How I Cut My Hot Tub Running Costs by 66% with an Air Source Heat Pump

Last updated: September 2026

This is the numbers post. After the energy price rises of April 2022, my hot tub went from expensive to run to eye-wateringly expensive to run. So I fitted an air source heat pump and measured what happened on my smart meter. The result: my weekly running cost fell by 66%.

Below I’ll show you exactly how I worked that out, what the same usage would cost at today’s UK and US prices, how long the heat pump took to pay for itself, and why your own saving might be bigger or smaller than mine. If you want to know how I plumbed it in, that’s in a separate post, Why I fitted an air source heat pump to my hot tub.

The background: an expensive tub to run

When I built my tub, John, my advisor at the pool supply store, warned me that I was at the limit of what a standard 3kW spa pack heater could handle for that volume of water. I knew that, but the alternatives at the time were big custom electric heaters that cost a fortune to buy, never mind run.

I also didn’t insulate the tub when I built it. I certainly don’t recommend my customers do the same now. So it lost heat easily and was slow to heat up.

Before April 2022, the tub cost me around £50 a week. I knew that was a lot, but it’s a luxury. Then electricity prices roughly doubled overnight, and so did my bill, to around £100 a week. I needed a better way.

What pushed me to try a heat pump

Just before this, I’d helped my good friend Paul fit a 7kW heat pump to his acrylic hot tub. You can read about that install in How I added an air source heat pump to an existing hot tub. What caught my attention was the numbers he texted me afterwards:

• His tub was heating about 2.5 times faster than on its 2kW electric heater.
• On the electric heater, his smart meter showed about 93p an hour while heating.
• On the heat pump it cost between 22p and 43p an hour, depending on the outside temperature. That’s between roughly a quarter and a half of the cost.
• He worked out it would pay for itself in about 17 months.

Those were exactly the kind of figures I needed. So I fitted a 21kW Comfortline heat pump to my own tub, and started logging my smart meter.

How I measured it

I split the running costs into three parts, because they behave differently:

1. Heat-up: taking the tub from its weekday temperature up to soaking temperature on a Thursday.
2. Using it: the weekend, when the tub is at 39°C and we’re in and out of it.
3. Holding it: the weekdays, when I keep it at a lower temperature rather than letting it go cold. Over the years I’ve found that’s cheaper than reheating from cold.

All the pound figures below use the roughly 28p per kWh I was paying in 2022. Because everyone’s price is different, I’ve given the kWh too. Those don’t change with the tariff.

1. Heat-up: 72% cheaper

Before the heat pump, I’d turn the tub up from 33°C to 39°C (91°F to 102°F) on a Thursday evening and leave it overnight. It took around 12 hours. The 3kW heater used about 36kWh doing that, costing roughly £10.80.

With the 21kW heat pump, the same 6°C rise takes about 1 hour 20 minutes at an outside temperature of 15°C (59°F). My smart meter showed roughly 8kW while it was heating, so about 10kWh in total, or £2.50. That’s a 72% saving on heat-up, and it’s ready the same evening.

2. Using it: no more heat loss

This is harder to put a number on, because I never logged exactly how long the old heater ran during a session. But the difference in use is huge. With the heat pump running at about 8kW on the meter (around £2 an hour at the time), I saw no temperature drop at all through a couple of 90–120 minute sessions at about 15°C outside. On the old heater, the water would drop from 39°C to 36°C (102°F to 97°F) in that time, and we’d have to get out because it was too cold. Then it would take hours to recover.

3. Holding temperature on weekdays

With the old heater, I held the tub at 33°C (91°F) during the week. Because the heat pump heats so much faster, I’ve dropped that to 31°C (88°F).

To measure it, I took my home’s average daily use for five days with the heat pump off: 22.94kWh a day. Over the same period with the heat pump holding the tub at 31°C, it was 35.86kWh a day. So holding the tub costs about 12.92kWh a day, or £3.62 at 28p.

The weekly totals

I didn’t log the old electric heater as carefully, so the “before” column is partly worked out. I took my £100 a week and worked backwards. The heat-up was 36kWh. The weekend, with three sessions at full power, reheating after each one and topping up the heat it lost, came to around 200kWh. That left about £33 a week for holding the tub at 33°C on weekdays.

Each week	3kW electric heater	21kW heat pump
Heat-up (Thursday)	£11 (36kWh)	£2.50 (10kWh)
Weekend use (3 days)	£56 (about 200kWh)	£16.50 (about 20kWh a day)
Holding temperature (4 days)	£33 (about 119kWh, at 33°C)	£14.48 (12.92kWh a day, at 31°C)
Total	£100 (about 357kWh)	£33.48 (about 122kWh)

That’s a saving of 66% on my hot tub running costs.

What would it cost at today’s prices?

Energy prices have moved since 2022, but the kWh haven’t. So here’s the same week at today’s rates. In the UK I’ve used the Ofgem price cap for 1 October to 31 December 2026, 26.32p per kWh. In the USA, I’ve used the EIA’s average residential price for July 2026, 18.31 cents per kWh.

Each week	kWh	UK (26.32p)	USA (18.31¢)
3kW electric heater	~357	~£94	~$65
21kW heat pump	~122	~£32	~$22
Saving	~235	~£62 a week	~$43 a week

Use your own unit rate from your bill: just multiply the kWh by your price. The percentage saving stays the same whatever you pay per kWh. What changes is how many pounds or dollars that is, and so how quickly the heat pump pays for itself.

How long did it take to pay for itself?

In 2022, the retail price of my heat pump was £2,564. I was saving about £266 a month, so it paid for itself in about 10 months.

Today, the nearest equivalent on parts4tubs.co.uk is the Comfortline 20kW, listed at £2,329 as I write, plus a connection kit from £225. At the current price cap, the same usage saves about £268 a month, so the payback is still roughly 10 months.

Paul’s smaller tub and 7kW unit worked out at about 17 months. That’s the point to take away: the more you use your tub and the more it costs to heat now, the faster a heat pump pays for itself.

Will you save 66%?

Maybe more, maybe less. My saving came from a few things stacking up:

• The heat pump’s efficiency. It moves heat from the air, so each kWh of electricity gives you several kWh of heat. That’s the biggest factor.
• A badly undersized old heater. My 3kW element struggled to keep up, so it ran for hours and couldn’t hold temperature during sessions.
• A lower weekday temperature. Faster heat-up meant I could hold the tub 2°C lower.
• Mild weather. My figures were taken at around 15°C (59°F). In winter a heat pump produces less heat and uses more electricity, so the saving shrinks. The US Department of Energy notes that heat pump pool heaters work most efficiently above about 45–50°F (7–10°C).

A small, well-insulated tub with a decent heater already won’t see as dramatic a drop in pounds. A big, uninsulated DIY tub like mine will. Also remember that your bill includes a daily standing charge, which doesn’t change whatever heater you use.

How to save even more

• Insulate. On a new build, insulate the floor and walls. It’s the thing I most regret skipping. See DIY hot tub insulation.
• Fit a good cover and keep it on. See the ultimate guide to hot tub covers.
• Hold a lower temperature on days you don’t use the tub, rather than letting it go cold or keeping it at 39°C.
• Size the heat pump generously so it can keep up in cold weather. See How to choose the right air source heat pump.
• Heat in the cheaper hours if you’re on a time-of-use tariff.

For more on how heat pumps work and what else to consider, read the ultimate guide to air source heat pumps for hot tubs and What is the best method to heat my DIY hot tub? To fit one yourself, follow Can I add an air source heat pump to my hot tub?

Planning your own build?

If you’re building from scratch, you can avoid my mistakes: insulate from the start and plan the heat pump into the plumbing and control room from day one. My DIY hot tub plans give you the layout, plumbing and parts list to do exactly that.

FAQs

How much does it cost to run a hot tub with a heat pump?

For my 2,800-litre tub, about 122kWh a week, which is around £32 at the late-2026 UK price cap or about $22 at the average US price. A smaller or better-insulated tub will use less. Winter use will cost more.

How much can a heat pump save on hot tub running costs?

I saved 66% compared with my 3kW electric heater. My friend Paul’s heat pump ran at between roughly a quarter and a half of his electric heater’s hourly cost. Savings are highest in mild weather and on tubs that are expensive to heat now.

How long does a hot tub heat pump take to pay for itself?

Mine took about 10 months, and would do again at today’s UK prices. Paul’s smaller setup was about 17 months. Lighter use means a longer payback.

Is it cheaper to keep a hot tub hot or let it go cold?

In my experience, holding it at a lower temperature during the week is cheaper than letting it go cold and reheating from scratch, especially on a big tub. With a heat pump, you can hold it lower because it heats back up quickly.

Does a heat pump save money in winter?

Yes, but less. Output and efficiency fall as the air gets colder, so expect a smaller saving in winter than the figures I measured at around 15°C.

Have you measured your own running costs? I’d love to see your numbers in the comments below.

Cheers,
Andi

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Hi, Andi here. I own Buildahottub.com and also write all of the articles and info pages on the site. Some years back now, I built my own hot tub but struggled to find the information I needed. So, once my tub was complete, I started this website to help others in their own pursuit of hot tub and plunge pools DIY building information.

Fast forward to 2025, I've helped over 1400+ DIY customers just like you all over the world build hot tubs and pools. Have a good look around the site, there are lots of resources here. Please do get in touch if I can help you. - Cheers, Andi