Log 2: Energy Costs of a Portable Spa

A project log for SmartSoak

Maximize hot tub enjoyment, minimize cost

alexwhittemorealexwhittemore 09/10/2019 at 05:210 Comments

When you spend 1/10 the cost on a spa, you have to wonder, what are you giving up?

The first 3 aren't a big deal. Yeah, you're sitting on the ground, but you're in a bath, who cares? If you do, you can get a seat on Amazon.

Insulation and Heater power are the real killers, and the reason for this project.

Portable spas have anemic heaters

Here's the thing - real hot tubs plug into the same circuit your electric car does. They have basically infinity power. Same if your tub is gas-fired. You want to soak but the water is cold? Set the temp and wait 30m. Maybe an hour.

Portable spas are designed to plug into a normal 120V outlet on a 15A circuit. That limits you to 1800W right there, which isn't a lot when you want to heat ~290 gallons of water. Add to that that individual appliances are meant to draw 20% less than that  (1440W) if they're "continuous" (on for >3 hours) and the hot tub doesn't have much to work with. And remember, the heater isn't even the only device in the box you plug in.

By my measurements, my Intex spa draws around 50W for the circulation pump, 1400W when the bubbles and heater are on, around 700W bubbles-only (man, that's a powerful compressor), and around 1200W for just the heater (so 1250 heater+pump).

TL;DR: The portable spa heater is only 1200W. It heats at about 1-2°F per hour. Long story short: you'll have to keep the water hot all the time if you want it to be ready for a soak any given day of the week.

This leads to a second problem:

Cost to maintain a set temperature is proportional to amount of thermal insulation, and the temperature delta to ambient.

Unfortunately, the requirement for dense, heavy, thick insulation is directly at odds with the requirement for packing everything neatly into a single box a single mortal human can lift by themselves (albeit briefly).

Real hot tubs are insulated TO THE NINES. Do you know why a good hot tub cover costs $500 easy? Because it's THICK, HEAVY, and unwieldy as hell. LOTS of foam and structure to support it. Similarly, if you take the side panel off, you'll find that nearly every gap between the tub and its housing is stuffed chock full of expanding PU foam.

The reason is, the (energy) cost to run the hot tub is driven entirely by the average heat loss to ambient.

You have two options for reducing heat loss:

  1. Use a ton of insulation (strike 1).
  2. Keep the water cold, so low delta-t, then heat it up quickly on-demand. (strike 2, with a heater less powerful than a hair dryer).

Looks like we just found our hacking targets.

Quickly, before we go on to the actual hacking, let's get a baseline. I know the heater takes 1200W, but it's not on ALL THE TIME. So how much power am I actually paying for? If I stick a temperature sensor in the water and log it with the built-in heater set to 104, I measure a heater duty cycle of about 42%. Give or take, keeping the hot tub full-heat takes about 500W. For now, take my word, and I'll show you numbers and graphs and stuff later. For reference, 500W is 12kWh/day, or about $3 at our local electricity prices, so about $90/mo. Or in different units, a lot of hacker project budget.