Hi, our invisble dog fence is rated "12V, 850mA" How do I figure out how much electricity this fence is using/unit time when it's on? Thanks much.

P = IV

= 12*0.85 W

it uses 12*0.85 Watt-hours per hour.

I think Capuchin lost track a bit, and gave a wrong answer by mistake.

Power= Volts times amps
P= 12 volts x .85 amps
P=10.2 watts

To calculate cost per month:
KiloWattHour=10.2 watts times 1 hour =.0102 KWH per hour of operation
.0102 KWH times 720 hours (30 day)=7.344 Total KWH per month
7.344 KWH at .10 cents per KWH= .74 cents per month cost to operate.

Insert your KWH cost in the place of ten cents to arrive at your true cost, we pay dearly for electricity here in the northeast.

I have no idea how electricity is measured, that's my problem ;)
But I think I was correct apart from that.