The calculation for watts to amps is : Watts / Voltage = amps.

 240 Volt example
 A 400 watt ballast plugged into ordinary house current (240v) would draw 1.67 amps.

120 Volt example
A 400 watt ballast plugged into ordinary house current (120v) would draw 3.33 amps.

Once you have worked out the amps a few more calculations that can be handy are :
Watts = Voltage x Amps
Voltage = Watts / Amps

SAFETY WARNING : It is a smart Idea to only  use 80% of your the circuit breakers capacity this gives you a little leaway for error !

Common Conversions British Commonwealth and other 240 volt countries

Lights :
1000W / 240V = 4.1A
600W / 240V = 2.5A
400W / 240V = 1.7A
250W / 240V = 1.1A

Circuits :
10A * 240V = 2400W and 80% safe usage is 1920W
15A * 240V = 3600W and 80% safe usage is 2880W
25A * 240V = 6000W and 80% safe usage is 4800W
30A * 240V = 7200W and 80% safe usage is 5760W

Common Conversions North American and other 110 volt countries

Lights :
1000W / 110V = 9.1A
600W / 110V = 5.4A
400W / 110V = 3.6A
250W / 110V = 2.3A

Circuits :
10A * 110V = 1100W and 80% safe usage is 880W
15A * 110V = 1650W and 80% safe usage is 1320W
25A * 110V = 2750W and 80% safe usage is 2200W
30A * 110V = 3300W and 80% safe usage is 2640W