To make an analog design using log amps or multipliers, four-quad is good for learning. Using a uP for math is better. Look at these links below. Build a Power Meter ! The Metrics for Going Green is with a Energy/Power/Watt meter. - delabs notes
The EnerJar is an easy-to-build, DIY device that accurately measures the power draw of electrical appliances. Sitting between any wall outlet and the appliance (think blowdryer versus laptop), the EnerJar helps users gain an understanding of power draw, investing them in the knowlege, and leading them to reduce their electricity use.