Solar Thermal vs Solar PV for Hot Water

How to use this calculator

Enter your household hot water energy demand and the backup fuel you’d otherwise burn (natural gas, propane, or electric resistance). The calculator runs two parallel ROI tracks and tells you which pathway returns more money over the system lifetime — solar thermal collectors, or solar PV plus a heat-pump water heater.

Each input has a tooltip-friendly meaning:

1. Annual hot water demand (kWh) — useful heat delivered to taps. A 4-person U.S. household uses about 4,400 kWh per year (ASHRAE 90.2 / DOE residential averages). If you don’t know yours, multiply household occupants × 1,100 kWh.
2. Backup fuel price (per kWh) — what you currently pay for the fuel that heats your water. U.S. natural gas residential averages $1.30 per therm ≈ $0.044 per kWh as of January 2026 (EIA Natural Gas Monthly). Resistance electric is your retail rate (national average $0.16/kWh).
3. Backup fuel efficiency — 85% for a mid-efficiency gas tank water heater (ENERGY STAR minimum 80%, condensing 90%+), 100% for electric resistance, 200-300% for a heat pump.
4. Electricity rate / export credit — used for valuing PV exports above hot-water needs. NEM 2.0 customers get retail rate; NEM 3.0 customers in California get avoided-cost (~$0.06-0.08/kWh).
5. Annual energy price escalation — historical U.S. average is 2.7% per EIA Form 861; EnergySage default is 3%. EIA’s 2025 STEO forecast is 3.5%.
6. System lifetime — 20 years matches both the thermal collector warranty and PV’s productive horizon after one tank/heat-pump replacement cycle.

How the math works

Both pathways are scored against the same baseline: the cost per kWh of useful heat delivered by your backup fuel. That’s the fuel rate divided by the appliance efficiency:

effective_rate_per_kWh_useful = fuel_rate / efficiency

For natural gas at $0.044/kWh in an 85%-efficient tank: $0.044 / 0.85 = $0.0518 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 - incentive%/100)
year_n_savings           = annual × (1 - 0.007)^(n-1) × (1 + escalation)^(n-1)
lifetime_savings         = Σ year_n_savings for n = 1..lifetime

Collectors degrade roughly 0.7% per year on a glycol loop, mostly from anti-freeze breakdown and seal aging — slightly faster than PV.

PV + heat-pump 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 × export_rate
year_n_savings    = annual × (1 - 0.005)^(n-1) × (1 + escalation)^(n-1)

PV degrades at 0.5% per year — the industry standard from NREL Standard Scenarios 2025.

The calculator picks a winner by comparing lifetime net gain (savings − net cost) on each pathway. If the two paths land within 5% of each other it calls it a tie and tells you to decide on roof space, fuel-availability plans, or aesthetics.

Worked U.S. example (Phoenix, AZ — natural gas backup)

Inputs:

• 4-person household, hot water demand 4,400 kWh per year of useful heat
• Gas at $0.045/kWh fuel input, 85% tank efficiency → $0.053 per kWh useful
• Solar thermal: 2 collectors + 80-gallon tank, $5,500 installed before ITC, 70% solar fraction in Phoenix
• Solar PV: 1.5 kWp, 2,500 kWh/year production, $4,500 installed, COP 3.0 heat pump, 30% ITC
• 3% escalation, 20-year lifetime

Solar thermal:

• Year-1 savings: 0.70 × 4,400 × $0.053 = $163
• Net cost after 30% ITC: $5,500 × 0.70 = $3,850
• Lifetime savings (with degradation + escalation): ~$3,930
• Payback: ~18 years
• Lifetime ROI: 2%

Solar PV + HPWH:

• DHW served: min(2,500 × 3.0, 4,400) = 4,400 kWh (PV fully covers demand)
• PV used for DHW: 4,400 / 3.0 = 1,467 kWh; excess = 1,033 kWh
• Year-1 savings: 4,400 × $0.053 + 1,033 × $0.13 (Phoenix export) = $367
• Net cost after 30% ITC: $4,500 × 0.70 = $3,150
• Lifetime savings: ~$9,950
• Payback: ~7 years
• Lifetime ROI: 216%

PV+HPWH wins by ~$5,600 over 20 years in this example. The pattern holds across most U.S. climates where gas is cheap.

When solar thermal still wins

Solar thermal beats PV+HPWH in three specific situations:

1. No roof space for full PV system — if you only have room for 2-3 panels’ worth of mounting area, dedicating that area to evacuated-tube thermal collectors produces more delivered hot water per square foot than the same area in PV. Thermal collectors typically yield 800-1,000 kWh of useful heat per m² per year vs PV’s ~200 kWh of electricity per m² (which after COP 3.0 becomes 600 kWh of heat).
2. High retail electricity rates and zero export credit — in places like Hawaii under NEM 3.0 or California IOU customers facing peak demand charges, the export-rate side of the PV equation collapses. Solar thermal isn’t penalized for “producing too much” because it self-throttles via the storage tank temperature setpoint.
3. Propane- or oil-heated homes — propane and heating oil run $2-$5 per gallon, equivalent to $0.10-$0.15 per kWh of fuel input. That makes the “backup fuel” so expensive that even a low-solar-fraction thermal install pays back fast.

When PV + heat pump wins (most U.S. homes)

PV+HPWH is the better bet when:

• Gas is your backup fuel (national average $0.04-$0.06/kWh)
• You have grid net metering at retail rate (most U.S. states still do under NEM 2.0)
• Your roof has room for a full residential PV system
• You’re already planning rooftop solar for whole-house electricity
• You want a system that does multiple jobs (water heating, EV charging, AC load reduction)

The big swing factor is that solar PV is a general-purpose energy asset — kWh from the panels can drive your dryer, your EV, your AC, or your water heater. Solar thermal is locked to one job. As more household end uses electrify, the optionality value of PV compounds.

Regional reference (4-person household, 4,400 kWh hot-water demand)

Hybrid approaches

A growing number of U.S. installers offer PVT (photovoltaic-thermal) hybrid collectors that produce both electricity and hot water from the same roof footprint. Brands like Sunmaxx and Naked Energy ship UL-listed PVT panels at $1,000-$1,400 per panel installed. PVT runs the math somewhere between the two pure pathways — it’s the right answer when roof space is the binding constraint.

For most U.S. homeowners with normal roofs and gas backup, solar PV + heat-pump water heater is the higher-ROI choice in 2026. Run the numbers above with your actual fuel rate and PV cost-per-watt quote before you sign anything.

Related calculators

• Solar pool heating calculator — same physics for pool water vs domestic hot water
• Solar panel payback calculator — payback math for whole-house PV
• Cost of solar panels calculator — per-kW installed cost benchmarks

Sources

• DOE / NREL — Solar Water Heating Comparative Analysis — pathway efficiency and lifetime modeling
• EIA Natural Gas Monthly + Form 861 — U.S. residential fuel and electricity prices
• SRCC OG-300 directory — solar thermal solar-fraction ratings by city
• EnergySage Solar Marketplace Report H2 2025 — installed cost-per-watt benchmarks
• DSIRE database (NC State) — state-level incentive details for both pathways