How Many Solar Panels to Run an Electric Water Heater? (Calculator + Examples)
A 50-gallon electric water heater uses 13.5 to 18 kWh per day -- its 4,500W heating element runs 3-4 hours per day across multiple cycles. You need 9 to 12 standard 400W solar panels to cover it at 5 peak sun hours. Water heating is the second-largest energy use in most homes, but switching to a heat pump water heater cuts the requirement to just 3-4 panels.
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 standard electric water heater using 4,500W for about 3.5 hours per day consumes 15.75 kWh, so 10 panels cover it. Households with lower hot water demand may need only 9 panels, while larger families may need 12.
| Peak Sun Hours | Light Use (3 hrs) | Average Use (3.5 hrs) | Heavy Use (4 hrs) |
|---|---|---|---|
| 3 PSH (very cloudy) | 17 panels | 20 panels | 23 panels |
| 4 PSH (cloudy) | 13 panels | 15 panels | 17 panels |
| 5 PSH (US average) | 9 panels | 10 panels | 12 panels |
| 6 PSH (sunny) | 8 panels | 9 panels | 10 panels |
| 7 PSH (desert SW) | 7 panels | 8 panels | 9 panels |
Formula: panels = daily kWh / (panel watts x PSH x 0.83 derate), rounded up.
Electric water heater energy breakdown
A standard electric tank water heater has one or two 4,500W heating elements (upper and lower). They do not operate simultaneously in most residential units -- the upper element heats first for quick recovery, then the lower element maintains the full tank temperature. Total daily heating time depends on household hot water consumption, incoming water temperature, and thermostat setting.
| Specification | Light Use (1-2 people) | Average (3-4 people) | Heavy Use (5+ people) |
|---|---|---|---|
| Element wattage | 4,500W | 4,500W | 4,500W |
| Heating hours per day | 3 hrs | 3.5 hrs | 4 hrs |
| Daily energy use | 13.5 kWh | 15.75 kWh | 18.0 kWh |
| Monthly energy use | 405 kWh | 473 kWh | 540 kWh |
| Yearly energy use | 4,928 kWh | 5,749 kWh | 6,570 kWh |
| Annual cost at $0.16/kWh | $788 | $920 | $1,051 |
Incoming water temperature has a significant impact on energy use. In northern states where incoming water is 45-50 degrees F, the heater works harder than in southern states where incoming water is 65-75 degrees F. This difference alone can account for 20-30% more energy use in cold climates.
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 standard electric water heater off-grid is possible but requires careful system design due to the high element wattage.
Battery bank sizing (for 15.75 kWh/day average use):
- Daily consumption: 15.75 kWh
- Autonomy target: 1 day (water heating can be reduced or skipped briefly)
- Total energy needed: 15.75 kWh
- At 48V with lithium (LiFePO4) batteries at 80% depth: 15.75 kWh / 48V / 0.80 = 410 Ah
- This requires roughly 4 units of 48V 100Ah server rack batteries
Inverter sizing: A 4,500W element is a pure resistive load with no startup surge. Your inverter needs to handle 4,500W continuous. A 5,000W pure sine wave inverter works well. Since the load is purely resistive, a modified sine wave inverter is also acceptable for water heaters.
Solar-direct heating strategy: Many off-grid homes skip the battery bank entirely for water heating by using a solar-direct diverter (like an Immersun or similar device) that routes excess solar production directly to the water heater element. This uses the water tank itself as thermal storage. The element runs only when solar production exceeds other household loads, heating water during the middle of the day.
The better off-grid option: A heat pump water heater draws only 500-600W and uses about 5 kWh per day. It requires a much smaller inverter (1,000W) and fewer panels (3-4 versus 9-12). For off-grid homes, this is almost always the better choice.
See our battery charging calculator for exact sizing.
Running it grid-tied
Grid-tied solar is the most practical way to offset electric water heater costs. The key strategy is timing.
With a timer: Install a $20-$40 timer on your water heater circuit to restrict heating to 10 AM-3 PM. This forces the water heater to draw power during peak solar production, maximizing the direct use of your solar energy (self-consumption) rather than relying on net metering. The insulated tank keeps water hot for 8-12 hours, easily covering evening and morning showers.
Without a timer: Your water heater heats on demand throughout the day. Net metering ensures your solar production offsets the cost, but the electricity flows are less efficient because you are exporting solar power during the day and importing grid power at night.
Over a full year, 10 panels producing about 6,050 kWh offset a water heater using 5,749 kWh. The surplus covers other loads or banks as net metering credit.
The heat pump water heater alternative
Before investing in 9-12 solar panels for a standard electric water heater, strongly consider upgrading to a heat pump water heater (HPWH). The math is compelling:
| Standard Electric | Heat Pump Water Heater | |
|---|---|---|
| Daily energy use | 15.75 kWh | 4.5-5.5 kWh |
| Solar panels needed (5 PSH) | 10 | 3-4 |
| Annual energy cost | $920 | $265-$320 |
| Annual savings | -- | $600-$655 |
| Equipment cost | $400-$800 | $1,200-$2,500 |
A HPWH uses a small heat pump to extract heat from surrounding air and transfer it to the water, achieving a UEF (Uniform Energy Factor) of 3.3-3.8. The Inflation Reduction Act provides up to $2,000 in tax credits for HPWH installations, which can make the upgrade nearly free.
Energy-saving tips for electric water heaters
These strategies reduce energy consumption regardless of whether you have solar:
- Lower the thermostat to 120 degrees F. Most water heaters ship at 140 degrees F. Dropping to 120 degrees F saves 6-10% on energy and is sufficient for all household uses.
- Insulate the tank. An insulating blanket ($20-$30) reduces standby heat loss by 25-45%, which is especially valuable for tanks in unheated garages or basements.
- Insulate hot water pipes. Foam pipe insulation on the first 6 feet of pipe from the tank reduces heat loss and delivers hotter water faster, reducing wait time and wasted water.
- Fix leaky faucets. A faucet dripping at one drop per second wastes about 1,661 gallons per year. If that is hot water, it adds roughly 1-2 kWh per day to your water heating load.
- Install low-flow showerheads and faucet aerators. A low-flow showerhead (2.0 GPM versus 2.5 GPM) reduces hot water consumption by 20% with minimal impact on shower quality.
- Use cold water for laundry. Modern detergents work effectively in cold water. Washing clothes in cold instead of hot saves 4-5 kWh per load.