I was speaking with John (N1OLO) of Hamsource (www.hamsource.com) today, and he told me about this Nifty-Neato site for helping you to determine just how much battery you need.
LINK
www.4sqrp.com/Battery_Capacity/index.php

I was amazed at how close my "gross" calculations and assumptions were.

The Average Mobile 144/440 MHz Radio ¹
Rx = 1.2 amps
TX (25 watts) = 10 amps

Field Operator Duty Cycle
Tx @ 5 min/hr = 8.3 %

Shift Duration: 12 hrs. ⁽²⁾

Plug the numbers into the boxes, and this is what you get:

FOR SLA (Sealed Lead Acid) Batteries (50% usable capacity)
Average Current Consumed - 1.93 Amp/Hours
AH Capacity Needed - 23.16
AH Capacity + Reserve - 46.32 (Min. Battery Size)

YOU NEED A 50 AH SLA Battery
50 AH SLA Battery gives:
25 Amp/Hours (AH) - Usable Battery capacity
and (@ given loads)
13 Hours - Expected Battery duration

FOR LiFEPO4 BATTERIES
Will require a battery of less AHr. capacity.

- Field Station @ 25W = LiFEPO4 Battery with minimum rated capacity of 30 AHr.

- Net Control @ 25W = LiFEPO4 Battery with minimum capacity of 80 AHr.
(Better to get 1 x 100 AHr, OR... 2 x 50 AHr batteries)

Notes:
(1) For HF, you will need a 100 AH SLA Deep Cycle battery

(2) ARES TEAM members NEED to plan for 12 hour shifts.
Additionally, it might be good idea to bring a second battery of the same AH rating in case you're there for an extended period (i.e. a Double Shift).

The BASIC
Battery Capacity Choice
If using Sealed Lead Acid (SLA) Batteries
If using Sealed Lead Acid (SLA) Batteries
If using Sealed Lead Acid (SLA) Batteries

BLUE SKY
Unfortunately, most people keep thinking in terms of a short "Blue Sky" Public Service events.
In these events, the local Repeaters are operational and 5 Watts (or less) will do the job - especially with a larger antenna than the stock "rubber duck" that came with your radio, AND you are within Line-Of-Sight of the Repeater.

In this scenario....
- All you need is the original HT battery PLUS a spare HT battery.
- Or, if you are running a portable mobile rig with 25 watts, you will need an 18 AH battery.


BLACK SKY
But, in a DISASTER ("Black Sky") situation, where ARES is REALLY NEEDED, we have to plan on not having any other source of power than what we bring with us.
That gives us THREE options:
- Batteries,
- Batteries and Solar Panels (w/Charge Controllers),
- Gasoline powered Generators.

When repeaters go down and off-the-air (as was seen in Hurricane Sandy (Oct. 2012), and given the topography of Region 2, you will (in most cases) need to pump out 20-25 Watts on 2m Simplex just to make yourself heard in the Net with a dual-band (144/440 MHz) "gain" antenna on a mast up 20 feet, fed with Flexible Low-Loss 1/2" Coax (LMR 400 Class).

BAD NEWS:
The 18 AH battery is NOT going to do the job for 12 hours out in the field.
- Especially, when you have no other power source available, Repeaters are down.

This means that, in a SHTF scenario with the increased transmit time due to hourly Nets and other communication needs, there is no way the 18 AH battery is going to last for your 12 hour shift.

So, you might want keep an 18 AH battery for Public Service Events, AND/OR keep a 55 AH battery available in a separate Battery Box (attached to a 10 gauge wire with Anderson PowerPole connectors).

Here is a quick video (15 min) that really simplifies the way to properly size your battery.
Please remember that there is a huge difference between the needs of an afternoon romp in the park versus a full scale / multiple day deployment.