Heatable Mini Heat Pump Tanks: Worth It with Solar?

Heatable and similar mini heat pump hot water tanks are a relatively new product category that's generating interest among UK solar owners. They heat water using a small built-in heat pump — far more efficiently than an immersion heater. Combined with solar, they could be a smart way to maximise your renewable hot water.

What Is a Mini Heat Pump Tank?

A mini heat pump hot water tank is an all-in-one unit that combines:

• A hot water cylinder (150–200 litres)
• A small air-source heat pump (typically 500–700W electrical input)
• Built-in controls and temperature management

The heat pump extracts warmth from the surrounding air (typically from the room it's installed in — utility room, garage, or kitchen) and uses it to heat the water. It works like a full-size air-source heat pump but dedicated solely to hot water.

The key advantage: a COP (Coefficient of Performance) of 2.5–3.0 means it generates 2.5–3 kWh of heat for every 1 kWh of electricity consumed. An immersion heater has a COP of 1.0 — it uses 1 kWh of electricity to generate 1 kWh of heat.

How It Works with Solar

This is where it gets interesting. If you have solar panels and surplus generation:

With an Immersion Heater + Diverter

• Surplus: 3 kWh of solar electricity
• Hot water heated: 3 kWh (COP 1.0)
• That heats roughly 60 litres from cold to 55°C

With a Mini Heat Pump Tank

• Surplus: 3 kWh of solar electricity
• Hot water heated: 7.5–9 kWh (COP 2.5–3.0)
• That heats roughly 150–180 litres from cold to 55°C

The same solar surplus heats 2.5–3 times more water. For a household that uses lots of hot water, or on days with less solar surplus, this efficiency advantage is significant.

Cost and Products

Prices include installation but vary significantly by installer and location.

The Financial Case

Scenario: Replacing a Gas Boiler for Hot Water

If you currently use a gas combi boiler for hot water and switch to a mini heat pump tank:

• Gas cost for hot water: ~£250/year (180L/day at 7p/kWh gas, 85% boiler efficiency)
• Mini heat pump electricity cost: ~£150/year (same hot water, COP 2.8, 26p/kWh)
• Annual saving: £100

At a cost of £2,000 installed, that's a 20-year payback. Not compelling on its own.

Scenario: Replacing an Immersion Heater (With Solar PV)

If you currently use an immersion heater (no gas) and have solar panels with a diverter:

• Current: Diverter sends surplus to immersion (COP 1.0), grid tops up the rest
• With mini heat pump: Same surplus heats 2.5–3x more water, less grid top-up needed

The saving depends on how much grid electricity you currently use for hot water top-up. A household buying 1,500 kWh/year of grid electricity for immersion heating could reduce this to 500–600 kWh with a mini heat pump — saving roughly £230–£260/year.

Payback: 7–10 years — more reasonable.

Installation Considerations

Space

These units are larger than a standard hot water cylinder — typically 600mm × 600mm × 1,600mm. They need adequate clearance for air circulation around the heat pump intake and exhaust.

Ventilation

The heat pump extracts heat from ambient air, cooling the room it's in by 2–5°C. In a utility room or garage, this is fine (and provides free cooling in summer). In a small kitchen or airing cupboard, it could be problematic.

Noise

Mini heat pump tanks produce 40–50 dB when running — similar to a fridge. They're not silent, so placement matters. A garage or utility room is ideal; a bedroom cupboard is not.

Plumbing

Installation requires connecting to your hot and cold water supply and (if replacing a gas system) potentially decommissioning the boiler's hot water circuit. A qualified plumber should handle this.

Electrical

The unit needs a dedicated electrical supply (typically a 13A fused spur). If connecting to a solar PV system for surplus diversion, additional wiring or integration may be needed.

When a Mini Heat Pump Tank Makes Sense

Good candidates:

• Homes with solar PV looking to maximise hot water from surplus
• Homes replacing an ageing hot water cylinder
• Homes moving away from gas (future-proofing)
• Off-gas homes currently using expensive immersion heating
• Properties where a full heat pump isn't practical but efficient hot water is wanted

Less suitable:

• Homes with a working gas combi boiler and no hot water cylinder (expensive to retrofit)
• Homes without solar PV (the financial case weakens significantly)
• Small flats with no suitable installation space
• Homes already using a PV diverter effectively (the upgrade from COP 1 to COP 2.8 may not justify the cost)

If you're considering a PV diverter as the more cost-effective first step:

myenergi Eddi Solar Diverter

£185

max power w

3000

modes

power_divert,timed_boost

outputs

2

priority

configurable

View on Amazon

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Solar iBoost+ Immersion Heater Controller

£150

max power w

3000

modes

auto_divert,manual_boost

outputs

1

buddy unit available

true

View on Amazon

Affiliate link — we may earn a small commission at no extra cost to you

Mini Heat Pump Tank vs PV Diverter

For most homes in 2026, a PV diverter is the better first step — far cheaper, faster payback, simpler installation. A mini heat pump tank is the upgrade path for homes that want maximum hot water efficiency from their solar investment.

For more on hot water cylinder integration with solar, see our hot water tank guide.

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