[Mandobart]

The diehard unit referenced cites "1150 peak amps for a jump start and larger capacity charging with 54 hours of 12V DC power." It definitely has an inverter if it has an AC outlet. What you really need to know is the amp-hour rating. How many amp-hours do you need? Check the input rating on your amplifier. It should be printed near the AC power connection. It may be expressed in Watts or VA. For our purposes here they can be treated the same. Lets say your amp is rated at 100 VA input. Most inverters are 90 efficient. So the DC end has to supply 100 ÷ 0.9 = 111 watts. 111 ÷ 12 VDC = 9.26 A coming from your battery. If you want to power your amp for 4 hours thats 9.26A X 4 hours = 37 A-hr. If you want to use a lead-acid deep cycle battery you don't want to run it down less than 50%. Also most inverters have a low battery supply cutoff if you run the charge down below ~11 VDC. So double that to 74 A-hr and you can be confident you'll have enough juice. Sound too big? It probably is.

The better way is to measure the actual current a battery needs to supply your amp thru your inverter. I calculated I needed no more than a 200 W inverter, so I bought it. I hooked up a 12 VDC battery and measured the DC current draw. You can't use an AC clamp-on meter for this! It cannot accurately read DC amps! So I measured ~3 ADC from the battery to power my amplifier through my inverter at normal performance volume. So 3 A X 5 hr = 15 A-hr. Doubling that so I won't run it down less than 50% means I need a 30 A-hr battery for my application. Your mileage will definitely vary, unless you're using the same amp and inverter that I am.