Originally Posted by westport_wayne
<snip>do those inverters drain the batteries qwick ? ive never had one. thanks
I'll stay with the "mini" theme here and keep it simple (as I can).
For the purposes of supporting an inverter you need to learn the amp-hour capacity of your batteries (this is like gallons of fuel in the tank). It's typically rated over a 20-hour period; that is, the battery can supply "X" amount of amperage for 20 hours. Sticking with dual purpose or deep-cycle batteries here a Group 24 (the size of a typical car battery) is often around 65 amp-hours, a Group 27 in the 80's, a Group 31 around 105, and a Group 4D about 245 amp-hours. There is a fair amount of variance...these numbers are just for illustration.
Take 35% of the rated capacity of your battery and that's what you have available for use day in and day out. There's a very long explanation that goes along with this having to do with charging and such but I don't want to get that detailed here. In short, the 35% number comes about by the limitations of charging and the fact that for most deep-cycle batteries you don't want to discharge below the 50% level (if you do the battery suffers physical damage and a shorter life).
The point of all this is to give you a quick way to assess the time you can run an inverter. To do this take the amp-hour rating of the battery (or battery bank (multiple batteries together)) and take 35% of the rating as available power. Take the wattage of your inverter and divide by 12 and that's the amperage that the inverter is taking from the battery at full rated watts. An expample...we have a 105 amp-hour Group 31 battery; that gives us about 37 amp-hours. We'll use a 175 watt inverter; at full load that pulls about 15 amps. Divide the battery amp-hours by amps and that gives us just about 2.5 hours of running time.
The above is quick and dirty but it doesn't give you really accurate info; it's a bit optimistic really. Is has to do with the fact that the amp-hour rating of batteries is done over a 20 hour period (typically). That means our example battery (105 amp-hours) can sustain a 5.25 amp load for 20 hours. It can NOT sustain a 20 amp load for 5.25 hours (it would be lucky to hit 4 hours). This is a result of Peukert's Law and has to do with percentages of load; it's an acceleration thing. Basically the higher the percentage of load on a battery the less time it lasts in an accelerating manner (sort of like you get worse gas mileage at 80 then at 55). The only reason this matters to us is that if you choose an inverter that will draw down the battery in less than 20 hours (or whatever time was used for your battery) it won't last as long as the simple "divide amp-hours by amps" would suggest.
If you have fairly high loads (and 15 amps qualifies) you won't have 20 hours worth of battery available (in this case 300 amp-hours) unless you're making a concerted effort to put together a really nice 12-volt house system. That's not always practical for space and budget considerations; especially in a weekender or camping style conversion. The really quick rule-of-thumb is that the inverter rated in watts should be supported by 20% of that rating in amp-hours of batteries. That means a 175-watt inverter should be connected to no less then 35 amp-hours of battery (175 watts times 20%); that's a pretty easy number to hit and typically results in a system able to sustain 1 hour of full power operation.
There is no one answer to this because every one of us operates in a different manner and has a different tolerance level for the "niceties". The easiest power to provide is the power that doesn't get used. It's always easier to conserve power then it is to generate it. My personal goal in my conversion is to install the things that add to quality-of-life and forgo the "toys" in order to avoid a large and expensive electrical system. The definition of which is which will vary greatly from person to person.
As an example, my system has a Xantrex MS2000 inverter/charger and is supported by a 490 amp-hour battery bank. I'd have chosen a smaller inverter if there had been one in the MS series at the time. The high rating is only there to support the microwave or espresso machine when in use (i'd have settled for 1000 to 1200 watts for that; both are not used at the same time) and to get the 100-amp charger this unit features. The appoximate system cost is about $2500 (more then the purchase price of my bus!) for the inverter, batteries, and wiring (but not including switches, monitors, etc) which is why I wouldn't suggest it for casual/camping/weekend use (unless you've got money crying to be spent!). My bus will be a full-timer and one of the primary goals is to be totally independent with no requirement for campground visits so I have a really robust electrical system. At the same time it had to be kept small enough that it could recharge from the engine's alternator in a resonable amount of time (while on the road) and also recharge from a Honda EU2000i. Battery charging will eventually be supplemented by solar panels.
Inverters do have low voltage shut-off but it's typically at 10.5 volts and that's considered a "dead" battery. At that point the battery has already sustained damage from over-discharge. A fully charged battery is 12.8 volts and a 50% discharged battery is 12.1 volts; you really need a digital volt meter to see that bit of difference. For longest life your house (deep-cycle) batteries should not be taken below the 50% level.
I sometimes hesitate to write all this for fear someone will think they "have to have" exaclty the right system or things won't work. That isn't my intent. Lots of folks use things that work for them that don't always fall within the parameters of what I've presented here. My goal is just to help folks learn about electrical systems in general and to learn what typical parameters are to enable them to make personal choices about what may or may not work in each specific instance.