How To Measure Energy (kWh) Of A Single Device With A Power Meter (Watts over Time)

April 15, 2015, by Ken Jorgustin

kill-a-watt-kilowatt-hour-meter

If you are considering or planning an off-grid or standalone alternative energy system such as a solar PV system for your home, your cabin or whatever else – one of the first things you need to do is to measure the energy usage or power consumption of the appliances and devices that you intend to power.

Logically, you first need to know the basic power consumption requirements in order to do much of anything else in the planning process. A very convenient tool (meter) that I have been using for this purpose has helped immensely while determining my PV solar array size, battery bank, charger/inverter, etc..

Here’s how it works and how simple it is to use:

 
KILL A WATT™ meter

I am currently in the process of designing my next solar PV system – having moved to my ‘BOL’ during the past year. While I have a background in electronics, electricity, and electro-mechanical, you don’t need to be an expert to grasp some of the basic concepts of a solar PV system (although you do need some fundamental knowledge of electricity – such as ohms-law and a few other fundamentals).

My assumption in this article (and other subsequent related articles) will be that you have some basic knowledge – so as not to explain basic electronic theory at every step of the way. If you don’t posses some of the fundamental knowledge in this area, hopefully you will still find the articles to be somewhat interesting 😉

That said, the goal here is to supply alternative energy to power up things like lights, refrigerator, freezer, and any other electrically powered appliance or device that suits your needs or desires. This might also include your furnace (its fans/blowers, ignitor, pumps, etc..), your computer, a TV, your phone system, a ham radio, a microwave oven, etc..

Each and every appliance or device requires (draws) it’s own amount of power to function. Power is the rate of doing ‘work’. Some devices draw a constant and same amount of power while other devices or appliances will vary over time with the amount of power they consume (e.g. a refrigerator which cycles its compressor pump on and off throughout the day, and/or its defroster).

In the United States, when you ‘plug something into the wall’, the appliance or device is ‘fed’ 120 volts of electricity. As the appliance or device draws power from that 120 volt line, it draws ‘current’ (picture ‘current’ as the water flowing through a garden hose) and it is measured in ‘Amps’. To represent the power that the appliance or device uses, we simply multiply ‘Volts’ times ‘Amps’. P=IE. Power is measured in ‘Watts‘.

If a light bulb draws 0.125 Amps, then you can calculate the power to be 0.125 (Amps) x 120 (Volts) = 15 Watts. Incidentally, the brightness of a typical 15 watt CFL light bulb (the energy-efficient bulbs) is approximately equivalent to the old 60 watt tungsten light bulb. While integrating an alternative energy system into your life, these efficiencies become very important!

 
Okay, let’s make it easier. Rather than having to measure the current of every appliance and device, and then having to multiply it out to obtain the Power (‘Watts’) of each device, there is a handy-dandy meter which will do it all for you automatically, and will simply display the power (watts) on its display readout. It’s called the ‘KILL A WATT™’ meter.

And in fact, it does even more than that. It will display the voltage (volts), the current (amps), the power (watts), and more importantly the ‘watts’ used over time (kWh) or kilowatt hours!

Knowing the kilowatt hours of an appliance is the accurate way to know the ‘real’ power consumption over time. While some devices will remain the same (e.g. a light bulb will always use the same power over time), there are some devices which vary – and the only real way to know how much power the device draws is to measure it over time in order to get a longer term ‘average’. Very important for some things…

For example, I wanted to discover the real power consumption of my refrigerator. I knew that if I plugged the refrigerator into the ‘KILL A WATT™’ meter and measured the instantaneous power consumption – I knew that it would not be accurate over time. I know that a refrigerator will cycle on and off throughout the day while maintaining temperature and performing other electrical functions. So the only accurate way to measure it is to leave it plugged in to the meter for say, 24 hours. The meter will then ‘accumulate’ the watt hours used and will display the running total!

In my particular case, I left the fridge plugged into the meter for 24 hours and the meter indicated that it had consumed 1.36 kWh of electricity. That’s 1,360 watts over 24 hours. By simply dividing 1360 by 24, I knew that my refrigerator on average draws 57 watts of power.

I did the same for my chest freezer. After 24 hours it had consumed (only) 0.264 kWh (or 264 watt hours) which comes out to be just 11 watts per hour on average!

You get the idea – you simply plug in various devices and measure them over time to get an accurate averaged measurement – which can then be used in one’s calculations of ‘real’ power requirements for the devices you wish to power via your alternative energy system.

I even measured the power consumption of my furnace… During the peak of this past winter when my propane furnace was operating the most (it also is used for my hot water), I wanted to know what the ‘worst case’ scenario will be with regards to the power consumption required – in order to calculate whether or not my existing battery bank (with associated inverter/charger) could supply enough energy (kWh) to keep it running all night long if needed. Here’s how I did it:

I constructed a set of ‘pig tails’ from having cut a short heavy duty (#12) extension cord in half (don’t try this at home unless you are qualified!) and went into my circuit-breaker panel to temporarily re-route my furnace ‘hot’ and ‘neutral’ line through the ‘KILL A WATT™’ meter. I let it run this way for several days to get a good average. It turned out that under the worst of conditions (it was about 20-below-zero each night) the furnace was drawing on average, 58 watts. Not bad! Certainly no problem for my battery bank 😉

 
In any event, I thought I would share with you the methodology I’ve used for calculating power consumption here at home while building a spreadsheet to use while deciding upon efficiencies, system integration, etc. While my current electric utility bill will provide an overall energy consumption for a given month, it helps immensely to understand the individual energy demands of each device. The solar PV system that I’m currently designing will not replace my existing 200 Amp electrical service, but instead will be built as an off-grid standalone source of energy which I will selectively integrate into the home to power up essential services.

Here’s the meter:
KILL A WATT™ meter