Water Misters, Part 2: How Much Money Can I Save?

The water misters can make the air conditioner perform better, but does it make sense to use them?  Of course, if you’re in a campground with unmetered water and electricity, there are no immediate savings other than comfort.

Electricity is one of the bigger costs in operating an RV park though, so we all pay in some form or another. The park doesn’t get it for free, and the cost is ultimately passed on to the consumer, so everything we can do to reduce our own usage benefits the community at large.

Calculating Air Conditioner Costs

If we make a few assumptions about air conditioner usage, we can estimate the cost of running one for an hour.

For now, let’s take 12 amps as the average load running at 120VAC, and assume a power factor of one.  Running the air conditioner one hour consumes 1.44kWh, and a national average residential electricity price of $0.129/kWh gives us a cost of about $0.19 per hour.  If the air conditioner runs about 1/4 of the time through the summer, it costs about $33/month to run.

Now let’s consider the scenario using the misters.  They reduce the power needed to run the air conditioner by about 10% in my testing, or about $0.02 per hour.  But if the air conditioner was previously able to keep up, it will also run less.  How much is heavily dependent on the weather, but it could easily be a 25% reduction.  That’s over $10 per month, per air conditioner.

But let’s also consider the water usage.  If we set it up so the misters only operate when the air conditioner is on, and that we use 0.5gal/h of water, we’d use 3 gallons a day.  At the national average price of water, that’s less than half a cent–a negligible amount.

So far, it looks like the misters make sense as far as utility costs.  The campground benefits (in most cases) from your using them.  What if we’re not grid-connected?

Misting While Boondocking

Here’s where it gets more complicated.  Most RVers probably only run air conditioners with their generators running when they don’t have hookups.  With a limited water supply, you’re particularly conscious of its use, but you may also want to cool the RV quickly after a day away from it.

Let’s look at the previous math slightly differently.  Instead of $0.129/kWh, let’s assume we have a generator that uses 0.5gal/h of diesel at idle, and 1.0gal/h with both air conditioners running.  If we only consider running the generator with both air conditioners on, each one is using 0.5gal/h.  At about $3/gal for diesel, that’s $1.50/h.

If we’re operating the misters effectively enough to reduce runtime by 25%, we reduce the hourly cost from $1.50 to $1.13.  That’s $0.38/hour.  Looking just at that part of the equation, the misters can pay for themselves really quickly.  The savings would pay for the misters in a couple of weeks running 2 air conditioners for 3 hours a day.

But we’re using water from our fresh water tanks, which has a much higher cost, especially in terms of convenience, than a municipal or campground supply.  If it’s 3 gallons a day, in the same two weeks we’ve used 42 gallons each of water and diesel.  You quickly arrive at a situation where the answer is it depends.  In my case, if the tanks were all full at the start, neither one is limiting.  Staying longer than two weeks, fuel and holding tanks would become the limiting factor before fresh water.  I’d run the misters.  How about you?

Next time I post on this subject, I’ll cover setting it up to run efficiently.  It’ll be triggered by the air conditioner’s compressor running, so the nozzles don’t run when the air conditioner doesn’t.

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