Solar Thermal vs Solar PV for Hot Water (UK)

How to use this calculator

Enter your household hot water energy demand and your current backup fuel (mains gas, LPG, oil, or electric immersion). The calculator runs two parallel ROI tracks and tells you which pathway saves more money over the system lifetime — MCS-certified solar thermal collectors, or solar PV plus a heat-pump hot-water cylinder.

What each input means:

1. Annual hot water demand (kWh) — useful heat delivered to taps. A typical 4-person UK household uses about 3,500 kWh per year (Energy Saving Trust field measurements). Multiply household occupants × 875 kWh if you don’t know yours.
2. Backup fuel price (per kWh) — Ofgem’s January 2026 price cap puts mains gas at roughly 6.2p per kWh. Heating oil ranges 7-9p per kWh; LPG 10-13p per kWh; electric immersion at the retail rate (27p per kWh under the current cap).
3. Backup boiler efficiency — 85% for a typical UK combi/system boiler (Boiler Plus minimum 92% for new installs); 100% for electric immersion; 250-300% for a heat pump.
4. Electricity rate / export credit — used for valuing PV exports above hot-water needs. Use your SEG export tariff if you want to be precise.
5. Annual energy price increase — UK retail electricity rose 4.5% annually 2020-2025 per Ofgem data. Gas roughly tracked the same path. Use 4% as a reasonable default.
6. System lifetime — MCS warranty for solar thermal is 5-10 years on collectors and 2 years on cylinder, but Energy Saving Trust says practical life is 20-25 years with one cylinder swap. PV panels carry 25-year output warranty.

How the math works

Both pathways are scored against the same baseline — cost per kWh of useful heat delivered by your backup fuel:

effective_rate_per_kWh_useful = fuel_rate / efficiency

For UK mains gas at 6.2p/kWh in an 85%-efficient boiler: 6.2 / 0.85 = 7.3p per kWh of delivered heat.

Solar thermal pathway:

annual_useful_heat_saved = solar_fraction × hot_water_demand
annual_cost_saved        = annual_useful_heat_saved × effective_rate
net_cost                 = system_cost × (1 - grant%/100)
year_n_savings           = annual × (1 - 0.007)^(n-1) × (1 + escalation)^(n-1)
lifetime_savings         = Σ year_n_savings for n = 1..lifetime

UK solar thermal collectors degrade ~0.7% per year, mostly from glycol breakdown over time and pump fatigue.

PV + heat-pump cylinder pathway:

DHW_served_by_PV  = min(PV_production × COP, hot_water_demand)
PV_used_for_DHW   = DHW_served_by_PV / COP
PV_excess         = PV_production - PV_used_for_DHW
annual_cost_saved = DHW_served × effective_rate + PV_excess × SEG_rate
year_n_savings    = annual × (1 - 0.005)^(n-1) × (1 + escalation)^(n-1)

UK PV degrades at 0.5% per year — the MCS Renewables Database benchmark.

The calculator picks a winner by comparing lifetime net gain on each pathway. Within 5% it calls it a tie and tells you to decide on roof space or boiler-retirement plans.

Worked UK example (Bristol, mains gas backup)

Inputs:

• 4-person household, hot water demand 3,500 kWh/year of useful heat
• Mains gas 6.2p/kWh, 85% boiler efficiency → 7.3p per kWh useful heat
• Solar thermal: 2 flat-plate collectors + 200 L cylinder, £4,800 installed, 45% solar fraction in Bristol
• Solar PV: 1.5 kWp, 1,450 kWh/year production, £4,500 installed, COP 2.8 heat-pump cylinder
• 4% escalation, 20-year lifetime, no grant for either

Solar thermal:

• Year-1 savings: 0.45 × 3,500 × £0.073 = £115
• Net cost (no grant): £4,800
• Lifetime savings (with degradation + escalation): ~£3,150
• Payback: never (within 20 years)
• Net loss: ~£1,650

Solar PV + HPWH:

• DHW served: min(1,450 × 2.8, 3,500) = 3,500 kWh (PV nearly fully covers demand)
• PV used for DHW: 3,500 / 2.8 = 1,250 kWh; excess = 200 kWh
• Year-1 savings: 3,500 × £0.073 + 200 × £0.15 (SEG) = £286
• Net cost: £4,500
• Lifetime savings: ~£7,800
• Payback: ~13 years
• Net gain: ~£3,300

PV+HPWH wins by roughly £4,950 over 20 years in this example. For households still on gas, this gap is typical across the UK.

When solar thermal still wins in the UK

Solar thermal beats PV+HPWH in three UK-specific situations:

1. Off-grid / no gas connection — if your backup is oil or LPG at 10p+ per kWh of fuel input, the gap between fuel cost and PV-driven heat closes. Solar thermal becomes the simpler, lower-capital choice.
2. Limited south-facing roof area — evacuated-tube collectors deliver around 700-900 kWh of useful heat per m² per year in the UK; PV panels deliver around 175 kWh of electricity per m², which becomes ~490 kWh of heat at COP 2.8. Per square metre, thermal wins on delivered heat.
3. High-occupancy households with daytime hot water draws — bed-and-breakfasts, large families with daytime baths and washing, holiday lets. The instantaneous summer solar fraction can hit 90%+, making thermal more useful than PV that has to fight self-consumption matching.

When PV + heat pump wins (most UK homes)

PV+HPWH is the better choice when:

• Mains gas is your backup at the Ofgem cap rate
• You have SEG export (Octopus, EDF, OVO all offer competitive tariffs)
• Your roof has room for a full residential PV system (typically 4-6 kWp)
• You’re planning to eventually move off gas under Future Homes Standard or net-zero retrofits
• You want flexibility — PV kWh can also drive an EV, heat-pump central heating, or just offset general consumption

The big swing factor is the UK’s Future Homes Standard ambitions and the gradual gas phase-out. PV is a transferable energy asset; solar thermal is locked to one job.

UK regional reference (4-person household, 3,500 kWh demand)

Hybrid approaches

PVT (photovoltaic-thermal) hybrid collectors are MCS-certified in the UK from manufacturers including Naked Energy (Virtu) and Sunmaxx. They produce both electricity and hot water from the same roof footprint at £900-£1,200 per panel installed. Worth investigating when roof space is tight — particularly on south-facing flats with limited mounting area.

For most UK homeowners with normal roofs and a mains-gas boiler, solar PV + heat-pump cylinder is the higher-ROI choice under 2026 Ofgem prices. Re-run the numbers above with your actual gas rate and any installer quote before you commit.

Related calculators

• Solar pool heating calculator — same heat-versus-electricity logic for pools
• Solar panel payback calculator — UK PV payback under SEG
• Cost of solar panels calculator — UK per-kW installed cost benchmarks

Sources

• Energy Saving Trust — Solar Water Heating field trial — UK solar fraction and lifetime data
• Ofgem default tariff cap — January 2026 gas and electricity unit rates
• MCS Microgeneration Installation Standard MIS-3001 — solar thermal performance methodology
• Solar Energy UK market report 2025 — UK PV cost-per-kWp benchmarks
• Boiler Upgrade Scheme guidance — grant scope and exclusions