Wilson, W4BOH

I’m really no less busy, but have used quarantine as an excuse to do a few fun things. Do you have a Kill-A-Watt, the great little $25 power meter. I do, but it’s been used almost entirely for measuring current drawn for various devices. But wait, there’s more! It does the obvious frequency, voltage, current measurements, but also POWER FACTOR (cosine of the phase angle between voltage and current). AND CUMULATIVE CONSUMPTION since last started.

[c] [y] photo by W4BOH, OCRA Inc

Here’s the tie to Covid: When I really think ahead, I can imagine power blackouts, short or long, leading to thoughts of generators to run things around the house. Of course there’s been lots of generator talk concerning Field Day too. So what’s my main concern about power? The weather is so nice that HVAC is not of interest. It’s too much load for to support for very long anyway. Next comes BEER! I like warm beer, but the same fridge also keeps our hamburger, bacon, etc., so it’s very important. Well, we just got a new fridge, a Frigidaire.  

BTW, are you old enough to remember when lots of people called any fridge a Frigidaire? That’s the reward for being early to a big market. I don’t remember not having a fridge, but I vividly remember neighbors who didn’t and, most interesting, the old man who drove an old wagon, pulled by an old horse, through our neighborhood. Houses using ice had a little sign they hung by their front doors to tell the iceman how much to leave. He would carry the ice in and put it in your icebox, if you wanted. He had bags of coal too. We kids would go along and chat with the old guy and eat ice chips from the bed of the wagon. I don’t think there was a sanitation grade on the wagon.

The previous fridge was a Whirlpool, a mechanical mess and a power hog. Maybe they are better now, but I wouldn’t bother with them. Getting back on track now…. Would it be practical to run the new fridge from a generator and/or batteries? YES, since it runs at an incredible low current of ONE AMP, 120Watts! Now, that’s way low power for any generator (although the inverter models idle down pretty well) and the efficiency of gasoliine use would be poor. So what to do?

There are lots of inverters available these days and even real sine wave models are reasonably priced. I don’t know, but a 500W model would likely handle the 3A or so starting current of the Frigidaire. SO what does the Kill-A-Watt tell us, besides the one Amp running current? I ran a 100 hour test period, four days, during recent temperate weather, with the kitchen running around 70 degrees F most of the time. Over the 100 hours we used 5KWh, so the average load is 50 Watts and the running time was 40 hours. That’s about 1.2KWh/day, costing about $55/year around here. And how does that relate to batteries? It’s almost exactly the rated capacity of the big AGM batteries we got from Adriano last year, 120Ahr at 12VDC.

But you don’t want to run your batteries all the way down, so we can think of two batteries we recharge once a day.. The problem then is to choose a reasonable charge rate and charger. A conservative charge rate of C/6 would be 20A per battery or 40A needed from the charger. That’s only about 500W, still low, but better. The Honda 2200W generator claims to use 0.17 gal/hr at 1100 W. That’s 21,250 BTU from gasoline to make 1.1KWh or 3750BTU of electricity. An efficiency of 18%, gas in to electricity out, NOT SO GOOD. Let’s call it 15% at lower load. Making electric heat from gasoline is NOT a good thing! Assuming we can get 15% at 500W (wildly optimistic), we need 11350 BTU of gasoline to make our 1700 BTU (0.5KWh) of electricity. That’s about 0.1 gallon! It’s also about a beer can full.

That seems too good to be true, to me, but it’s what the numbers say, if Honda isn’t lying. It’s 20 cents, if gas is $2/gal and you’re getting about 6 cents worth of power, if you got it from the grid. That shows that you can’t beat the power company, unless you get some PV panels! And don’t forget, you’ll have to run like that for 5 hours to get your batteries back up, costing you about a buck. What ho, that’s $365 for a year, versus the $55 running on the grid, but your beer is cool and your steak doesn’t spoil!

And we neglected the $1000 for the generator and the $200 for the inverter, but we won’t charge for them, because we need them for Field Day!

Or you could run off the 12V battery in your Prius, but that recharge would be much less efficient.

Now, I did this quickly, for fun, so feel free to point out my errors. There’s some rounding and some assumptions, so we’re looking for 25% accuracy, at best. If I’m in that range, I’m happy, but I still worry about the Honda claim.

73,

WL