Questions on Sizing Project

kh56

New Member
So I'm trying to properly size both batteries and an inverter. I plan on starting off powering a small upright freezer....and depending on my success, expanding that to several more appliances including a side-by-side refrigerator, furnace, WiFi router, computer...etc.

For each of these appliances I'm trying to gather the following information:
  • Running watts (for sizing the inverter)
  • Surge watts (for sizing the inverter)
  • Daily watt hours (for sizing the batteries)
I received a lot of great advice on my last related thread. My understanding is that I need at least two devices to measure these values.

I'd like to ask a couple more questions and see if you can help clarify my understanding a little further:
  1. A clamp meter with inrush should be used to measure surge watts. The meter will measure that value in amps which can be multiplied by the volts (measured by the same clamp meter). For surge watts, a line splitter should be used. I can either pull out and separate the wires in an extension cord or else purchase something like this. What is a good brand of clamp meter that measures inrush?
  2. For running watts, I've seen something like the Kill-A-Watt recommended several times. Is this something that I can plug in and determine the running watts after just a few seconds?
  3. For daily watt hours, just multiply the running watts by the number of hours the appliance runs daily. For the refrigerator, freezer, and router, those would be 24 hours per day. For the other appliances, I would need to make an estimate.
  4. I've heard duty cycle mentioned in another thread. How do you all determine that? Is it better to let the Kill-A-Watt run for a longer period of time like a week or so and then just get the average watts?
  5. In the next couple of years, I think that I want to try making a small portable amateur radio solar setup. If possible, I would be interested in a clamp meter that does DC too (for your recommendation on #1 above). Otherwise I'll worry about that down the road.
Thanks all for your help!
 

FilterGuy

What, me worry?
For running watts, I've seen something like the Kill-A-Watt recommended several times. Is this something that I can plug in and determine the running watts after just a few seconds?
I am not sure what you mean by running watts.

The Kill-a-watt will measure both Watts and Watt-hours. It will measure the instantaneous watts, but you have to leave it hooked up over a period of time for Watt-hours.

For something like a refrigerator that cycles on and off, using the Kill-a-watt to measure it's watt-hour consumption is a great way to go. Leave it plugged in for a couple of days and then divide the watt-hours by the number of hours it was measuring and that gives you the Watt-hours/hour. Multiply the Whr/Hr by 24 and that gives you a pretty good number for the energy consumption per day.

For daily watt hours, just multiply the running watts by the number of hours the appliance runs daily. For the refrigerator, freezer, and router, those would be 24 hours per day. For the other appliances, I would need to make an estimate.
No the refrigerator and freezer cycle on and off. That is why you need to measure the watt-hours over a long period of time to get a good value.
For appliances you turn on and off, you can go with an estimate of the number of hours run per day and multiply that by the measured wattage.

I've heard duty cycle mentioned in another thread. How do you all determine that? Is it better to let the Kill-A-Watt run for a longer period of time like a week or so and then just get the average watts?
Duty cycle refers to how long the device is on vs how long it is off. Like I said before things like your refrigerator and freezer will automatically cycle on and off and you should use a kill-a-watt to measure the Watt-hours over time and then calculate the average daily.

For things like your blender or lights that you turn on-or-off, you can measure the watt-hours over time with a kill-a-watt and then calculate the average, but I would just do an estimate of how long you run it.

A lot of things like the blender, toaster, microwave, etc. Are high wattage, but typically are not run for a very long time. Consequently, they don't end up using a lot of Watt-hours and therefore are not typically a driving factor in battery size, but they can impact your inverter size.

BTW: I use this spreadsheet for sizing systems:
 

Hedges

I See Electromagnetic Fields!
When there is a nameplate rating for watts/amps of a motor, multiply that by 5 to get a good estimate of surge.
In the case of a refrigerator, nameplate includes defroster and maybe icemaker, so this doesn't give a good surge figure. But it does give wattage that inverter needs to sustain during those cycles.

Consumer energy labels should give a decent estimate of daily consumption, if your conditions are typical.
Fridge cycles on and off, so use that estimate or else Kill-a-Watt over a period of time.
If you have a utility bill, that also lets you determine average.

Batteries are for nighttime consumption, which can be a fraction of daytime. Except if you want to last a few days without sun.
Lithium can handle deep cycling, so sizing for one night and starting a generator on cloudy days is one way to go. Save on up-front cost.
Lead-acid can last a decade or so with shallow cycling, so sizing for 3 days gives it an easy life most of the time, and capacity for days without sun.
 
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kh56

New Member
Filter Guy,

Thank you for your detailed reply. Running watts is what I meant by (what your spreadsheet describes as) "watts." I believe this is what the appliance is pulling at any one time.
 

FilterGuy

What, me worry?
Filter Guy,

Thank you for your detailed reply. Running watts is what I meant by (what your spreadsheet describes as) "watts." I believe this is what the appliance is pulling at any one time.
OK. I suspected that but was not sure. Did we answer your questions?
 

Hedges

I See Electromagnetic Fields!
So far I'm happy with the $100, 1000A AC/DC clamp meter from Harbor Freight.
I haven't used the Inrush feature, but loaned it to a coworker who did.
 

FilterGuy

What, me worry?
Yes, everything except for a recommendation on a good inrush clamp meter. Thanks!

If you want top-of-the line, go with fluke.... but they are crazy expensive.

For a long time I had a cheapo AC/DC clamp on that did not have inrush capture. It worked great (and still works great). I wanted a unit with inrush so when I spotted a fluke at a garage sale for a steel I nabbed it.....It was nice but frankly, the only thing it did that the cheapo did not do just as well was the inrush.

When the fluke walked away at a job site I picked up the harbor freight Aims 1000A. I had an initial problem with it and took it apart to fix it (A cold solder joint needed to be sweated).... it has been fine since. It is less likely to walk away from a job site and if it does I am not out that much.

I would be interested in a clamp meter that does DC too (for your recommendation on #1 above). Otherwise I'll worry about that down the road.

Being able to measure DC current is far more important than measuring inrush. *Start* with a unit that will measure DC current.
 

Hedges

I See Electromagnetic Fields!
Many of the DC clamp meters I considered, and I think most of the Fluke, had resolution of 0.1A (and accuracy 5 counts).
The HF had resolution of 0.01A (and accuracy a few more of those smaller counts.)


I wanted to be able to read a 5 amp PV string at some other time of the day when it's only delivering 1.xx amps and still have decent resolution to compare similar strings. The HF meter does that.

I was tempted for a moment to splurge on the FLIR meter with IR camera, but it had the poorer resolution, and AC only.
 

kh56

New Member
Question here on the Kill-A-Watt. I just purchased it and am getting watt hours on various appliances throughout the house. I went to my furnace and realize that it's hardwired. Is there an ideal way to get the Kill-A-Watt plugged into this?
I'm assuming that the easiest way is to cut an extension cord in half and wire half to the furnace and plug it into the Kill-A-Watt.
 
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