All you need to do is download the spreadsheet and enter a few bits of info.
How it works
To calculate your cost per month you will need a copy of your last few power bills, and
this spread sheet. If you just want to estimate and know how your power billing works just check out the pic.
First you need to find out how much your power company charges you and how much you allready use. Take your power bills out to do this.
There are three things you will need Total Usage, a Baseline power allotment (this is important as you go over this Base the price per kWh will increase), and the Pricing.
Next find your setup, or enter your own on the Lights sheet.
Lastly enter your info. Start with info from your bills.
Previous Usage cells, enter your Total Usage from your last 3 bills.
Baseline, find and fill in you baseline power allotment.
Pricing, Dont worry if all the entries aren't on your bill, just fill them ALL in.
Your Lights,enter the number you looked up on the Lights sheet.
On to Version 2. Let me start by adding this post
Quote:
Originally Posted by babygro
Ohms law is relatively straight forward and can be used to calculate an accurate figure of wattage useage from any light system, and this would probably have been a better way to go. To calculate total wattage used from a ballast HID system you need to use the amperage figure not the bulb wattage figure. To show you how your figures are inacurate, OHMS Law is Volts x Amperes = Watts, so if we know two of those figures we can calculate the third.
The real wattage of a 400w HID system is Volts x Amperes or 120 x 3.8 = 456watts for an average 400w system others would be 
250w 120 x 2.5 = 300w
400w 120 x 3.8 = 456w
600w 120 x 5.7 = 684w
1000w 120 x 9.5 = 1140

By using the new Ohm's Law section you will be able to get even more exact with your estimate. I recomend useing this section to get your actual watt usage. As pointed out by babygro and A Good Keen Man using only bulb comsumption and not power loss in the ballast will throw your results by up to 15%. By useing Ohms Law i think it will be more around 5% Give or Take so I added a range of costs derived from 5% to +5% less or more KWh usage.
98% of electronic equipment will have its Amps posted on it somewhere. As for your voltage just about everyone should know their local voltage but it is usualy posted to.
References:
GCSE Physics: kilowatthour <How to do the math yourself.
Electric Bill Detail  PG&E Bill <Sample Bill.
download: Download Central <Free Open Office
Well I hope this is informative and helps you find your right setup for your homes Power Base. To unlock document toke.