Kill A Watt is a power monitor which you can use to determine the actual power your appliances and other devices are using, since the usage rarely matches the nominal value given on the label. This is helpful when determining your power needs to correctly size your electrical system. The Kill-A-Watt is manufactured by P3 International.
You may be pleased to find that some appliances run at much lower power than their labeling would suggest. The label likely has to list the highest-possible wattage draw under any conditions. It is rare to find a device that pulls more than the label states.
Kill A Watts may be available for loan from your public library[1], and sometimes used at thrift stores.
The buttons on the traditional Kill-a-Watt have two functions each. You press the button repeatedly to see the information:
- Volt - Not directly useful for load testing, Later when you have an inverter and plug the meter into the outlet you will be able to see your inverter's output voltage. It is generally quite close to your grid power voltage.
- Amp - Amps at 110vac. Not particularly useful for load testing.
- Watt / VA: Watt - this reading is the most important thing for load assessment. Add 10%-20%((depending on your inverter's efficiency)) to tell how much power the appliance will actually consume. Volt-Amperes is apparent power, normally not directly useful. If you have a device that is unexpectedly tripping an inverter (200w load tripping 300w inverter when powering on) you might watch VA to see if apparent power is >300w; this can happen with some loads. For this reason some inverters are rated in VA rather than W. See Power Factor below.
- Hz / PF: Hertz - as with Volts, you can check your inverters AC frequency with this. In the US it is 60Hz. Power Factor - this reading can be indirectly useful. It is a relationship between Watts and VA (see above). One rule of thumb is that devices with a PF of >=.90 are good candidates for MSW inverters[2]. A resistance heater may have a perfect 1.0 power factor and works great on MSW. A laptop charger or other wall wart might have a PF of .040 - .60, which suggests it might overheat on MSW. Also see this video about power factor.
- kWh / Hours: kWh are 1,000w-hours. This is the actual amount of power used over time. Hours are clock hours. Dividing kWh by Hours will give you your hourly consumption. Multiply by 24hrs for daily consumption. Run over multiple days to get better averages.