What power of backup power supply do I need for my boiler
Why the correct power of the backup power supply for the boiler is a key question
When a customer comes with the question "which backup power supply should I buy for my boiler," the first thing we ask them is not the boiler brand, but what exactly the power supply needs to power and for how long. The power of the backup power supply is a parameter that determines whether the device can even start up and maintain the operation of connected appliances — and the battery capacity then determines how long it can do so. These are two distinct quantities and are often confused in practice. A customer buys a power supply with a large battery but insufficient power, and the pump or boiler circulation pump simply won't start because the power supply cannot handle the inrush current. Or, conversely, they buy a power supply that is oversized in power but has low capacity, which lasts only 20 minutes, even though they needed to cover a three-hour power outage.
That is why in this article we will focus exclusively on power — that is, how many watts (W) or volt-amperes (VA) your boiler and circulation pump actually need, how to calculate this value, why you need to account for the peak load during startup, and how to choose a specific Avansa model so that the power supply operates reliably for one or even ten years. Topics such as battery capacity selection, backup duration, or installation are discussed in more detail in separate articles in this section; here we focus on the core issue — power.
What all the backup power supply powers in the boiler
The common perception is that a "boiler" is a single device with one power consumption. In reality, it is a system of several appliances that may or may not run simultaneously:
- Boiler control electronics — display, printed circuit board, fan in condensing boilers, ignition electrode. Power consumption is usually 50 to 150 W, and in gas condensing boilers with modulated fans, even more during burner startup.
- Heating circulation pump — modern electronically controlled pumps (e.g., Grundfos Alpha, Wilo Yonos) have a power consumption of 3 to 45 W in operation, but older asynchronous pumps are 60 to 90 W and during startup can draw 2 to 3 times the nominal power for a fraction of a second.
- Domestic hot water (DHW) circulation pump, if part of the system — another 20 to 60 W.
- Zonal valves and actuators in the heating manifold — units to tens of watts per piece, but they switch on sequentially, not all at once.
- Thermostat and control — negligible power consumption, in the range of a few watts.
The total of the nominal power consumption in a typical family house usually comes out to somewhere between 150 and 400 W in a steady state. The problem is that the "steady state" is not what the backup power supply must handle first — it must handle the moment of startup, when the motors of the pumps and fans start up simultaneously.
Apparent power, active power, and why it matters
When selecting a power supply, you will encounter two units — watt (W) and volt-ampere (VA). For resistive loads (e.g., a light bulb or resistive heating element), they are equal. For motors, such as circulation pumps and fans, this is not the case — these have an inductive character, and their apparent power (VA) is higher than the active power (W) by a value determined by the power factor (cos φ), which for small motors is around 0.6 to 0.8.
In practice, this means that a pump with a nominal power of 80 W actually "requests" 100 to 130 VA from the backup power supply. If the power supply is dimensioned on the edge, during motor startup — when the load is several times higher for 0.1 to 0.5 seconds — overload can occur, and the power supply may shut down for protection, or in the worst case, the waveform of the voltage (in cheaper inverters with modified sine wave) may cause the boiler electronics to not recognize the startup as proper power supply, and the boiler may go into a fault condition.
Why the type of voltage wave is important
High-quality UPS power supplies such as Avansa operate with a pure sine wave (Pure Sine Wave), which is especially important for condensing boilers with controlled fans and modern circulation pumps with electronic commutation (EC motors). These have their own electronics sensitive to the shape of the voltage waveform and, when powered by a cheap inverter with a square or trapezoidal wave, may report a fault, buzz loudly, or not start at all. This is one of the most common reasons for complaints when a customer buys a cheap "solar" inverter instead of a power supply specifically designed for boilers.
How to calculate the required power — step by step
The recommended procedure for selecting the power is as follows:
- Determine the nominal power of the boiler from the label or documentation (usually 80–150 W for gas condensing boilers, note — electric boilers are generally not included in UPS backup, as their power consumption is in kilowatts).
- Determine the nominal power of the circulation pump (on the pump label, typically 25–90 W).
- Add the power consumption of additional components — zonal valves, DHW pump, control (usually 10–40 W in total).
- Sum these values — you get the steady-state (operational) power consumption.
- Add a reserve for the startup current — we recommend at least 50–100% extra, and for older asynchronous pumps, even up to 150%.
- Compare the result with the rated (not peak) power of the backup power supply — the rated power of the power supply must be higher than your calculated steady-state power consumption, and the peak (peak) power of the power supply must cover the startup peak.
Practical example: A typical family house with a gas condensing boiler (power consumption 120 W in operation, up to 180 W during fan startup) and an electronic circulation pump Grundfos Alpha 2 (6–45 W). The total steady-state power consumption is approximately 150–165 W. With a reserve for peaks, we are around 300 W of required power supply. In this case, the Avansa 500 W is fully sufficient, as it has enough reserve even for the possible purchase of an additional appliance in the future.
Second example: An older house with a standard boiler and a classic asynchronous circulation pump (power consumption 80 W, startup current up to 200–250 W for a fraction of a second) plus a DHW pump (45 W) and two zone actuators (2×5 W). The steady-state total is approximately 135 W, but the startup peak when the pumps start simultaneously can briefly exceed 400–450 W. In this case, we recommend the Avansa 700 W, which has a comfortable reserve and will not operate at the edge of its capabilities, which prolongs its lifespan.
Connection diagram of the backup power supply to the boiler
Below is a simplified diagram of a typical connection — the backup power supply is connected between the electrical socket (230V grid) and the boiler with the circulation pump. In the event of a power outage, the power supply automatically switches to the battery in the order of milliseconds (typically within 10 ms), so the boiler does not even notice the transition.
It is important that the power source is connected directly to the socket from which the boiler and the pump are powered - not via an extension cable with a stabilizer or via a power strip with devices that do not belong there (refrigerator, TV). A detailed procedure for connecting and recommendations for electrical installation can be found in the article Installation and Connection of a Backup Power Source to the Boiler.
Power comparison in the Avansa range
The Avansa range offers several power levels precisely so that the source can be accurately "matched" to the actual consumption without unnecessary overdimensioning (which costs money) or underdimensioning (which causes outages and faults). Approximate overview:
| Model | Rated power | Suitable for |
|---|---|---|
| Avansa 300 W | 300 W | Small boiler without a circulation pump, only control electronics, boiler rooms with minimal consumption |
| Avansa 500 W | 500 W | Standard condensing boiler + 1 electronic circulation pump |
| Avansa 500 W + battery 46Ah | 500 W | Same application as above, but with the requirement for a longer backup time (extended battery) |
| Avansa 700 W | 700 W | Boiler + older asynchronous pump + TÚV pump + zone valves |
| Avansa 1050 W | 1050 W | Larger boiler rooms, multiple circulation pumps, combination with other backup devices (e.g. pool circulation pump, network router, alarm) |
A more detailed power comparison can be found in the article Comparison of Avansa backup power sources by power, where we also discuss the differences in battery capacity of individual models.
Why to calculate with a reserve - peak vs steady power
One of the most common technical mistakes is comparing only the steady power consumption of devices with the rated power of the source. In practice, it is also important to consider the so-called peak (peak) power that the source can provide for a short moment - usually a few seconds. High-quality Avansa sources typically have a peak power 1.5 to 2 times higher than the rated power, which is precisely the reserve that covers the motor start of the pump.
Below is a typical power consumption curve of a circulation pump at startup - a short peak lasting a fraction of a second to about 1-2 seconds, after which the consumption drops to a steady operating value.
If the source does not have sufficient peak reserve, it will either shut down in protective mode or cause a voltage drop, which the pump or boiler electronics will evaluate as a network fault. Therefore, we always recommend choosing one power category higher than the pure sum of the rated power - especially if you have an older pump without electronic regulation in the system.
Connection between power and battery capacity
Power and battery capacity are related, but they are not synonyms. Power (W) refers to how much energy the source can deliver at a given moment. Capacity (Ah or Wh) refers to how long it can do so. A source with high power but a small battery can reliably start even more power-hungry devices, but it will only cover the outage for a short time. Conversely, a source with a large battery but low power will last long during an outage, but may not be able to start devices with higher immediate power requirements.
In a standard configuration (boiler + 1 circulation pump, steady consumption of approx. 100-150 W), for example, Avansa 500 W with a standard battery will last approximately 2-4 hours, while the version Avansa 500 W + battery 46Ah will extend this time to approximately double due to the larger battery capacity at the same power. Precise calculations of endurance according to specific consumption can be found in the article How long will the backup power source last during a power outage and recommendations for choosing the capacity in the article What battery capacity is suitable for a circulation pump.
Practical scenarios from standard installation practice
Scenario 1: Apartment with a combined gas boiler
A small condensing boiler in an apartment, no additional pump (circulation is handled by an integrated pump in the boiler with a power of approx. 40-60 W). The steady power consumption of the entire boiler including electronics is around 90-120 W. In this case, even Avansa 300 W is often sufficient if the customer only wants to cover short outages (up to 30-60 minutes) and does not want to handle additional devices.
Scenario 2: Family house with floor and radiator heating
Two circuits, two circulation pumps, a mixing valve with a servomotor, a boiler with a fan. The sum of the steady power consumption is usually around 220-280 W here, and peaks during the simultaneous start of both pumps can temporarily exceed 500 W. In these cases, we recommend Avansa 700 W, which provides a comfortable reserve.
Scenario 3: House with an air-to-water heat pump as an additional source and a boiler as a backup
Here you need to be careful - the heat pump itself usually has a power consumption in the range of kilowatts and a standard backup power source of the Avansa range is not intended for its power supply. In such a case, we only back up the gas boiler and the circulation pumps of the heating circuit, not the heat pump compressor. This distinction is crucial and in practice it is the most common source of misunderstanding - the customer thinks that "backup for the boiler" will keep the entire heating system running including the heat pump, which technically is not possible with standard power of 500-1050 W.
Scenario 4: Boiler room with multiple zones and TÚV circulation
An apartment building or a larger family house with multiple heating zones, TÚV circulation pump, circulation pump and multiple servomotors. The total steady power consumption can exceed 350-400 W here and during the simultaneous start of multiple components even 700-800 W. Here, Avansa 1050 W is appropriate, which has sufficient reserve for the future as well, for example when expanding the system with an additional circuit.
Selection procedure - decision diagram
For quick orientation, we present a simplified procedure on how to reach the correct power category of the source.
Common mistakes in estimating power
- Ignoring the starting current - especially with older circulation pumps without electronic control, where the difference between the start-up and steady operation is the greatest.
- Forgetting the TÚV pump or circulation - the customer calculates only the heating circuit, but forgets that the same manifold also has a hot water pump.
- Trying to back up compressor-based devices - heat pumps, air conditioners, large pool circulation pumps - these belong to a completely different power category of backup power sources.
- Underestimating the quality of the voltage waveform - cheap inverters with a modified sine wave can cause fault messages on the boiler even with sufficient power.
- Connecting the power source via an extension cable with a stabilizer or surge protector, which distorts the measurement and can reduce the actually available power.
More about the faults that arise from these mistakes can be found in the article Common faults of backup power sources for boilers.
How to verify the actual consumption of your boiler and pump
If you are unsure about the exact power values, the most reliable method is:
- Look directly at the boiler's nameplate (usually on the inside of the cover) - the electrical power in watts is indicated there.
- Look at the circulation pump's nameplate - it is usually attached directly to the pump body, the value P1 (power) is the one that matters, not P2 (shaft power).
- If the labels are unreadable, use a wattmeter (energy meter) connected between the socket and the boiler, which will measure the actual consumption during normal operation and during the start-up of the circulation pump.
- If in doubt, contact a technician or boiler service, who has the technical documentation available.
We discuss this topic in detail in the article How to determine the consumption of your boiler and pump for choosing a backup power source, where you will also find a specific guide on how to use a wattmeter and a table of typical values according to boiler brands.
Recommendations according to the type of heating system
| Type of system | Estimated steady power | Recommended model |
|---|---|---|
| Apartment gas combined unit, without external pump | 60-120 W | Avansa 300 W |
| Family house, 1 heating circuit, electronic pump | 100-180 W | Avansa 500 W |
| Family house, requirement for longer backup during outages | 100-180 W | Avansa 500 W + battery 46Ah |
| House with 2 circuits, older pumps, TÚV pump | 180-300 W | Avansa 700 W |
| Larger boiler room, multiple zones, future reserve | 300-500 W | Avansa 1050 W |
Common questions about the power of a backup power source for a boiler
Is a power source with exactly the same power as the boiler sufficient?
We do not recommend it. Even if the boiler and pump power is lower than the power of the source, you need to account for the starting current when the circulation pump and fan start up, which can be significantly higher for a short time. Therefore, we always choose a model with a reserve of at least 50-100 % above the steady power.
Can I connect a router to the same power source so that my internet and WiFi thermostat work?
Yes, as long as it stays within the power reserve of the source. A router has a power consumption of about 5-15 W, which is negligible compared to the boiler's power, so with models of 500 W and higher, it is usually not a problem.
Is there a difference between powering a gas and an electric boiler?
Yes, it is essential. A gas boiler has an electrical power only for control, ignition and pumps - in the range of hundreds of watts. An electric boiler heats the water directly with electrical energy and its power is in kilowatts up to tens of kilowatts, which a standard backup power source like Avansa cannot cover. These types of backup power sources are intended for gas, solid fuel or pellet boilers, where electricity is used only for control and circulation, not for direct heating.
How do I know if the power source is undersized?
Typical signs are: the boiler throws an error when switching to the battery, the pump starts and immediately stops, the power source beeps and indicates overload, or the power source turns off shortly after the pump starts. If this happens, it is necessary to switch to a higher power model.
Is it worth always buying the highest power to be sure?
Not necessarily. Higher power usually means a higher price and sometimes larger device dimensions. The goal is to choose a power that has a reasonable reserve (50-100 % above the steady power), not an unnecessarily oversized power source that you would use only 10-20 % of its capacity.
Does the required power change if I buy another radiator or extend the heating circuit?
Adding a radiator does not increase the power of the electrical appliances - what changes is the thermal power of the heating, not the electrical power of the pump. The power of the circulation pump changes only if the type, speed or additional separate pump in the new circuit changes.
Conclusion
When choosing a backup power source for a boiler, power is the first and most important property to assess correctly - before even starting to deal with battery capacity and backup duration. The key is to add the actual power of all appliances (boiler, circulation pump for heating, possibly TÚV pump and actuators), add an appropriate reserve for starting peaks and then choose a specific model. For a typical family house with one modern circulation pump, Avansa 500 W is sufficient, for older pumps or multiple circuits we recommend Avansa 700 W, and for larger boiler rooms or future reserve requirements Avansa 1050 W. If you are unsure about the exact consumption values of your specific system, we are happy to help you choose - just send us the type of boiler and circulation pump and we will recommend a model with an appropriate power reserve.
Do you have a question about this topic?
Not sure or dealing with a specific situation in your household? Write to us - we are happy to advise.