Energy efficiency and cost to operate of appliances

humidifier with data logger

In my last post I talked about a new humidifier I purchased after careful deliberation.  You see, when I add an electrical appliance to my home I give very serious thought to how much energy it uses and what that will cost.  By cost I mean actual dollars per month, but also the cost to the planet when considering carbon footprint of the emissions needed to power it.  For most of the year my solar power system covers all my electrical needs, but due to the lack of sun hours in the winter, I do use utility power.  I write this blog to raise awareness for those who do not yet have solar or other renewably sourced electricity.

When I got the humidifier, I took a basic set of energy readings and found that it uses 16 to 24 Watts depending on the setting.  I generally use the middle setting that uses about 22 Watts.  As such, this is an extremely efficient humidifier.  Since I only use it for 8 hours each night in my bedroom I assumed the cost would be quite low.  A quick calculation showed that it would use 5.2kW/month if used every night (which I don't).  I pay about 16 cents/kW here in Maine - so my monthly cost would be about 84 cents.  Not bad!  And here in New England our utility power is sourced partially from renewables (it's around 40-50% currently) so the carbon footprint is modest.

Data logger showing Watts of humidifier

But wait! There's more!  This particular humidifier has a humidistat (showing 20% in the image above), so it probably cycles on and off as needed to maintain the 40% humidity I prefer at night.  So I connected my data logger to the power cord (see image at top of the page) and logged readings every second for one night.  Here's what I saw:

At first glance it seemed like it was jumping from 20 to 22 Watts all night.  But let's zoom in on that data:

Aha! Now we see that it is cycling on and off with a duty cycle of roughly 45-50% and dropping down to a baseline of about 5W to power the electronics.  (That's actually about 2W - my logger calibration is a bit off).  So the actual cost is around half what I had calculated.  Neat!

Lets look at the cost calculations.  To get the monthly cost I started with the actual Watts and multiplied it by hours/day to get daily Watts used:
W per Day = W X Hours per day  (22 X 8 = 176)
Then multiplied that by 30 to get Watts per month
W per month = W per Day X 30  (176 X 30 = 5280)
This can be expressed as 5.28 kWh (we pay the utility company by the kWh)
Here in Maine I pay 16 cents/kWh, so I just multiply
$/month = kWh X $/kWh (5.28 X .16 = 84.48 cents/month)

But thanks to my data logger, I see that in reality I'm using about half that or less than 43 cents if I use it every night.  So even in the winter when I use utility power, the carbon footprint of this humidifier is relatively token especially when compared with all the other types (evaporative and steam) that use MUCH more power.

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Data logging refrigerator power

Since I had got my data logger out, I decided to log some other appliances.  Generally the biggest energy using appliance in a home is the refrigerator and my 20+ year old one is relatively efficient.  Shown above is my data logger connected to an adapter cable that can measure AC current in Amps that is  converted to Watts in the logger.  And yes, that cool blue night light doubles as a Volt meter - I'm really that geeky!

As you may know, your fridge does not run all the time.  Typically the compressor will be on about 25-30% of the time, and that is illustrated by my log above. The on time is about 10 minutes, an it's off about 33 minutes.  Those big spikes represent the starting energy of the compressor - they only last about a second and the utility meter can't respond that quickly, so it does not factor in to my energy costs.

So here's the math on the cost to operate my fridge
300 Watts X 24 = 7200W/day (if always on)
7200W X .25 = 1800W/Day = 1.8kWh (actually on 25% of the time)
1.8kWh X 30 = 54kWh/month (30 days)
54kWh X 16 cents/kWh = $8.64 per month (your cost per kWh will be different).

So if you use a KillAwatt meter or any other energy monitor to try and calculate your energy cost for a given appliance, you should always leave it running for at least 24 hours to get the average power usage.  Just looking at a power meter in the moment can give you a false impression of actual power consumption.

At the low end of the power spectrum is this heated cat bed that I got recently for my 16+ year old cat Maxx.  She loves it and stays in it all the time.  It is rated at 4 Watts and the surface temperature is just a few degrees above ambient.  I measured 73F with my thermal camera while she was not in it:

When I connected it to an energy monitor, I saw actual readings of 3.5 to 3.7W. Here's the heating pad inside:

Since this in on 24/7, it uses about 2.66 kWh/month at a cost to me of about 42 cents.  It's a small price to pay to keep Maxx happy in her old age.  And she really likes it a lot!