How Many Solar Panels to Run a Tankless Water Heater (Electric)? (Calculator + Examples)
An electric tankless water heater uses 6 to 12 kWh per day -- but here is the catch: it draws 18,000 to 36,000W instantaneously while running, even though it only runs 20-40 minutes per day. You need 4 to 8 standard 400W solar panels for the daily energy, but the massive power spike makes off-grid operation extremely challenging and expensive.
Quick answer
A 400W solar panel produces about 1.66 kWh per day at 5 peak sun hours (400W x 5h x 0.83 derate). A typical whole-house electric tankless water heater uses about 9 kWh per day (18,000W running for about 30 minutes), so 6 panels cover the daily energy. The real challenge is not the number of panels -- it is handling the 18,000-36,000W instantaneous demand.
| Peak Sun Hours | Light Use (20 min) | Average (30 min) | Heavy Use (40 min) |
|---|---|---|---|
| 3 PSH (very cloudy) | 7 panels | 10 panels | 14 panels |
| 4 PSH (cloudy) | 5 panels | 8 panels | 10 panels |
| 5 PSH (US average) | 4 panels | 6 panels | 8 panels |
| 6 PSH (sunny) | 3 panels | 5 panels | 7 panels |
| 7 PSH (desert SW) | 3 panels | 4 panels | 6 panels |
Formula: panels = daily kWh / (panel watts x PSH x 0.83 derate), rounded up.
Electric tankless water heater energy breakdown
Unlike a tank water heater that slowly heats stored water, a tankless unit heats water on demand as it flows through the unit. This requires enormous instantaneous power but for very short periods. Total daily runtime depends on how much hot water your household uses.
| Specification | Light Use (1-2 people) | Average (3-4 people) | Heavy Use (5+ people) |
|---|---|---|---|
| Instantaneous wattage | 18,000W | 24,000W | 36,000W |
| Daily runtime | 20 min | 30 min | 40 min |
| Daily energy use | 6.0 kWh | 9.0 kWh | 12.0 kWh |
| Monthly energy use | 180 kWh | 270 kWh | 360 kWh |
| Yearly energy use | 2,190 kWh | 3,285 kWh | 4,380 kWh |
| Annual cost at $0.16/kWh | $350 | $526 | $701 |
The wattage required depends on flow rate and temperature rise. A shower at 2.5 GPM with a 60-degree F temperature rise requires about 24,000W. Running two fixtures simultaneously can push demand to 36,000W or more in cold climates. Whole-house units typically need 2-4 dedicated 40-amp, 240V circuits.
Try the calculator
Adjust the panel wattage and your location's peak sun hours to see exact production numbers for your setup.
Benchmarks: U.S. avg 4.98 · Phoenix 6.54 (highest) · Seattle 3.95 · Anchorage 3.17 (lowest). Above ~5.5 = sunny · 4.5–5.5 = average · below 4.5 = cloudy.
Tap to see sensitivity analysisSensitivity analysis
| Scenario | Value |
|---|---|
| Low (-20%) | 1.3 kWh |
| Expected | 1.6 kWh |
| High (+20%) | 1.9 kWh |
Your daily production scales linearly with both panel wattage and peak sun hours. A 10% change in either input changes your result by 10%.
Running it off-grid
Running a whole-house electric tankless water heater off-grid is one of the most difficult appliance-solar combinations. The issue is not daily energy (which is moderate) but instantaneous power demand.
The power problem: An 18,000W unit requires an inverter that can deliver 18,000W continuously at 240V split-phase. Most residential off-grid inverters top out at 6,000-12,000W. You would need either a single commercial-grade inverter ($5,000+) or multiple residential inverters stacked in parallel. A 36,000W whole-house unit is essentially impossible to run from a standard off-grid inverter setup.
Battery demand: Even with adequate inverter capacity, the battery bank must deliver extremely high current. At 48V, an 18,000W load draws 375 amps. Most lithium battery banks are rated for 100-200A continuous discharge. You would need multiple batteries in parallel just for the discharge rate, not for capacity.
Practical alternatives for off-grid:
- Propane tankless water heater. Uses no electricity for heating (only a small amount for ignition and controls). This is the standard choice for off-grid homes.
- Heat pump water heater. Uses only 500-600W continuously -- easily handled by a standard off-grid inverter and battery setup. Needs just 3-4 solar panels.
- Point-of-use electric tankless units. A small unit for one sink (3,000-7,000W) is much more manageable for solar systems than a whole-house unit.
- Standard electric tank heater with timer. Heat water only during peak solar hours. The 4,500W load is large but manageable with a 5,000W inverter.
See our battery charging calculator for exact sizing.
Running it grid-tied
Grid-tied solar is the only practical way to pair solar panels with a whole-house electric tankless water heater. The grid handles the massive instantaneous power demand while your solar panels offset the daily energy through net metering.
Here is how it works: When someone turns on the shower, the tankless unit draws 18,000-24,000W from the grid instantaneously. Your 6 solar panels cannot deliver this power directly -- they produce about 2,400W combined at peak. But over the course of the day, those 6 panels produce 9.96 kWh total, which exceeds the 9 kWh the tankless heater consumed across all its short cycles.
The grid acts as a buffer -- absorbing your solar surplus during the day and delivering high-power bursts to the tankless heater on demand. Net metering ensures the math works out at the end of the billing cycle.
Important note: Some time-of-use (TOU) rate plans charge more for electricity during peak hours. If your tankless unit runs mostly during off-peak hours (mornings and evenings), while your panels produce during peak hours, you may actually come out ahead financially due to the rate differential.
Energy-saving tips for tankless water heaters
These strategies reduce the total runtime and daily energy consumption:
- Lower the temperature setting. Most tankless units default to 120 degrees F. If you can tolerate 115 degrees F, the unit requires less temperature rise and draws less power per minute.
- Install low-flow fixtures. A 1.5 GPM showerhead versus a 2.5 GPM head reduces flow by 40%, which directly reduces the wattage required and the total energy per shower.
- Avoid running multiple hot water fixtures simultaneously. Each additional flow point increases instantaneous demand. Stagger showers and dishwasher use.
- Consider point-of-use units for distant fixtures. Instead of running hot water pipes 30+ feet from the main unit, a small point-of-use tankless heater under a distant sink eliminates pipe heat loss and reduces the main unit's workload.
- Descale the unit annually. Mineral buildup reduces heat transfer efficiency, forcing the unit to run longer for the same temperature rise. Flushing with vinegar takes 30 minutes and maintains rated efficiency.
- Insulate hot water pipes. Foam pipe insulation reduces heat loss in transit, which means the tankless unit needs to heat water to a slightly lower temperature to deliver the desired temperature at the faucet.