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Generally electrical appliances such as an electric bulb, geyser, heater, etc. are rated with power and voltage. A bulb rated as 100W - 250V, means that when a bulb is lit on a 250V supply, it consumes 100 W of electric power. This means that, 100 J of electrical energy gets converted into heat and light energy in 1 second.
From this rating we can calculate the resistance of the filament of the appliance while it is glowing and also the current which flows through it.
Since P= 
R= and I= 
For the bulb rated as 100 W - 250 V, the resistance of its filament
R= = = 625 
The resistance of the filament of a bulb is much less than this value when it is not lit because the resistance of a filament increases with the increase in temperature.
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