If I have a device rated at 0.7w (lets say for the purposed of this it's using the 0.7w all the time) and a battery that delivers 200mah at 9v, how long will that battery last?

So the device needs 0.077A (given that A=W/v)

And the battery provides 0.2amp hours.

You can run the battery for about 2.6 hours... Although in reality the time would be less as it is not a linear scale

[Edited on 2/5/13 by tegwin]

Current drawn for 0.7W at 9v = 0.7/9 = 78mA

200/78 = 2.5 hours.

This is assuming the device is powered directly from the 9v battery, no regulators etc.

Actually that sounds about right.

Thankyou both