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Solar Tankless Water Heater Calculator

Solar panels offset daily kWh — not peak kW demand. Enter your heater rating and usage to see panels needed, annual savings, and how solar thermal compares.

Tankless water heater setup
min/day
people
$/kWh
Solar offset for your tankless heater
14 × 400W panels to offset daily usage
Important: Solar panels offset your daily kWh usage — they do NOT power the peak 24 kW demand. A 24 kW heater running requires 24x more instantaneous power than panels produce. Grid connection is non-negotiable for any tankless electric water heater.
Peak draw (grid required)24 kW instantaneous
Daily kWh consumption24.00 kWh/day
Monthly cost (grid)$94.97/mo
Annual cost (grid)$1,138.80/yr
Solar annual savings$1,138.80/yr
Solar add-on system cost$16,480
Solar payback period14.5 yrs
Tank heater annual kWh (same household)1285 kWh/yr ($167/yr)
Solar thermal alternative~$3,300 installed, 32.9 yr payback — more efficient for hot water
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How to Use This Calculator

Select your heater's kW rating

The kW rating is the most critical input. A point-of-use unit under a single sink draws 8 kW; a whole-house 240V unit can draw 24-36 kW. This is the instantaneous peak demand — the power the heater pulls from the electrical panel the moment hot water is requested. No battery or solar array of reasonable size can supply this on its own. Solar panels on a grid-tied system offset the daily energy (kWh) while the grid supplies the peak power (kW).

Enter usage minutes and household size

Enter total daily run time across all uses — showers, dishwasher, hand washing, laundry. A typical 10-minute shower runs the heater for about 10 minutes. The calculator converts this to daily kWh and sizes a solar array to offset that energy via net metering or battery storage.

Read the results and understand the key insight

The calculator shows panels needed to offset daily kWh, annual savings, and payback. It also compares your tankless heater's energy use to a traditional tank heater for the same household, and shows a solar thermal alternative cost and payback. Solar thermal (roof-mounted collectors that heat water directly) is often more cost-effective than PV for pure hot water heating.

The Formula

Daily kWh = Heater kW × (Usage Minutes ÷ 60) Monthly kWh = Daily kWh × 30.44 Annual kWh = Daily kWh × 365 Annual Cost = Annual kWh × Electricity Rate System Watts = Daily kWh × 1000 ÷ Peak Sun Hours ÷ 0.80 Panels Needed = ⌈ System Watts ÷ 400W ⌉ Tank Heater Daily kWh = People × 18 gal × 0.045 kWh/gal ÷ 0.92 EF Solar Thermal Payback = System Cost ÷ (Tank Annual Cost × 0.60)

The critical distinction: kW is peak instantaneous power (the heater's demand), kWh is energy consumed over time (what solar offsets). An 18 kW heater running 30 minutes uses 9 kWh. Solar panels sized to generate 9 kWh per day (in net metering) effectively cover that energy bill — but the heater still draws 18 kW from the grid in the moment it runs.

Example

The Johnson family — 4 people in Dallas

The Johnsons installed a 24 kW whole-house tankless heater. With 4 people, they use about 60 minutes of hot water daily. They pay $0.13/kWh in Dallas (5.4 PSH).

Heater24 kW whole-house
Daily usage60 min/day
Rate$0.13/kWh
LocationDallas, TX (5.4 PSH)

Result

Daily kWh24 kW × 1 hr = 24 kWh/day
Annual cost$1,139/yr
Solar panels to offset14 × 400W panels (5.6 kW)
Solar annual savings~$1,139/yr
Payback~15 years (high usage = high cost)

The 24 kW heater requires that 24 kW peak draw from the grid — solar cannot replace this. But 14 solar panels offset the daily energy bill through net metering. The Johnsons might also consider solar thermal collectors ($3,200 installed) which would save $680/yr on their tank heater equivalent, paying back in 4.7 years — typically the smarter choice for pure hot water.

FAQ

Practically, no. A 24 kW tankless heater needs 24,000W of instantaneous power. You would need 60+ solar panels and a battery bank large enough to supply that surge — a system costing $50,000+. The math simply doesn't work off-grid. For off-grid hot water, use a propane tankless heater, solar thermal collectors, or a heat pump water heater with a modest battery bank. Grid-tied solar can offset your daily kWh via net metering, but off-grid electric tankless is not practical.
Electric tankless heaters are 98-99% efficient (no standby losses) vs. 85-95% for tank heaters. However, they draw enormous peak power and require thick wiring and a large breaker (100-150A for whole-house units). For the same household, a tankless heater's kWh consumption can be lower than a tank heater because there's no standby heat loss (tanks lose 1-3 kWh/day keeping water hot). The trade-off is the huge peak demand requiring grid connection.
Solar thermal systems use roof collectors to directly heat water using the sun's heat — separate from electricity generation. For hot water specifically, thermal collectors are 3-5x more efficient at converting sunlight to heat than PV panels converting to electricity. A solar thermal system costs $2,500-4,500 installed and can offset 60-80% of a household's hot water heating costs. Solar PV is more versatile (powers everything), but for pure hot water reduction, thermal gives better ROI. For tankless heaters specifically, solar thermal doesn't help with the peak draw problem — only with a tank-based system.
A 24 kW unit requires a 100A dedicated 240V circuit; a 36 kW unit needs 150A. Many homes have only a 200A main panel — adding a 150A circuit for a water heater alone may require a panel upgrade ($2,000-4,000). This is why many homeowners who switch to electric tankless discover the electrical upgrade costs more than the heater. Always get a load calculation done by an electrician before purchasing a whole-house electric tankless unit.
Best to worst for solar + hot water integration: (1) Heat pump water heater — most efficient, 3-4x COP, 120V/240V, works great with solar during the day via smart scheduling; (2) Solar thermal + tank — directly uses solar heat, excellent ROI; (3) Standard tank water heater with solar PV — simple and reliable, solar offsets all tank energy; (4) Tankless with grid-tied solar — works but requires grid for peak draw; (5) Tankless off-grid — not practical for electric units.

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