How many solar panels do you need to run a water heater?

By the SmartSolarSizer team · Published 20 October 2026

How much electricity does a 50 gallon water heater use?

About 14.6 kWh a day, going by the label. The GE 50-gallon electric tank below is rated at 5,334 kWh a year on its EnergyGuide data, which is the same as its 4,500 W element running roughly 3.2 hours every day of the year. For any hot water heater, the yearly kWh on the yellow EnergyGuide label (or the kWh/annum figure on a UK or EU label) is the number to trust; divide it by 365.

Labels: GE GE50S10BMM electric tank and Rheem ProTerra heat pump water heater. Panels on an average day, 430 W panels.

People often size from the element instead, multiplying 4,500 W by a guess at the hours, or assume a bigger tank means a bigger bill. The tank doesn't work that way. The element only runs to reheat the hot water you draw off and to replace the heat lost through the tank walls, so a household's habits move the number far more than the size of the cylinder.

Solar panels for an electric water heater, city by city

In New York, one 430 W panel makes 1.66 kWh on an average day. The 50-gallon tank's 14.6 kWh therefore takes 9 panels, and covering it in December would take 13. Across 14 locations, with the month of least sun in the last column:

That's a sizeable chunk of a home system for one appliance. In London the tank alone would need 13 panels on an average day, more than the roughly 12 panels of a typical UK home system (Energy Saving Trust).

On a grid-tied home with net metering, use the average-day column: summer surplus pays for the dull months. The worst-month column only matters if the water heater has to run from your own panels every day of the year, and at 28 panels for a London December it shows why that is rarely practical with a plain electric tank.

A heat pump water heater cuts the panels before you buy them

A heat pump water heater moves heat from the surrounding air into the tank instead of making it with an element, and the US Department of Energy says that makes it two to three times more energy efficient than a conventional electric model (DOE). ENERGY STAR puts the saving at about 3,760 kWh a year for a family of four (ENERGY STAR). On the labels above, the Rheem uses 3.4 kWh a day: 3 panels in New York against 9 for the plain electric tank.

If your electric tank is near the end of its life and you're planning solar, this is where we'd spend the money first. Space is the limit: DOE says these heaters need a location that stays between 40 and 90 °F (4–32 °C), such as a basement or garage.

How many solar panels for a geyser (150 L or 200 L)?

The litres on the label don't tell you the energy. What matters is the element's power and how long it actually heats each day:

In South Africa a typical geyser element is about 3 kW, with older units up to 5 kW, and BusinessTech's sponsored LookSee piece puts water heating at 40–50% of a typical home's electricity bill (BusinessTech). If a 3 kW element runs for two hours a day, that's 6 kWh: 4 panels of 430 W in Lahore or Delhi, and 3 in Johannesburg or Karachi.

To find your own hours, time how long the geyser's indicator light stays on, or plug the reading from a timer or energy monitor into the estimator above. As a reference point, Which? says a 3 kW immersion element takes about three hours to heat a 180-litre tank from cold (Which?). A 150 L geyser with the same element heats a full tank a little faster and a 200 L one a little slower, but day to day both only reheat what the family uses.

Geyser season is winter, and in these cities that hurts less than you might expect. Panels in Lahore still make 4.24 kWh per kW a day in January, close to the yearly average of 4.40. In Johannesburg, tilted panels make slightly more in June (4.87) than across the year (4.76). The geyser needs the same 4 and 3 panels in the coldest months.

Can you convert a normal geyser to solar?

Yes, and you don't have to replace the tank to do it. There are three practical routes, from cheapest to most involved:

• Put the existing geyser on a midday timer on a hybrid or on-grid system, so it heats while the panels are producing and the insulated tank carries the hot water into the evening.
• Fit a DC solar geyser element and controller, which heats the water straight from its own dedicated panels with no inverter or battery in between.
• Replace it with a heat pump or solar thermal geyser, which needs far less electricity for the same hot water.

On a grid-tied system with export, a solar diverter (covered in the next section) does the timer's job automatically, feeding the element only when there's spare solar. For most homes with an existing geyser and a grid connection, the timer or diverter is where we'd start, because it costs least and changes nothing in the tank.

Immersion heater kWh, and using one with solar panels

A UK immersion heater is typically 3 kW and costs about 78p an hour to run, according to Which?, which also says a 180-litre tank takes about three hours to heat. That's about 9 kWh per full heat from cold, or 8 panels on an average London day.

You rarely need to size panels for the immersion, though. The usual approach on a UK solar home is a solar diverter, which watches for power that would otherwise be exported and sends it to the immersion element instead. Which? puts diverters at about £300–500 installed. The tank soaks up spare sunshine, mostly from spring to autumn. Don't expect much from it in December: a London array makes 1.24 kWh per kW a day that month against 2.80 across the year, and the house will use much of that before any reaches the cylinder.

Why a battery is the wrong place to store hot water

A tank of hot water is already a battery. It stores energy as heat, and a well-insulated one holds it for hours, so the cheapest plan is always to heat it while the sun is shining. Storing electricity first and heating water with it later means paying for battery capacity to do a job the tank does for free.

The numbers make the point. Running a 3 kW geyser for two hours at night takes 6 kWh, which needs a 7.4 kWh lithium battery for hot water alone. Covering a whole day of the 50-gallon electric tank from a battery would take 18.0 kWh, all of it spent on a job an insulated tank and a timer do for nothing.

There's a quieter problem off-grid, even at noon. The 9 panels that cover a 50-gallon tank in New York add up to 3.87 kW of rated output, less than the 4,500 W element draws. While the element is on, the battery and inverter make up the difference. On a grid-tied home the grid fills that gap without you noticing; off-grid, it's a reason to heat in short bursts at midday or to choose a heat pump water heater, which draws far less while it runs.

What size inverter does a water heater need?

A heating element has no start-up surge, so the inverter's continuous rating only needs to sit 25% above the element: 6,000 W for a 4,500 W US tank (on 240 V), 4,000 W for a 3 kW immersion heater or geyser, and 8,000 W for an older 5 kW geyser element. Because the start-up watts equal the running watts, the main calculator's stricter rule (25% above start-up watts) lands on the same sizes.

Hybrid heat pump water heaters still carry a backup element, so size for it unless you lock the heater in heat-pump-only mode. Tankless electric water heaters are the awkward case. They heat water as it flows instead of over hours, so they draw far more power than a tank element while a tap is open, which makes them a poor match for an off-grid inverter.

Common questions

Sources

Solar output for every location comes from the European Commission's PVGIS 5.3 (1 kW system, panels at the best fixed angle facing the equator, 14% system losses), retrieved 20 October 2026. Panel counts assume 430 W panels, the most common size in 2025 home solar quotes (EnergySage).

• GE GE50S10BMM electric water heater, ENERGY STAR / EnergyGuide data
• Rheem: understanding your EnergyGuide label (ProTerra heat pump water heater)
• ENERGY STAR: heat pump water heater savings
• US DOE Energy Saver: heat pump water heaters
• Which?: immersion heaters (power, heat time, running cost, diverters)
• BusinessTech: geyser electricity use in South Africa (sponsored content, 17 June 2024)
• Energy Saving Trust: solar panels (typical UK system size)

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