I've used the Kill-A-Watt in the past for 120V stuff, but the tub is 220V and hard wired into an outside breaker box, then it goes to the main breaker box. So I'm thinking I'll need some sort of clamp meter and calculate some stuff.

1) what reasonably priced clamp meter should i get?
2) what do i need to check for, and how (mathematically) can I get to the point where I can say the tub costs me $X per month to run?

or is there a better way to approach this?

3/17/2010 5:09:22 PM

Do you actually need a measured value, or is a calculated value fine?

3/17/2010 6:26:26 PM

is it in time machine mode or not?

3/17/2010 8:36:09 PM

P=VI*cos(theta)

cos(theta) = power factor (phase difference between voltage and current)


ask joe#'s or eluisis(sp). they are the resident AC power experts.

3/17/2010 8:52:46 PM

^^

3/17/2010 9:07:46 PM

Shouldnt this be in the manual of the hot tub…?

3/17/2010 9:41:22 PM

Finding the power factor is the hard part--a standard clamp-on CT won't tell you the phase angle. You could assume a pf or perhaps find a value on the datasheet. There are clamp-on power meters, but they cost a few bucks.

Personally, I'd either read the manual, find nameplate data for the pump motor and heaters, or use the kw-hr meter that came with the house.

[Edited on March 17, 2010 at 9:59 PM. Reason : ]

3/17/2010 9:47:16 PM

Quote :

"I'd either read the manual, find nameplate data for the pump motor and heaters, or use the kw-hr meter that came with the house."

whats the manual gonna tell me? the heater turns on and off as it pleases...the pumps turn on and off all the time. the only way to get an accurate estimate is to measure whats actually being used over a certain time period. and the meter that is on the house is for the entire house, not just the hot tub. i don't see how that would be much help.

3/17/2010 10:06:23 PM

how accurate do you need to be? Does the tub have any 120V equipment(such as thermostats or controls), or does everything run on 220V? If there are no 120V attachments and you don't need something extremely accurate, you could put an ampmeter on one leg of the circuit and record the current reading. multiply that by 240V and then multiply that by an assumed 0.9 power factor to get watts.

You will need a clamp ammeter that is capable of recording data in order to measure energy usage this way. If you don't have one, it'd probably be cheaper for you to wire in an actual watt-hour meter at the outside breaker box. Then you'd really know what you're consuming.

[Edited on March 17, 2010 at 10:30 PM. Reason : .]

3/17/2010 10:29:24 PM

Turn off the hot tub.

Measure household power use over some period of time using the kw-hr meter.

Turn on hot tub.

Measure household + hot tub power use over some period of time using the kw-hr meter.

Subtract power use without the hot tub from power use with the hot tub.

Multiply hot tub power by some period of time to find energy use.

Multiply hot tub energy use by cost per kw-hr.

Repeat a few times and find a good average.

3/17/2010 10:36:48 PM

hot tub energy usage is going to be extremely dependent on the time of year if it's an outdoor hot tub, so none of these are good methods of measuring energy usage.

3/17/2010 10:53:37 PM

there's 100 ways this can be done, and the best have been mentioned here already. it all depends on what you want to pay and how accurate you want it.

if i were doing it (and didnt have access to the measuring equipment i have at work that would do this) i would just hook up a clamp-on CT and measure the current when the heaters are on, off, and when the pump is running. multiply each value by 240*0.9 and those are the individual power consumptions during those three scenarios in watts. Then I'd try to estimate the duty cycle of the heaters, and then calculate accordingly.

or if you want to be super accurate install a w-hr meter, or better yet install a pqube so you can trend your power consumption and show your carbon footprint!

remember to clamp the CT around a single conductor only because if you do them all you won't measure anything.

3/17/2010 10:59:32 PM

gotta get that smart grid tech

3/17/2010 11:07:45 PM

These are indispensable for tasks like this.

They also make 220 Kill-A-Watt units if you're like me and just want it to spit out a "this is how much it's costing you a month" number. It's also going to be a lot more accurate because you can leave it plugged in for a week and get an idea across various weather conditions.

3/17/2010 11:40:32 PM

You never said whether you needed to know exact numbers or not so if a good estimate is enough, search here: http://www.poolspaforum.com/forum/index.php?s=6f321f9bb139a68abb01ffc5bcc72cb6&showforum=7

If it's Watkins (Hot Spring / Limelight / Tiger River / Solana / Hot Spot) let me know and I can have someone look it up at work.

3/18/2010 7:47:40 AM

Quote :

"is it in time machine mode or not?"

3/18/2010 8:04:15 AM

I can tell you my power went down a f**kload when mine was broken for a couple of months.

3/18/2010 1:06:33 PM

^ isn't your older though (just saying that since I remember hearing about it a long time ago)? who makes it, what year?

3/19/2010 11:10:58 AM

use the kw-hr meter that came with the house.

Just turn everything off, say at the breaker, then wait a few hours in the hot-tub, go back and see the energy used.

3/19/2010 1:41:07 PM

then go reset all of your clocks

3/21/2010 2:58:02 AM

^ quite right. Should take far less time than re-wiring the hot tub to include a power meter.

3/21/2010 8:14:35 AM

1.21 gigawatts

3/23/2010 4:49:08 PM