Hot water at the touch of a button is taken for granted in many households. But how quickly does the water in the storage tank actually heat up when the heating element, heat pump, or gas boiler is running? We'll show you the most important factors and reveal how you can noticeably reduce the waiting time.

How to correctly calculate and shorten the heating time of a hot water storage tank

The heating time of a hot water storage tank determines how quickly you can draw hot water again after a long shower or a full bath. As the electrical engineering team at PVundSO, we receive this question almost daily, especially from customers with new PV systems and supplementary immersion heaters. In this guide, we explain the underlying physics, provide practical comparisons, and show you how to reduce waiting times in everyday life. You'll learn what power output an immersion heater should have, how hot water heat pumps compare, and why the classic legionella protection circuit is outdated under the latest standards.

What determines the heating time of the hot water storage tank

Four factors determine the heating time of a hot water storage tank in almost every household. The storage volume determines how much water needs to be heated. The desired final temperature and the starting temperature define the temperature difference, and the heating output of the heat source transfers the necessary energy to the water. Small heat losses through insulation and supply lines also occur.

Storage volume and water quantity

The larger the storage tank, the longer it takes to heat up at the same heating output. A two-person household usually manages with 80 to 120 liters, while families of four plan for 200 to 300 liters. The decisive factor is the actual hot water demand, not the purely theoretical size of the storage tank.

Desired final temperature

The standard temperature at the hot water outlet is 60 °C, which corresponds to the specifications of the DVGW worksheet W 551. Higher temperatures cost exponentially more energy and are only worthwhile in exceptional cases, such as very large circulation systems.

Heating power in watts or kilowatts

A typical electric immersion heater operates at 2 to 9 kW, while a hot water heat pump draws 300 to 800 watts. Gas and oil heating systems often achieve 15 to 25 kW thermal output, which correspondingly accelerates the heating process. Those who want to convert their surplus PV energy directly into hot water will find suitable options with the... MY PV DC ELWA (2 kW) an entry-level direct heating element for DC systems.

Heat loss and insulation

Even a well-insulated storage unit loses one to three kilowatt-hours per day through its surface. These standby losses slightly extend the heating time because the heating element constantly has to compensate for the losses.

Formula for calculating the heating time

Anyone wanting to calculate the heating time of a hot water storage tank will inevitably encounter a simple physical equation. It links the amount of water, the temperature difference, and the heating power to a clear result and can be performed with a calculator in just a few seconds.

Physical basis Q equals m times c times delta T

The core of every calculation is the amount of heat. This is derived from the mass of water in kilograms, the specific heat capacity of water (4,18 kJ/kg·K or 1,16 Wh/kg·K), and the temperature difference in Kelvin. A rule of thumb is that one liter of water requires 1,16 Wh to heat up by one Kelvin.

Example calculation for a 200 liter storage tank

A 200-liter storage tank needs to be heated from 10 to 60 °C. The temperature difference is 50 K, and the energy requirement is 200 × 50 × 1,16 Wh = 11.600 Wh or 11,6 kWh. With a 3 kW heating element, this takes approximately 3,9 hours; with a 2 kW element, the heating time is 5,8 hours.

Efficiency and safety margin in practice

In reality, you'll experience losses of around 10 percent because the storage tank loses heat through its walls. Therefore, you should factor in an additional time on top of the theoretical heating time. In our example, instead of 3,9 hours, you'll likely end up with 4,3 to 4,5 hours to reach the full target temperature.

Typical hot water storage tank heating time according to volume

The following table shows typical values ​​for hot water storage tank heating times for different tank sizes and heating types. All figures refer to heating from 10 to 60 °C, a practical standard value for tap water used for hot water preparation. Values ​​are rounded and include approximately a 10 percent allowance for losses.

Heating a 300-liter storage tank from 10 to 60 °C requires approximately 17,4 kWh of energy. With a 2 kW heating element, this equates to 8,5 hours of pure heating time; with a 9 kW heating element, it's only about 2 hours. A hot water heat pump like the Bosch CS7001i air-to-water heat pump Depending on the volume, it takes between 6 and 9 hours, but it works much more efficiently per kWh of hot water.

In our installations, we often see the 3,5 kW version as a good compromise between heating time and self-consumption of solar power. It heats up quickly enough for everyday use, but utilizes the solar power for a sufficient duration to convert it efficiently into heat, even with fluctuating solar irradiance.

Hot water heating time varies depending on the heating method

The type of heating system determines not only the time it takes to heat hot water, but also the operating costs over many years. We compare the four most common options and highlight typical strengths and weaknesses from practical experience.

Electric heating element

A conventional immersion heater converts electricity directly into heat. With a power output of 6 to 9 kW, it can heat a 300-liter storage tank in 45 to 90 minutes. However, using grid electricity at standard rates, this is the most expensive option per kilowatt-hour of hot water produced. Those who prefer to utilize surplus PV power will find more information in... PV heating element set 3,6 kW a turnkey solution without feeding power into the grid.

Gas and oil

Gas condensing boilers and oil-fired heating systems have thermal outputs of 15 to 25 kW and heat water very quickly. For households with existing central heating, this is often the standard. However, CO2 pricing and emissions regulations are increasingly making this solution less attractive.

Heat pump and hot water heat pump

A hot water heat pump draws only 300 to 800 watts of electricity, but delivers three to four parts heat from one part electricity. Heating takes longer; a Bosch CS7001i hot water module takes around 8 hours and 49 minutes to heat a 260-liter tank. However, operating costs are reduced to about a quarter.

PV heating element powered by solar energy

A PV heating element like the my-PV AC ELWA 2 or the my-PV AC-THOR It utilizes surplus solar power that would otherwise flow into the grid. It operates with stepless modulation and adapts to the current solar irradiance. In combination with a PV system, there are practically no running costs per kilowatt-hour of hot water. Those looking for a complete system including solar modules and an optional 200-liter storage tank will find what they need with the... Photovoltaic system for hot water preparation 2,6 kW with MYPV ELWA DC heating element and 6 solar modules A complete, ready-to-use set.

How the PV heating element affects the heating time

The PV heating element has been one of our most popular products in the hot water segment for years. In many installations, we see that it is particularly useful when the PV system produces large surpluses in the summer and there is no other suitable use for the energy.

How my-PV ELWA and AC-THOR work

The AC ELWA 2 It is a complete heating element with 3,5 kW output and integrated control, screwed directly into the storage tank flange. my-PV AC-THOR It is a power controller for conventional heating elements and processes up to 9 kW. Both measure the PV surplus at the house connection and adjust continuously.

Continuous modulation instead of simple on/off control

Conventional heating elements only know on and off. A modulating PV heating element also utilizes up to 800 watts of excess power, which a standard element couldn't process. This noticeably increases your PV system's self-consumption and strategically shifts heating to periods of maximum sunshine. You can find a suitable combination of solar panels and heating element in the category Photovoltaics with heating element.

Practical values ​​under varying levels of sunlight

On a clear summer day, 5 kWp of PV power heats a 300-liter storage tank to the target temperature in about 3 to 4 hours. In winter or on cloudy days, the excess power is often only sufficient for partial heating; conventional heating sources then take over. Our PV systems They are built with inverters that accurately detect and pass on the excess energy.

Tips for reducing the heating time of a hot water storage tank

Three adjustable settings can noticeably reduce the heating time of a hot water storage tank in everyday use. None of them cost much, and some even save electricity on an ongoing basis.

Properly insulate and insulate storage tanks

Proper insulation of the supply lines and an ErP A label for the storage tank help retain heat in the system for longer. This reduces the frequency with which the heating element needs to reheat, and the effective reheating time after a large draw-off is shortened. Pay particular attention to the flange insulation below the heating element.

Set the correct target temperature according to DVGW W 551

The standard specifies a minimum temperature of 60 °C at the storage tank outlet and 55 °C in the circulation loop. For smaller storage tanks under 400 liters without circulation, you can operate them at 55 to 60 °C using a thermostatic valve and regular flushing. Significantly higher temperatures unnecessarily prolong the heating process.

Dual storage as a clever combination

A dual-energy storage system has two heat exchangers. One is supplied by the heat pump, the other by the PV heating element or a solar thermal system. Under high solar irradiance, the most efficient source operates first, heating occurs in stages, and you make maximum use of your solar power. PV heating element set 3,6 kW It is particularly well suited as a second heat source in such a bivalent setup.

Quick checklist to take with you:

• Choose a moderate target temperature, not unnecessarily high.
• Check flange insulation and pipe insulation
• Control circulation according to demand instead of letting it run continuously.
• Use excess PV power primarily for hot water
• Plan for dual-purpose storage for larger households

How long will the hot water stay in the storage tank?

From a customer's perspective, it's not just the heating time that matters, but also the storage time. How long does the water stay usably warm before the heating element kicks in again? The answer lies in two factors: standby losses and insulation class.

Standby losses per day in kWh

Depending on the storage capacity and quality, losses range from 0,5 to 2 kilowatt-hours per day. For a 200-liter unit with good insulation, this equates to approximately 1 kWh, while with poorer insulation it can be up to 1,8 kWh. Good storage tanks can keep hot water at an acceptable temperature for up to two days.

Insulation class and ErP label

Since the ErP Directive, hot water storage tanks have been assigned efficiency classes from A+ to F. Class A or better means significantly lower standby losses over the years. When purchasing a new tank, the extra cost for better insulation often pays for itself after just two to three heating seasons.

A rule of thumb from practical experience is that storage systems older than 15 years often benefit from replacement. Insulation materials and manufacturing tolerances have improved significantly, and the savings in standby losses quickly pay for themselves.

Frequently asked questions about the heating time of hot water storage tanks

We frequently receive the following questions from customers. Detailed consultation with our electrical engineers is part of our service.

Why is the heating taking longer than calculated?

In practice, actual values ​​are often 10 to 20 percent higher than the theoretical calculation. Reasons for this include heat loss through the walls and flange, cold tap water below 10°C in winter, and temperature stratification within the storage tank. Measure at the upper sensor, not the lower one.

Can I generate hot water exclusively with solar power?

From March to October, this is usually possible with a suitably sized PV system and a modulating heating element. In winter, the yield is often insufficient, so a dual-energy storage system with a heat pump or gas as a backup is practical. A year-round option is the... Photovoltaic system for hot water preparation 2,6 kW with MYPV ELWA DC and 6 solar modules an, which is also optionally available with a 200-liter storage tank.

How large should the hot water storage tank be?

Plan for 30 to 50 liters of water per person per day. A two-person household will manage with 80 to 120 liters, a family of four with 200 to 300 liters. Higher quantities only prolong the heating time without providing any real benefit.

Do I need to include a legionella protection circuit in my plans?

The current DVGW guideline W 551 no longer recommends the classic weekly heating to 70 °C across the board. Crucially, a constant minimum temperature of 60 °C at the storage tank outlet is required for smaller systems, and continuous temperature maintenance is essential for large systems with circulation.

If you are unsure about sizing, standards or equipment selection, you will find help in our PV system checklist A free orientation guide. Alternatively, contact us directly; we will advise you personally.

Conclusion

The heating time of a hot water storage tank depends primarily on its volume, temperature difference, and heating output. Using the simple formula Q = m · c · ΔT and adding approximately 10 percent for heat losses, you can calculate realistic values ​​for your household. A hot water heat pump heats more slowly but saves three to four times the electricity compared to a simple immersion heater. PV immersion heaters like the my-PV AC ELWA 2 are most economical when combined with a dedicated solar thermal system. PV immersion heaters like the MY PV AC ELWA 2 or the MY PV DC ELWA in combination with your own solar thermal system. When choosing the temperature, refer to the 60 °C specified in DVGW worksheet W 551; this ensures your hot water remains hygienic and your heating time stays within a reasonable range.