Calculating total power draw of device (camera) and accessories
Generally, you must determine the current used at your lowest battery voltage - 24 volts in this case. The data sheet states that the Alexa "in base configuration" draws 85 watts. 85/24 is 3.6 amps. You must find the current draw for every accessory you plan to attach, calculate its current, and then add them all together to find a peak total current, call it X amps. Power (watts) is equal to voltage (volts) times current (amps).

Now find the capacity of your battery in amp-hours. Divide this by X and you will have the approximate time, in hours, that your battery will power the camera before it runs out of juice.

Be aware of a few things. Most batteries will perform better at higher temperatures, so if you're planning to operate in a cold environment you will need to derate the battery. Most battery capacities apply to a new battery, and most batteries will gradually lose capacity as they get charged and discharged. Take this into account. Battery capacity is generally specified at a specific current. If you are drawing more current than the specification, the battery will provide less capacity than you expect.