Saturday, February 23, 2019

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.

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!

Wednesday, February 6, 2019

Energy efficient humidifier

Recently my humidifier stopped working despite relatively frequent maintenance (OK I could have done better).  They all require frequent cleaning and rinsing out with vinegar.  I bought it in a hurry a year ago because I was having sinus infections due to the very dry winter air in Maine and this was relatively inexpensive and off-the-shelf at my local pharmacy.  But I liked being able to stick my face in the warm mist to soothe my sinuses and I could add scent to it which was pleasant.  In retrospect I should have done more research and made a smarter choice to get a more energy efficient unit.

So this time I took my time and studied up on humidifiers and how efficient they are.  I found a very helpful blog post: "How Much Energy Does My Humidifier Use?" that really clarified things.  It verified what I already knew intuitively as an engineer.  Basically there are 3 types of humidifier:

1. Warm mist units that boil water to create steam.  This is the type that I had and it used over 250 Watts.  Not very efficient!  Actually this type are the least energy efficient.  Vicks make several models that I often see in the local pharmacy.

2. Evaporative units blow air across a wet wicking material using a fan that consumes around 50 Watts.  I had one like this years ago and had issues with it.  First the fan was quite loud - even on low.  

And second I had to replace the wick material every few months as they built up mineral deposits and mold.  Also, I did not like the idea of creating waste.  The wicks cost around $7-10 each and were not available in-store when I needed them.

3. Ultrasonic humidifiers use an ultrasonic device that agitates water so it creates water vapor directly, then a small fan pushes it out into the room.  These units are very quiet and relatively affordable - starting at around $45, but you can spend a lot more.  They use a lot less energy.  At 20-30 Watts they are the most efficient type of humidifier.

So I settled on a model I like:

This model TT-AH001 is made by TaoTronics, and has good Amazon reviews.  I did some energy tests and found these readings:
Standby 2W
Low 16W
Medium 20W
High 24W
I also like the control features - they have geek appeal.  Not many humidifiers in this price range have a built-in humidistats and this one let's you set the humidity in 5% increments and displays it in 1% resolution.  It has 3 speeds, none of which make any noticeable sound, I just hear the occasional burble as air comes up into the tank like you would hear from a water cooler.  It has a timer so you can set it to run a certain number of hours.  I only use it at night, so I just turn it on as I go to bed, and set the humidity to 40 and run time for 8 hours.  It works perfectly and maintains an accurate humidity.  And to cap it all off there's a blue LED night light "feature" that I never use, but the geeky designers just had to throw that in!

I also looked specifically for a model that has a flat top for the water tank.  Very few of them do, and it is really helpful to be able to set it upside down in the sink for filling where it will be stable while filling it.  I mean seriously, how are you supposed to fill this one (shown at left)?

As a product designer, I am shocked by the poor ergonomics of most humidifier tanks that have useless "sexy" curves that simply fall over when you place the tank upside down in the sink - requiring you to hold it while filling a gallon or more.  Some of them have dark plastic water tanks that don't let you see the water level which is really dumb!  This is another reason I chose the model I'm using, I can clearly see the water level.

What is impressive about all ultrasonic units is that they start generating cool mist almost immediately.  No waiting for the water to boil and the mist is quite visible so you know it's working.

I'm filling my tank with filtered water and find that it uses 1/4 to 1/2 tank every night in my small bedroom.  So I can go 2-4 days before needing to re-fill the tank.

I'm looking forward to a reduced electric bill next month.  I only pay for electricity for about 4-5 months of the year due to my solar panels, and it really bugs me to have to pay the utility company.

See my next blog post for a detailed evaluation of the power consumption of my humidifier.