What is the formula for converting 12 volt (100ah) batteries into one kilowatt hour? Then I could find the number of batteries it would take to take care of my usage. Has anyone tried using AGM batteries for this purpose? Any brand name in particular better than others? Also what kind of lifespan should one expect if only drawing the voltage down 70% (or recommended value)?
Couple of things.
Most storage systems use heavy duty 6 volt cells rather than 12 volt and string them together to add up to whatever voltage the inverter needs. some smaller ones are 12v but more common are 24 and 48 volt systems.
I've never tried AGM batteries, the simpler the better, I have flooded wet cell batteries with water miser caps which trap the vapor/mist coming off the cells and recombine it to a drop of electrolyte and drip it back into the cell. They are also flip top which makes checking and adding to the cells easier.
I bought my inverter used from a guy up the coast and since I came to pick it up he threw in 4 Trojan T105 batteries which if you check out their web site will tell you all about them. I have no experience with other manufacturers but I'm very happy with the ones I have.
http://www.trojanbattery.com/Products/ProductSpec.aspx?Name=T-105Not sure of the maths but this battery will give you 447 mins of 6V @ 25amps so if Watts = Volts * Amps that's 150 watts for 7+ hours
I have 4 strung together so that would be 600 watts for 7 hours
If I use more power the batteries will provide 115 mins at 75 amps = 450watts for almost 2 hours. with 4 of them that gets me 1800 watts for almost 2 hours - and so on. The more you draw the shorter you can draw it for.
my inverter (trace DR 3824) will take whatever you give it DC @24 volts and generate 3800 watts of AC out the back. Ignoring efficiency loss for a second that works out to be around a 31 amp draw from the batteries which means I'd have less than an hour at max draw.
You can add capacity fairly easily by adding more batteries. my next purchase is going to be 4 T125's which can be had for about $140 each - these have a slightly higher capacity which will more than double my total capacity.
Care and feeding of these is pretty simple. Keep them clean, keep the corrosion away (see keeping them clean) and make sure they have enough electrolyte in them, distilled water being the replenishment) avoid deep discharge cycles and blast them with an equalize charge once in a while which shakes up the sediment and busts up the sulfated parts of the cells. My inverter has a selectable equalize mode to do this.
Rather than figure out how many KWH you need to run your house you should figure out how many watts total you consume at any given time. Using a "kill-a-watt" device to learn the power use of various 110v plug in things is a very good start or check out.
Here's a handy page to give you some estimates of appliances and their requirements
http://www.city.ames.ia.us/ElectricWeb/energyguy/appliances.htmUsing the above values, figure out how many total watts you're going to use at peak times and then you will know (A) what size inverter you need to supply it and for how long and subsequently how much battery supply you need. (B) what size generator you need to run the house AND recharge the batteries.
There may be some loads you just can't run on batteries OR even the generator unless the gen is huge like the electric dryer or the 3 ton A/C compressor.
One more note before I put you to sleep. I can't stress conservation enough. Since I found out that my grid power is going up by 50% starting Jan 1'st I've embarked on a power use detection and reduction assault on my house. I've replaced almost all my incandescent bulbs with compact fluorescents, changed temperature settings on the thermostat and hot water heater (even though both are oil fired, the less they run the less they use). Compare microwave draw to toaster oven draw - encourage the use of the lesser of the two and so on. change the kids room nightlights from the little bulbs to the luminous green lights or LED based nightlights.
Every $ you save due to conservation will be worth way more than that to you in cost avoidance in sizing your system to supply your needs.
My ultimate goal is to put a new roof on the house and then slap some solar panels on it to generate *some* of my daily needs from solar. The payback will be quite a while but if the price of electricity keeps going up perhaps it won't take all that long after all.
My generation plans are to get a small diesel genset which I can run at least a subset of my house on in the event of a grid failure or when I want to burn up some old engine oil and use the power there to run the house.
sorry you asked?
Robert