SWG Cost Calculator - seeking feedback

[needsajet]

I started out wanting to make a chart to illustrate the difference between paying for an SWCG vs. paying for chlorinating liquid.

I ended up with a calculator that compares SWG to chlorinating liquid economics. It might be useful for people who are considering a SWG.

If you have a chance to review this, please let me know anything you want to share with me... good, bad or indifferent, like it, hate it, especially as a new user. I'd love to hear first impressions of things that aren't intuitive or easy to use.

I also welcome comments on the approach to the costs and math (helping me find my mistakes especially!), because I was surprised along the way how complex a comparison this is!

Cheers and thanks in advance if you get a chance

SWCG FC Cost Calculator.xlsx - Google Drive

SWCG FC Cost Calculator.xls - Excel 97-2003 version - Google Drive

 

[needsajet]

All going well, here's what you should see when you open the file:

 

[Alwayswantedapool]

Is there also an additional cost for acid due to increasing ph potential with a swg? I just bought swg so will plug in my numbers and see what I get.

 

[needsajet]

Yep, you can scroll down to see the amount, but it's fair to say that's a weakness in the calculator. The user can increase or decrease the effect depending on their sense of it (the variable is on the 'Calcs' worksheet. It's currently set around two gallons of acid a year for a 16K gal pool.

It's a tough one because not everyone sees the increase. You can make it bigger or smaller, depending on your objective. I tied it to cell run time, because the general belief is that it's a function of aeration caused by the SWCG.

 

[needsajet]

Is there also an additional cost for acid due to increasing ph potential with a swg? I just bought swg so will plug in my numbers and see what I get.

And thanks for picking that up. The annual dollar value is there, but maybe I should have it right up front.

 

[JamesW]

A SWG will not create any pH rise as long as the TA is not too high.

 

[needsajet]

Thanks, James, it's helpful to have that aspect noted here.

So if you're able to keep your TA down, set cell AX8 on the "Calcs" worksheet to zero, to eliminate "increased acid addition arising from SWCG aeration". Right now it's set at $0.01 per hour of SWCG run-time, or on average $9 or $10 per year.

 

[Alwayswantedapool]

I played with it a little and overall seemed to work well. A couple of things I saw

1. When doing the Custom, it took me a little while to figure out that I needed to input Custom1 to get it to work. Kept trying to key in at the top the name I used for Custom1

2. Repair Allowance - I cant tell what you are doing here, my expectation was I could basically input an expense per year over a lifetime that I defined, but I couldn't get it to work.

3. In the SWCG Chlorine cost, you define an annual cell wear cost, but I don't see that carried forward into the annual operating costs.

4. In the cumulative Sanitizer cost your SWCG other costs which I think should be your annual operating cost doesn't seem to be snyching up.



The good news is it shows my new SWG is a huge save I like that answer.

 

[CaptainCannonball]

I'm curious about the amp draw figures. In my research I noticed Pentair claims 220watts for an ic40, is that just a max rating? How did you estimate those?

For us $0.39 a kWh people, the electricity cost is one of the biggest factors/deterrents and would account for almost $1.50 of the "price per pound of chlorine" cost.

It would be cool to incorporate a cost for an automated bleach dosing system instead of factoring in labor costs for dumping jugs.

 

[needsajet]

Thanks very much for the suggestions and feedback, much appreciated!

Repair allowance: my thinking is that pouring chlorine has no repairs, so at the least it should be recognized that a SWCG is a piece of equipment that can break. So it's just a user choice. If you think you might need a new flow switch or board someday, you can allow for it down the track. The cost comes in at whatever year you guess, or it can be ignored by zeroing it out.

Annual cell wear: I showed the annual number for illustration, but it's actually a larger periodic cost when the cell gets replaced, so you'll see a bump in the cumulative cost (on the bar chart). Same with the repair allowance, so I should have treated those two the same. The bars in the bar chart are correct, but good point, I've displayed those inconsistently down below, so it's on my list to fix.

Haha, yeh, the cost is close enough (in most cases) that the SWCG decision delivers convenience rather than dollars, so if you're saving, that's even better

For electricity I used 2.25 kWh per lb FC. I was hoping to get some feedback on that I just used my own, but it got me digging and I realized I was using cell output, which ignores heat loss from the transformer. That got me digging some more and I found a thread with some info from pooldv and JamesW. It's now raised from 2.25 to 3.8 kWh per lb. The user can customize it, so if you have the file, go to the 'Calcs' worksheet and enter 3.8 in cell AN8.

On power cost, I need to use the marginal rate when looking at cost. If I just divide my bill by total kWh, I get a higher figure than what I pay for additional kWh. I don't know anything about California, but I've seen $x/kWh consumed (everyone), plus $x per day or month for delivery/transmission (residential or commercial), and for commercial also $x for peak demand, and $x for power factor. For a homeowner adding a SWCG, it should be just the first figure (plus tax, minus discounts, if applicable).

Well, since I included everything but the kitchen sink for the SWC, it seems to me that you have to allow something for the effort of going to the store and buying chlorinating liquid. The user can zero it out, or even enter a negative number if it's a chance to visit your brother, haha! That's a great idea to add a chlorine pump option, and I've added it to my future wish-list, thanks

 

[needsajet]

Did the updates suggested above, so the new versions on Google should be better Thanks again for the input.

Anyone else? Ideas most welcome

 

[JoyfulNoise]

Dang Aussies and their silly metric system!!

We only buy salt here in the States in 40lb bags, not 50lbs.

Thread should be re-titled to “Bored Dude Plays with Excel Waaaay Too Much”

Nice work

 

[needsajet]

Can't remember who, but someone told me to keep my mind active to avoid memory loss.

I'll change it to 18.1436948 kg bags

Ours are 55 lbs, possibly because we have more ocean.

 

[JoyfulNoise]

I think this is a great spreadsheet as it bares out a simple principle - chlorine costs money to produce and, whether your producing it on site or buying it as a manufactured good, your paying about the same.

In layman’s terms - There ain’t no free lunch!

 

[needsajet]

40 lb bags of salt are in.

I also changed the Hayward GL chlorinator outputs to:
T-3: 0.53 lbs per day (was 0.6)
T-9: 0.9 lbs per day (was 0.7)
T-15: 1.45 lbs per day (was 1.4)
...per comments from JamesW and likely closer than the original discount salt pools numbers.

To get the latest you can re-click the links in the first post.

 

[mitchellb]

This is a great analysis. I am ambivalent about my SWG. I definitely have an increased cost for MA. Plus I live in Florida and have frequent, heavy rains which requires the addition of salt through most of the summer months when I am forced to lower the water level in my pool.

To me, the cost is likely a wash, but the convenience of not having to run to the store and buy liquid chlorine is well worth it in the end. My free time is priceless, and I would rather spend it floating in my pool than running to the store for jugs of chlorine.

 

[Nectarologist]

This is awesome! Thank you for taking the time to make this. I'm likely getting a SWCG this spring. I agree with mitchellb's conclusion so if I can install it myself I'm going to move forward with it. Timer was installed for future Polaris but I went with a robot so it's just wiring the salt cell to it. Thanks again.

 

[Green Goblin]

I love this effort. I did this same calculation a few years ago when managing my pool in Florida. Like Mitchellb, I had to add salt at least a few times per year.

I came to the conclusion that a SWG did not save me money, but was still worth it in terms of overall experience. A big portion of that was that I'm not very handy, so for me installation and many repairs were paying others to do it, and that wasn't cheap. Also, where I was living we had a lot of power surges from lightning and from wonky electrical service, so the boards on the SWG would rarely make it more than 2-3 years.

Some thoughts on your calculator-- why use miles travelled per jug? Am I wrong that the needed information is the distance to the chlorine jug store and the number of trips. Whether you buy 1 jug per trip in the winter or 4 per trip in the summer your cost driving per trip is the same.

Am I thinking about this wrong too, don't most people keep the CYA level higher in an SWG pool? So your CYA costs should be higher for the SWG pool. My pool certainly seemed to consume CYA-- some of that due to the rainy season pump outs that were required, and some was just drift over time.

Despite the comment above that the system should not raise pH, it certainly seemed that I added more acid to the pool with the SWG system.

My other observation was that the higher salt content of the water seemed to cause failure more rapidly in my pool accessories-- for example I never wanted to let my fill hose touch the pool for example because the metal coupling would rust very rapidly afterward. Pool bushes and poles seemed to deteriorate more quickly, but maybe this was due to other factors-- hardly scientific stuff here.

Lastly there may be a few accessories that you buy for an SWG that you wouldn't otherwise own-- I owned a cap/stand to put the cell on so I could clean it with acid periodically, and I had to buy a belt wrench because sometimes I couldn't get the cell unscrewed by hand (I only hand tightened that thing on, but sometimes it would be hard to open again later).

- - - Updated - - -

Also, for my calculations, I limited the whole comparison to 20 years. My assumption was that 3 cells probably can span 20 years, and that the main unit would probably not last or be desired beyond 20 years.

 

[Green Goblin]

And... I also factored in that even with the SWG I still needed to add liquid chlorine occasionally. I still had to SLAM the pool occasionally.

I know some people get so dialed in with the SWG that they never need to add liquid chlorine, I wasn't that good. Probably 1-2 times per year I'd do a SLAM. It was a heated pool, used regularly for 10-11 months of the year.

 

[mitchellb]

A SWG will not create any pH rise as long as the TA is not too high.

I do not think this is correct. As you lose chlorine through outgassing from the pool, you will end up with a relative increase in NAOH as a byproduct of the electrolysis reaction. This gradually raised the pH in SWG pools.

The cost of lowering pH on a regular basis should be factored in here and can be substantial in some cases. I have to add quite a bit of MA regularly.