Stuff below copied from a boaty forum, worth a read through as gives a better idea what you can get from a leisure battery.
"You also need to consider depth of discharge and rate of discharge. Not having enough batteries in the bank will cause both to significantly increase and shorten battery life.
Most leisure batteries are sold quoting ah, but what does that really mean? If you take the example of a typical 130ah leisure battery sold on eBay it has three ratings, C5 80ah, C20 100ah and C100 130ah. These are the maximum ah it will supply at differing discharge rates.
The C100 which is often the quoted headline capacity means it will supply 130 amps over 100 hours, or 1.3 amp per hour. The best way to get the maximum from a battery is a slow discharge. The C20 means it will give a maximum 100 amps over 20 hours, so a faster discharge rate has reduced the overall amount of current you can draw. Once you get to the C5 rate it will only give you 80 amps drawn over 5 hours. I would suggest that for your average invertor usage you are exceeding the C5 rate and therefore will get even less ah.
Then the real killer is that you should never discharge your battery below 50% depth of discharge. So, you can halve all of the above figures if you don't want to kill your battery early.
Finally, you need to factor in that unless you have an advanced alternator regulator or a battery to battery charger you are probably only getting the batteries 100% when on shore power and battery charger. Once away from shore power and just relying on the standard alternator for charging you are probably only charging to about 90% capacity. That means that for a heavy load you have about 40% or less of your C5 rate to play with. For the 130ah leisure battery above that means about 32 amps if you don't want to damage or shorten the life of your battery. So, what does 32amps get you? Well at 12V about 400 watts for an hour. Run a 1.25kw element in a calorifier and you'll get about 18 mins run time on one battery without killing it. The invertor low voltage alarm will let you discharge a lot further than that, but you will be shortening the life of your battery. Add a second battery and you will half the discharge rate and double the runtime.
You need to start by looking at your anticipated load, work out what you are trying to run and how many watts it will take, then work that back into amp hours and then work out how many batteries you need. It will be a lot more than you were hoping for in most cases.
"
"You also need to consider depth of discharge and rate of discharge. Not having enough batteries in the bank will cause both to significantly increase and shorten battery life.
Most leisure batteries are sold quoting ah, but what does that really mean? If you take the example of a typical 130ah leisure battery sold on eBay it has three ratings, C5 80ah, C20 100ah and C100 130ah. These are the maximum ah it will supply at differing discharge rates.
The C100 which is often the quoted headline capacity means it will supply 130 amps over 100 hours, or 1.3 amp per hour. The best way to get the maximum from a battery is a slow discharge. The C20 means it will give a maximum 100 amps over 20 hours, so a faster discharge rate has reduced the overall amount of current you can draw. Once you get to the C5 rate it will only give you 80 amps drawn over 5 hours. I would suggest that for your average invertor usage you are exceeding the C5 rate and therefore will get even less ah.
Then the real killer is that you should never discharge your battery below 50% depth of discharge. So, you can halve all of the above figures if you don't want to kill your battery early.
Finally, you need to factor in that unless you have an advanced alternator regulator or a battery to battery charger you are probably only getting the batteries 100% when on shore power and battery charger. Once away from shore power and just relying on the standard alternator for charging you are probably only charging to about 90% capacity. That means that for a heavy load you have about 40% or less of your C5 rate to play with. For the 130ah leisure battery above that means about 32 amps if you don't want to damage or shorten the life of your battery. So, what does 32amps get you? Well at 12V about 400 watts for an hour. Run a 1.25kw element in a calorifier and you'll get about 18 mins run time on one battery without killing it. The invertor low voltage alarm will let you discharge a lot further than that, but you will be shortening the life of your battery. Add a second battery and you will half the discharge rate and double the runtime.
You need to start by looking at your anticipated load, work out what you are trying to run and how many watts it will take, then work that back into amp hours and then work out how many batteries you need. It will be a lot more than you were hoping for in most cases.
"
Current toys: '99 XT600E, 2000 4TX, '82 Princess 30DS (where the username comes from), No longer a '03 Fazer thou.
Save the planet, it's the only one with beer!
Save the planet, it's the only one with beer!