Heating capacity by wattage over time

Does anybody know where I can find an appropriate calculator to help me determine load capabilities of a heating element? I am trying to determine if it is possible to build a system that uses a RIMS tube to heat the strike and sparge water within a reasonable time frame.

I am thinking something my wife could start when she gets home from work and two or three hours later would have a specific volume up to temp. I know the math for btu's into water but I need to calculate watts into BTUs.

Try this calculator http://gnipsel.com/beer/software/calculators/electric-heat.xls

That's a great spreadsheet. Full of good info.

a good rule of tumb for doing things in your head is 1000w = 3400BTUs

and for those that dont know the BTU calculation its:
(weight of water, in pounds) x (end temp - start temp, in degrees F) = BTUs required
or
(#gallons) x (8.3) x (temperature change, in degrees F) = BTUs required

1 kW = 3412 BTU per hour
or
1 kW hour = 3412 BTU

watts are a measure of power, BTU is a measure of energy.