Voltage conversion part 2

I have purchased a (US) 110Volt battery charger to charge the power tool battery in my country 220V (UK). The battery is 18V (2.4Amps).
I would like to purchase a step-up converter but do not know the minimum power requirements I need.
Charger details:
Input: 120 V AC ~60Hz 2.0 AMPS (NOM 2.0A) __ Output: 2.8 AMPS DC (NOM 2.8A ---)
My question is: What are the minimum Watts that I require for my Voltage Converter? 45, 100, 200, 300, etc...
I hope that my explanation is a bit clearer this time 'round.
Thanks again
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you will want a step DOWN transformer (220 > 110)

many rechargeable battery packs are of the slow charge (12 to 18 hours type) the charging current will depend on the amp-hour rating. a quick survey of some popular brands tells me the batteries are 2.4 to 2.6 A-H. for a 12 hour charge with a 2.6 A-H battery you would use about .2 amps. now .2A @ 18V = 3.6W now the transformer usually warms up a bit so there is some loss involved but its hard to guess how much. also if its a one hour fast charger the power goes up (by a factor of 12).
a correctly sized the transformer will be about the same size and weight as the transformer in the charger (or a little bigger for a safety factor)
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