Kill-A-WattPlug this gadget into a wall outlet and then plug your appliance or other gadget into it. Measures how much energy (kWh) your appliance has used since the kWh reading was reset. Will also measure voltage, current, frequency, etc. This is the simplest model (costs about $20) , but they have others that will calculate your electric costs if you enter your electric rate (cents/kWh for example).
I'm trying to see how much "Vampire" electric load my house has with TVs, VCR/DVRs, computers, modems, programmable thermostat, etc., all drawing a little bit of current 24 hrs a day.
I was telling my kids the other day - when I was growing up in the 50s, our house had a gas stove, refrigerator, and water heater. We didn't have a TV or tablets or cell phones to charge, and there were no digital displays glowing all night. We had wind-up clocks, except for a wall clock in our kitchen. On a cool summer evening, when the house was shut down for the night, that kitchen clock might have been the only thing in the house drawing any electricity. Big difference today!