Can I Heat a Hot Tub with Solar Power? Panels, Batteries and Heat Pumps Compared

Last updated: September 2026

Can you heat a hot tub with solar power? It’s a question I get asked a lot, especially whenever energy prices jump. The idea is lovely: a steaming tub on a cold evening, heated by sunshine you collected earlier in the day. The reality is more nuanced. Solar can definitely help, but the way most people imagine doing it (panels, a big battery and the tub’s own heater) is the least practical option of all.

In this post I’ll go through the three realistic ways to use the sun on a hot tub, what each really costs, and the one I’d actually recommend.

In the video above I go through the pros and cons of each approach. Here’s the written version, with the numbers checked and brought up to date.

First, how much heat does a hot tub need?

It helps to know the size of the job. It takes about 1.16kWh of heat to raise 1,000 litres of water by 1°C. My own tub holds about 2,800 litres (740 US gallons), so every 1°C rise needs about 3.25kWh. Heating it from 15°C to 38°C from a fresh fill needs roughly 75kWh, before you count any heat lost along the way. After that, you’re topping up whatever the tub loses through the cover, walls and pipework, which depends hugely on insulation and weather.

That’s why solar on its own struggles. A hot tub needs a lot of heat, and it needs most of it in winter, when the sun is weakest. If you want to run the numbers for your own tub, my heater size calculator does the maths.

Option 1: panels, a battery and the spa pack heater

This is what most people picture: solar panels charge a home battery by day, and the battery runs the hot tub by night. It’s technically possible, but it’s expensive and it doesn’t go very far.

Take the Tesla Powerwall 3. Its datasheet says it can store up to 13.5kWh. A typical UK spa pack heater is 3kW, so a completely full battery runs it for about 4½ hours and nothing else in the house. On my tub, that would raise the water by about 4°C. On a US 5.5kW heater, it’s under 2½ hours.

And the panels aren’t cheap either. The Energy Saving Trust says an average UK home system (about 4.5kWp) costs around £7,600 to install and typically pays for itself in 9–12 years. In the USA, EnergySage puts the average at $2.60 per watt, or about $31,000 for a typical 12kW system before incentives. Then add the battery on top.

So, for most people, powering a hot tub element from a battery isn’t worth it. If you already have panels and a battery for the house, great, use the daytime surplus. But I wouldn’t buy them just for the tub.

Option 2: solar thermal panels and a heat exchanger

Solar thermal collectors don’t make electricity. They heat water directly. You’ll have seen the long black panels on rooftops, or the rows of glass tubes. Water (or a heat transfer fluid) is pumped through them, heats up in the sun, and comes back much warmer. On a good day, the water coming off the panels can get to around 50°C (120°F).

You then pass that hot water through a heat exchanger. A heat exchanger has two separate circuits: the hot water from the panels flows through one side, your hot tub water flows through the other, and heat passes between them without the two ever mixing. So the sun heats the panel water, and the panel water heats your tub. There’s more on how exchangers work in Can I heat my hot tub from my home boiler?

It works, and it’s cheap to run, since the only running cost is a small pump. But be realistic about three things:

• It isn’t cheap to buy. The US Department of Energy puts a solar pool heating system at $2,500–4,000 installed, with a payback of one to seven years depending on fuel prices and sunshine. In the UK, the Energy Saving Trust says a typical domestic solar water heating system costs around £5,300.
• It’s seasonal. The same EST page says a home system provides around 90% of a household’s hot water in summer but only around 25% in winter. A hot tub at 38–40°C is harder still. The DOE notes that cheaper unglazed collectors suit warm weather only, while glazed collectors can work year-round in many climates. In the UK, you’d want glazed flat plates or evacuated tubes, and you’d still treat it as a top-up.
• Control is trickier. You need a differential controller that only runs the solar loop when the panels are hotter than the tub, and something to stop the tub overheating on a hot, sunny day. It’s all doable, but it’s more plumbing and more to go wrong.

For sizing, the DOE suggests collector area of 50–100% of a pool’s surface area, with more needed in cooler climates. A hot tub runs far hotter than a pool, so plan for the top of that range at least, and get the system designed for your location.

Option 3: solar PV powering an air source heat pump

This is the one I like. You use solar panels to make electricity during the day, and you use that electricity to run an air source heat pump rather than the spa pack element.

The difference is huge. An electric element turns 1kWh of electricity into 1kWh of heat. A heat pump moves heat out of the air, so it gives you several kWh of heat for every kWh it uses. The DOE says pool heat pump COPs usually range from 3 to 7, although that’s measured with warm 80°F (27°C) air and 80°F water. At hot tub temperatures and in cooler weather, expect less, so I use about 2.5 in cold weather and 4 in mild weather as rough working figures.

Put that together with the battery example above. The same 13.5kWh that raised my tub by about 4°C through the element would raise it by roughly 10°C through a heat pump at a COP of 2.5, and around 17°C at a COP of 4. Better still, run the heat pump in the middle of the day while the panels are producing, and you may not need the battery at all. The tub’s cover and insulation hold the heat for the evening.

In the video I mention a customer who heats his tub for free this way, by timing his heat pump to run off his solar panels. I’d be careful with the word “free”, though. The panels cost money up front, and if you’d otherwise export that electricity, you’d be paid for it. Every kWh you put into the tub is one you’re not selling. I’ve gone through this in How to heat a hot tub for free. It’s still an excellent use of daytime surplus, and it’s the option I’d recommend if you already have panels or are putting them in for the house anyway.

A heat pump is worth it even without solar. On my own tub, my 21kW Comfortline cut running costs by 66%. The numbers are in How I cut my hot tub running costs by 66%. For the full background, read the ultimate guide to air source heat pumps for hot tubs, and to pick one, see How to choose the right air source heat pump for your hot tub or pool.

The three options compared

Approach	Upfront cost	Works in winter?	My verdict
PV + battery + spa pack element	Very high	Poorly (little sun, and the battery runs out fast)	Not worth buying for the tub alone
Solar thermal + heat exchanger	Medium to high	Only a little in the UK	A good summer top-up in sunny areas
PV + air source heat pump	High (PV) + medium (heat pump)	The heat pump works year-round; solar helps most in summer	The best option if you have or want PV anyway

UK vs USA

• UK: winter sun is weak and short, and hot tubs get the most use in the cold months. Solar will cut your bill most from spring to autumn. At the Ofgem price cap for October to December 2026, electricity is 26.32p per kWh, so every kWh of your own solar you use instead is worth having.
• USA: in the sunbelt states, solar thermal can do far more of the work, and the DOE’s figures on payback are more encouraging. In the north, it’s the same story as the UK. How you’re paid for exported power varies by state and utility, which changes the maths.

Common mistakes

• Buying a battery to run the element. It’s the most expensive way to get the least heat.
• Using unglazed pool collectors for a hot tub in a cold climate. They lose too much heat at 38–40°C.
• No overheat protection on solar thermal. On a hot summer day it can push the tub above the 40°C (104°F) maximum the US CPSC recommends.
• Forgetting the cover. For pools, the DOE says covers can save 50–70% of heating costs. On a hot tub, a good cover is the cheapest “renewable” there is.
• Skipping insulation on a DIY build. I didn’t insulate my own tub, and it’s the one thing I’d change. See DIY hot tub insulation.

If you’re going fully off-grid, it’s a different challenge again. See Off-grid hot tub: luxury in the wild or impossible dream? And for every heating method side by side, with running costs, read What is the best way to heat a DIY hot tub?

Planning your own build?

If you want solar thermal, a heat pump or both, it’s far easier to plan the heat exchanger loop, bypass and controls on paper than to retrofit them later. My DIY hot tub plans give you the plumbing layout and parts list, and for a custom setup I can draw up an architectural CAD and 3D plumbing design with the heating circuit included.

You’ll find air source heat pumps and connection kits at parts4tubs.co.uk, and heating parts at parts4tubs.com in the USA.

FAQs

Can solar panels run a hot tub?

Yes, while the sun is shining and if the system is big enough. Running a hot tub from a battery overnight isn’t practical, though. A full 13.5kWh battery runs a 3kW heater for only about 4½ hours.

How many solar panels do I need to heat a hot tub?

It depends on the tub, the insulation and the season, and in a UK winter no sensible number of panels will do it alone. Pairing panels with a heat pump means you need far fewer, because the heat pump gets several kWh of heat from each kWh of electricity.

Is solar hot tub heating free?

No. The sun is free, but the kit isn’t: around £7,600 for an average UK PV system and around £5,300 for solar water heating, according to the Energy Saving Trust. Running costs are low, though.

Can I use solar pool panels on a hot tub?

Unglazed pool panels will warm a tub in summer, but they lose a lot of heat at hot tub temperatures. Glazed or evacuated tube collectors with a heat exchanger and a proper controller work better.

What’s the best way to use solar with a hot tub?

Run an air source heat pump from your solar panels during the day and keep the tub well covered. That gets the most heat out of every kWh you generate.

Using solar on your tub already? Tell me how it’s working in the comments below. I read them all.

Cheers,
Andi

You can also find me on my socials;

Can I Help You?

If I can help you in any way I would love to hear from you. You can get in touch using the form below.

Thanks - Andi

 

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