Well I had time today to crunch a few numbers, I like numbers.
This is all based on a 10g batch with a 13 gallon pre-boil volume. Starting water temp is 65F. Grain bill is 30 pounds and the assumed kettle loss to trub is ONE gallon.
17 gallons to start at 65F needs to be heated to 170F (some for strike, some for the "sparge".
142 pounds of water raised 110F is 15,620 BTUs of energy
During the mash, the RIMS heater will run on and off, but how much is hard to determine. So for now I am ignoring the "maintain temp" power usage.
After the mash and "sparge" I will have to boil 13 gallons (3 gallons lost to absorption).
109 pounds of water raised from 160F to 210F requires 5,450 BTUs of energy
Now that it is boiling, I want to boil off about 1.5 gallons, or 13 pounds of water.
13 pounds of water requires an additional 12,610 BTUs of energy to boil off
Through use of my previous electric kettle I observed approximately 95% eff. when heating with electric (this is because of heat loss through kettle walls for example)
So ignoring the heat loss during the mash and the subsequent RIMS heater firing during the 60 minute mash (I cannot quantify this very well) it will take about 33,680 BTUs of energy to complete the entire brewing session from a COLD start, to finish.
Each kWh for me runs about $.12-$.14.
Each kW is equal to 3412 BTUs
This means that I will need to consume 9.9 kW to complete the brewing session, sorta... Remember this is assuming that electric is 100% eff. and it is not due to radiant heat losses. So basically I need to increase this by 5% to get my actual electric requirement. I need to consume 10.4 kW, at a cost of approximately $1.25 to $1.46 for a complete brew session.
This is asked a lot... how long will it take to heat though?
11 gallons of strike water from 65F to 170F will take 20 minutes
13 gallons of boil volume from 160F to 210F (boiling) will take 12 minutes
If the system is able to raise the temp. of my garage 20F (which it cannot) due to heat loss:
3600cu feet of air
Multiplied by 20F temp. delta
Multiplied by .018BTUs per cu. foot to raise 1F
This would indicate a heat loss of 1,296 BTUs during the 60 minute mash, which is only about .4kW or $.06