs around 220 Ah. I would need 10 in
series-parallel or roughly two for each day of cloudy weather.
Larger batteries would probably work better. The alternative
would be to forget running the fridge in cloudy weather and
just buy enough batteries to run for one or two days.
How much PV would I need? Well, PVs produce as little as 80%
of their STC rating in hot weather and they would have to have
enough to recharge the batteries (80% efficient themselves) and
we might have as little as 3 'sun-hours' of light during the
winter so say, (1.65 kWh/day / .8 / .8 / 3) .860 kW or 860 W.
Call it 900W at $8/W installed that would cost $7200.
Then again, if we drop the fridge from the list then I'd only
need .35 kWh/day which works out to (.35 /.8 /.8 *5) 2.75 kWh
of batteries or (2,750 / 12) 230 Ah which is very close to
220 Ah and I could get away with just two golf cart batteries
in series. The PV's might cost ( 350 / .8 / .8 / 3 * 8) $1500
and maybe less if I buy seconds and mount them myself.
> Generator systems - also a very good alternative... but we have a
> fairly small lot and I'd prefer to not keep fuel on it. Plus... if the
> grid is down, and the fuel runs out... the generator would de-generate
> (so to speak)
Generators are much less expensive than PV, especially if only
needed for the 100 years earthquakes. Most people have no problem
keeping propane on their lot for BBQs, having natural gas piped
in or storing gasoline in their cars gas tank.
Just the point out the obvious, PV panels only work when the sun
is shining. Everything has it's good and bad points.
Anthony