This is something that definately falls into the category of your mileage may vary. There are many variables. It will depend on what mode you are using. CW will use the least power overall, then SSB and then AM or FM. You can first measure receive current, and then add an estimated duty cycle of how much transmitting you anticipate. But your best estimate will be actual usage, and that will vary depending on transmit duty cycle. I would assume receive current probably will not exceed one amp. Even at full power SSB is not going to come anywhere close to 22 amps and CW has a short duty cycle. AM and FM probably are not going to be 22 amps either. In AM or FM my FT857D draws about 17 amps at 100 watts. Place an ammeter inline with the power supply and measure actual current transmitting. If your battery is a gel cell or any form of lead acid battery, except some deep cycle batteries, expect to get about 50% or a bit more than full rated current capacity also depending on the amount of time transmitting. If your battery is LiFePO4 then close to full rated capacity.
