How do I convert watts to amps and amps to watts?

The formula for converting Watts to Amps is : Watts = Amps * Volts

The formula for converting Amps to Watts is : Amps = Watts / Volts

SAFTEY WARNING : Only use 80% of the circuit breaker capacity!

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

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

Remember that a circuit services more than one power outlet.
A two outlet wall plug will use only one circuit.
There will be more outlets on 240V systems per circuit than there will be in 110V systems.