Sizing an array for a single appliance is the clearest way to understand solar, because it strips the question down to one load, one number of hours, and one amount of sunlight. Here is that calculation for a electric water heater, with the assumptions stated so you can change them.
A electric water heater drawing 4,000 W for 3 hours a day uses 12.00 kWh of electricity. At 4.5 peak sun hours and a 20 percent system derate, that needs about 9 standard 400 W panels, an array of roughly 3.60 kW. Fewer in Arizona, more in Seattle.
Calculate it for your own electric water heater
Change any box. The result updates as you type.
Planning estimate. The rate box is prefilled with a regional average; the figure on your own bill is the one that matters. Power is draw while running, not while switched on.
The arithmetic, with this appliance in it
Four steps, and none of them are difficult:
- Daily energy. 4,000 W multiplied by 3 hours, divided by 1,000, gives 12.00 kWh per day.
- Panel output. A 400 W panel at 4.5 peak sun hours produces 1.8 kWh on paper.
- Apply the derate. Inverter conversion, wiring, heat, dust and imperfect orientation take roughly 20 percent, leaving about 1.44 kWh per panel per day.
- Divide. 12.00 kWh divided by 1.44 kWh gives 9 panels, rounded up.
Peak sun hours is not the same as daylight hours. It is the number of hours of full-strength sun the day is equivalent to, which is why a 14-hour summer day might only count as five.
Panels needed by panel size
Larger panels mean fewer of them, not less roof area. Output per square metre has barely changed; the modules simply got bigger.
| Panel rating | Output per panel per day | Panels needed | Array size |
|---|---|---|---|
| 300 W | 1.08 kWh | 12 | 3.60 kW |
| 400 W | 1.44 kWh | 9 | 3.60 kW |
| 450 W | 1.62 kWh | 8 | 3.60 kW |
| 500 W | 1.80 kWh | 7 | 3.50 kW |
| 600 W | 2.16 kWh | 6 | 3.60 kW |
How many panels where you live
Location changes this more than any other input. The same electric water heater needs roughly twice the array in the Pacific Northwest that it needs in southern Arizona.
| Peak sun hours | Roughly where | Panels needed | Array size |
|---|---|---|---|
| 3 h | Seattle, Anchorage | 13 | 5.20 kW |
| 3.5 h | Chicago, Boston | 11 | 4.40 kW |
| 4 h | New York, Portland | 10 | 4.00 kW |
| 4.5 h | Atlanta, Kansas City | 9 | 3.60 kW |
| 5 h | Dallas, Los Angeles | 8 | 3.20 kW |
| 5.5 h | Phoenix, Albuquerque | 7 | 2.80 kW |
| 6 h | Southern Arizona, Hawaii | 7 | 2.80 kW |
For a figure based on your actual coordinates rather than a regional band, use the location-based solar production calculator.
Panels alone will not run it
Water heating is the single best use of surplus solar, because the tank is a battery that stores heat instead of electrons. Diverter controllers exist specifically for this.Solar produces during the day. If the load runs when the sun is down, that energy has to come out of a battery, and the battery has to be larger than the energy it delivers because you never discharge it fully.
| Autonomy | Energy required | Usable battery at 80% depth of discharge | At 48 V |
|---|---|---|---|
| Overnight only | 6.67 kWh | 8.33 kWh | 174 Ah |
| One full day | 13.33 kWh | 16.67 kWh | 347 Ah |
| 2 days | 26.67 kWh | 33.33 kWh | 694 Ah |
| 3 days | 40.00 kWh | 50.00 kWh | 1,042 Ah |
Inverter sizing for a electric water heater
4,500 W resistive. Either divert surplus to it or leave it on the grid.| Running power | 4,000 W |
|---|---|
| Start-up surge | 4,000 W (1x running) |
| Minimum continuous inverter | 5,000 W |
| Minimum surge capability | 4,000 W |
| Current at 48 V DC | 92.6 A |
Inverters are rated for continuous output and, separately, for a brief surge. An inverter that can run the load but not start it is a common and frustrating way to undersize a system.
Hot water is storage you already own
A hot water tank stores energy as heat, and heat is far cheaper to store than electricity. Diverting surplus solar into the tank rather than exporting it is one of the highest-value things a household array can do, and controllers that do this automatically are inexpensive.
The sizing above assumes the load runs on solar directly. If you are diverting surplus instead, the question becomes how much excess your array produces rather than how many panels the tank needs.
What it costs to run on the grid
For comparison, running the same electric water heater from the grid costs $2.21 a day and $808 a year at current average rates. The full breakdown is on our electric water heater running cost page.
Questions people actually ask
How many solar panels does it take to run a electric water heater?
How much electricity does a electric water heater use per day?
What size inverter do I need for a electric water heater?
Can I run a electric water heater on solar without a battery?
Size the whole system, not one appliance
- Solar System Size Calculator for the whole house rather than one load.
- Solar Panel Count Calculator to work backwards from a target array size.
- Usable Battery Capacity Calculator for depth of discharge and real usable kWh.
- Inverter Size Calculator for combined loads and surge.
- Off-Grid Solar System Sizing Calculator if there is no grid connection at all.
Costs on this page use a national average residential rate of 18.44¢ per kWh (August 2026, EIA-derived state residential averages). Your own rate is printed on your electricity bill and is the number that actually matters. Every figure here is recalculated from one central rate setting, so these tables move when the national average moves.