What battery capacity is suitable for a circulation pump
Why battery capacity is a key parameter of a backup power supply
When a customer is choosing between different models of backup power supplies for a boiler and circulation pump, they usually first ask about the power in watts. This is logical, as power determines whether the device is even able to start up and operate the connected load. Less attention is paid to the second, equally important parameter – battery capacity. It is precisely the capacity that determines how long the boiler and circulation pump will function during a power outage before the battery runs out. Power and capacity are closely related, but they are not interchangeable terms, and in practice I often see customers confusing them.
For boilers, a simple rule applies: the boiler itself (gas, electric, or solid fuel with ignition and control electronics) usually does not require high power – just tens of watts for the control electronics and fan, and possibly a short-term addition of a circulation pump with a power consumption of 40 to 90 W. The problem arises when the customer wants the heating system to function during longer power outages, which in rural areas often last 2 to 8 hours, and in worse cases even the whole day. In such cases, battery capacity becomes the limiting factor of the entire solution – even the most powerful backup power supply is useless without sufficient battery capacity, as it will only operate for a few tens of minutes.
Basic units: Ah, Wh and battery voltage
In order to be able to talk about the selection of capacity, it is necessary to clarify three basic quantities that appear in the technical specifications of backup power supplies:
- Ampere-hour (Ah) – indicates how much current the battery can supply over one hour. For example, a 46 Ah battery can theoretically supply 46 A for one hour, or 4.6 A for ten hours (in reality, this does not work linearly, we will get to that below).
- Voltage (V) – most backup power supplies for boilers use batteries with a nominal voltage of 12 V, while some larger units operate with 24 V or have multiple battery cells connected in series.
- Watt-hour (Wh) – the actual energy capacity, which is obtained by multiplying voltage and capacity in Ah. This is the most important value for practical calculation of runtime, as it directly indicates how much energy we have available.
The formula is simple: Wh = V × Ah. A 12 V / 46 Ah battery thus theoretically contains 552 Wh of energy. If the connected load (boiler + circulation pump) draws, for example, 80 W, the theoretical runtime would be 552 / 80 = 6.9 hours. In practice, it is less, as the efficiency of the inverter (from 12 V DC to 230 V AC) is around 85–92 %, and the battery should not be discharged to 0 %, but only to a certain safe limit.
How to realistically calculate the required battery capacity
When selecting capacity, I recommend following an exact sequence of steps that I also use when designing custom backup solutions for customers:
Step 1 – determine the actual power consumption of the appliances
The power consumption of the boiler can usually be found on the production label or in the technical documentation – the value is given in watts (W) or in current and voltage (e.g. 0.5 A at 230 V, which is approximately 115 W). The circulation pump typically has a power consumption between 20 and 90 W, depending on the power stage and type (classic three-stage versus electronically controlled with automatic regulation). If you are unsure of the total, we have dealt with this topic in more detail in the article How to determine the consumption of a boiler and pump for backup selection.
Step 2 – determine the required backup time
This is a subjective decision depending on the location. If you live in an area with short outages (up to 1 hour), a smaller capacity will suffice. If your area is prone to longer outages during storms, disasters, or planned outages of the distribution system, you should plan for a reserve of 4 to 8 hours, or even more in extreme cases.
Step 3 – calculate the required energy in Wh
Multiply the total power consumption (in watts) by the required backup time (in hours). Example: boiler and pump power consumption together 100 W, required runtime 5 hours → required energy 500 Wh.
Step 4 – add a reserve for losses and depth of discharge
Add a reserve of 25–40 % to the calculated value for inverter efficiency, battery aging, and the recommended maximum depth of discharge (Depth of Discharge). For lead-acid batteries (AGM, gel), it is recommended not to discharge below 50 % capacity to significantly avoid reducing the battery life. For lithium batteries (LiFePO4), it is safe to use 80–90 % of the capacity.
Step 5 – choose a model according to available capacity
Based on the calculated value, select a specific backup power supply model, or a combination with an external battery.
Practical example of calculation in two model scenarios
Scenario A – small apartment / house with a gas boiler and no large pump: The boiler with control electronics and a circulation pump has a total power consumption of about 70 W. The required runtime during an outage is 3 hours. Required energy: 70 × 3 = 210 Wh, with a 30 % reserve = about 273 Wh. At 12 V, this corresponds to a capacity of approximately 23 Ah. For such a scenario, a smaller backup power supply with a lower built-in capacity is sufficient, for example Avansa 300 W or Avansa 500 W, depending on the built-in battery of the model – always check the product's technical specifications.
Scenario B – family house with a larger boiler, two circulation pumps (heating circuit + domestic hot water circuit) and a requirement for longer backup: The total power consumption is about 150 W, the required runtime is 6 hours. Required energy: 150 × 6 = 900 Wh, with a 35 % reserve = about 1215 Wh. This corresponds to a capacity of about 100 Ah in a 12 V system. In such a case, it is worth choosing a model with higher power and larger or expandable battery, for example Avansa 1050 W, or a combination of Avansa 500 W + 46 Ah battery, which allows connecting an external battery and thus significantly extending the backup time beyond the built-in capacity.
How runtime changes with load – why it is not linear
A common mistake when estimating runtime is assuming that if a battery lasts three hours at a load of 50 W, it will last exactly half that time, i.e., 1.5 hours, at a load of 100 W. In reality, it is a bit different because at higher current draws, the effective usable content of the battery decreases (so-called Peukert effect in lead-acid batteries), and losses increase due to internal resistance and the inverter. At low loads (up to about 30 % of the inverter's rated power), the runtime is more favorable than a simple linear calculation would suggest; at high loads close to the inverter's maximum power, on the other hand, the runtime decreases faster than expected.
That is why it is important to always calculate battery capacity based on real, measured or documented power consumption from technical specifications, and not on a rough estimate. If you have doubts about the exact power consumption of your boiler + pump setup, I recommend using a wattmeter (socket power meter), which for a few dozen euros will provide accurate numbers directly from real operation, including startup peaks.
Types of batteries used in backup power supplies and their impact on capacity selection
Capacity in Ah alone does not tell the whole story – equally important is the type of battery chemistry, as it affects how much of the stated capacity you can safely and repeatedly use in practice.
| Battery type | Safe depth of discharge (DoD) | Lifespan (cycles) | Note |
|---|---|---|---|
| Lead-acid AGM | 50 % | 300 – 500 | Most common in cost-effective backup power supplies |
| Gel | 50 – 60 % | 400 – 600 | More resistant to deep discharge than standard AGM |
| Lithium LiFePO4 | 80 – 90 % | 2000 – 4000 | Higher purchase price, significantly longer lifespan and lower weight |
This means that two batteries with the same stated capacity (e.g., 46 Ah) do not actually provide the same usable energy volume in practice if they are of different chemistries. With a 46 Ah lead-acid AGM battery, you should expect a safely usable capacity of about 23 Ah (50 % DoD), while with a lithium battery of the same nominal capacity, you could safely use up to 37 – 41 Ah. When designing a backup system, it is therefore always necessary to look not only at the labeled Ah value, but also at the recommended depth of discharge for the specific type of battery.
Connection diagram of the backup power supply to the boiler and circulation pump
To better understand how battery capacity fits into the entire system, it is useful to see a simplified diagram of energy flow. The backup power supply is connected to the electrical grid in normal operation, from which it charges the internal (or external) battery and at the same time passes the voltage to the boiler and pump through itself. In the event of a power outage, the device automatically (within a few milliseconds, so-called no-break – offline/line-interactive type) switches to power supply from the battery via the DC/AC inverter.
A detailed installation and connection procedure, including recommendations for placing the power supply and cable lengths, can be found in the article "Installation and connection of a backup power supply to a boiler." Here, it is important to emphasize only one thing directly related to battery capacity: long or undersized supply cables between the battery and the inverter increase losses and can actually reduce the effective usable capacity, so it is worthwhile to follow the manufacturer's recommendations regarding wire cross-sections, especially for external batteries with higher capacity.
When a smaller capacity is sufficient and when it is worth investing in a larger one
From practice, I see several typical customer groups and their real needs:
Short-term outage bridging (up to 1 hour)
If power outages in your area usually last only a few minutes to tens of minutes (typically when switching to backup grid lines), a smaller battery capacity is sufficient. The goal here is simply to bridge a short outage without the boiler entering a fault state and requiring manual restart. For this purpose, smaller models such as Avansa 300 W or Avansa 500 W are fully sufficient.
Medium-length outages (2 – 5 hours)
This is the most common requirement for customers outside large cities, where longer outages occasionally occur during storms or line faults. In this case, it is necessary to plan for a larger battery capacity, or a model that allows the connection of an external battery, such as Avansa 700 W or directly Avansa 500 W + 46Ah battery.
Long outages (6 hours or more) or critical operation
For cottages, chalets, or facilities where freezing of the system would cause significant damage, or for locations with a weaker distribution grid, it is advisable to design a system with higher capacity and the possibility of connecting one or more external batteries simultaneously. In this case, the model Avansa 1050 W is recommended, which has sufficient power reserve to operate the boiler, multiple pumps, and possibly also the circulation pump of a solar system simultaneously.
Impact of battery age and condition on real capacity
The nominal capacity stated on the battery label applies only to a new battery under standard test conditions (usually at 20 °C and with a ten-hour or twenty-hour discharge current). In real operation, capacity changes due to several factors:
- Age of the battery – lead-acid batteries gradually lose capacity; after 2–3 years of operation, their actual capacity may be only 70–80% of the original value, and after 4–5 years it may be less than 60%.
- Environmental temperature – at low temperatures (below 5 °C), the usable capacity of the battery drops significantly; at freezing temperatures below -10 °C, it can decrease by 20–30%. Therefore, it is not recommended to place the backup power supply in an unheated garage or outdoor area.
- Number of discharge cycles – each deep discharge shortens the battery's lifespan. If the battery is regularly discharged below the recommended limit, its capacity decreases more rapidly.
- Charging and maintenance method – incorrectly set charging voltage or long-term storage of the battery in a partially discharged state (sulfation) significantly reduces the actual capacity.
Therefore, when designing a backup power supply, I recommend calculating with so-called construction reserve – if the required capacity comes out to 40 Ah, it is reasonable to choose a power source or battery with a capacity of at least 46–50 Ah, so that the battery can still cover the planned backup time after two or three years of operation. More on how to extend the battery life and keep it in good condition can be found in the article Maintenance and Battery Life of a Backup Power Supply.
Practical experience with customer orders – what is most often underestimated
Over the years of supplying backup power supplies for boilers, similar situations keep repeating, worth mentioning:
Underestimating the total power consumption. A customer calculates only the boiler, but forgets that the circulation pump for heating is running at the same time, and possibly a second pump for the hot water circuit or floor heating. The combined power of two pumps at 60 W each can mean an additional 120 W, which is higher than originally calculated and significantly shortens the actual runtime during longer outages.
Forgetting about the inrush current. Circulation pumps, and especially those with asynchronous motors, have a short-term higher draw at startup than their rated power (sometimes 2–3 times higher, for a fraction of a second). This does not directly affect the battery capacity needed for long-term runtime, but it does affect which power (not capacity) model should be chosen to handle the startup without overloading.
Incorrect estimation of the outage duration in a given location. Many customers base their calculations on an average, but in reality, their area experiences repeated longer outages (for example, due to overhead lines in wooded terrain, where falling branches cause faults). I recommend increasing the capacity by one step higher during design rather than relying on an optimistic estimate.
Combining old and new batteries. When expanding capacity by connecting an external battery to an already used power supply, it occasionally happens that a customer combines an older used battery with a new one. Batteries of different ages and conditions should not be combined in a single parallel connection, as the older battery with lower internal capacity can be overcharged or unevenly loaded, which shortens the life of both units.
How to verify the capacity after installation
After installing the backup power supply, I recommend performing a simple test of the actual runtime – disconnect the power supply from the grid (simulate an outage) during normal boiler and pump operation and measure how long it takes before the device signals low battery or turns off. This test should be repeated once a year, as it provides a real picture of the battery's current condition and early warning of its degradation before a real long outage occurs in winter. Common faults that can be detected during such a test are thoroughly discussed in the article Common Faults of Backup Power Supplies for Boilers.
Summary of recommendations by household type
| Type of installation | Estimated power consumption | Recommended backup time | Estimated required capacity |
|---|---|---|---|
| Apartment, small boiler, 1 pump | 50 – 80 W | 1 – 3 h | 15 – 25 Ah (12 V) |
| Family house, standard boiler | 80 – 120 W | 3 – 5 h | 30 – 50 Ah (12 V) |
| Family house, 2 circuits (heating + DHW) | 120 – 180 W | 5 – 8 h | 60 – 100 Ah (12 V) |
| Cottage/critical operation, long outages | 150 – 250 W | 8 h and more | 100 Ah and more, possibility of expansion |
This table is only an estimate – I always recommend calculating the exact values based on the actual power consumption of your boiler and pumps, as described above. If you are unsure which specific model to choose, our comparison in the article Comparison of Avansa Backup Power Supplies by Power or the general guide How to Choose a Backup Power Supply for a Boiler and Circulation Pump can also help.
Common questions about the battery capacity of a backup power supply
Is it better to have higher power (W) or higher capacity (Ah)?
Both parameters address different issues. Power (W) determines whether the device can even start and maintain the connected load, including the inrush current. Capacity (Ah, or Wh) determines how long this operation can last. You need both parameters correctly set at the same time – high power with low capacity will allow you to power even more demanding devices for a short time, but only briefly.
Can battery capacity be increased later?
For models that allow it structurally, yes – by connecting an external battery with the appropriate voltage and compatible terminals, as is the case with the model Avansa 500 W + battery 46Ah. For power supplies with a fixed, non-expandable built-in battery, this is not possible, and you need to choose a larger model right away.
How long will a backup power supply with a 46 Ah capacity last in reality?
With a typical boiler and one circulation pump load of around 80–100 W and a safe depth of discharge of 50% (lead-acid battery), the actual runtime is approximately 2.5 to 3.5 hours. The exact time depends on the inverter efficiency, battery age, and ambient temperature. A more detailed calculation and real measured values can be found in the article How Long Will a Backup Power Supply Last During a Power Outage.
Is it worth buying a larger capacity "as a reserve," even if I don't need it right now?
Yes, if the budget allows. Considering that battery capacity naturally decreases with age (typically by 20–30% over 2–3 years for lead-acid types), a slightly oversized capacity ensures that the power supply will still perform its function after a few years of operation without the need for immediate battery replacement.
Do lithium batteries always have a higher capacity than lead-acid batteries?
Not necessarily at the same stated Ah – the difference is not in the raw capacity, but in how much of it can be safely and repeatedly used. A lithium battery with the same rated capacity as a lead-acid battery will provide more usable energy in practice, as it can tolerate deeper discharge without significantly affecting its lifespan.
What happens if I choose a battery with too low capacity?
The device will work properly while connected to the grid and shortly after an outage, but during a longer outage, the battery will discharge before the power is restored, and the boiler and pump will turn off. In some boiler types, this may require manual restart or, in the worst case during freezing weather, pose a risk of system freezing if the outage lasts significantly longer than the backup capacity allows.
Conclusion
Selecting the correct battery capacity for a boiler and circulation pump backup power supply is not a matter of chance or a rough guess that "it should be big enough" – it is a specific technical calculation based on the actual power consumption of your appliances, the required backup time, and the type of battery used. The recommended procedure is always the same: determine the power consumption, define the required time, calculate the necessary energy in Wh, add a reserve for losses and battery aging, and then choose a specific model. Do not forget that the capacity stated on the label is only a theoretical value under ideal conditions – the actual usable capacity is lower and gradually decreases with battery age. For higher backup time requirements, it is worth choosing a model with the option to connect an external battery, which provides room for future expansion without the need to replace the entire device. If you are unsure about the specific numbers in your household, do not hesitate to use other articles from the Knowledge Center, which discuss power selection, installation, maintenance, and troubleshooting of backup power supplies in greater depth.
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