How long will a backup power supply last during a power outage
How long will the backup power last during a power outage
The question "how long will it last" is absolutely crucial when it comes to backup power sources for boilers and circulation pumps - and at the same time the most frequently misunderstood. Many customers imagine that a backup power source (UPS) can power their boiler for several hours or even through the whole night. The reality is different and depends on a combination of several factors: battery capacity, power consumption of the appliance, type and condition of the battery, ambient temperature, and the way the boiler loads the battery during burner startup. In this article, we will practically, using real numbers and scenarios, go through everything that affects the runtime and how to calculate how many minutes or hours you can rely on a specific Avansa model.
If you are only dealing with the selection of a suitable model, we recommend reading the separate articles How to choose a backup power source for a boiler and circulation pump and What power of backup power source do I need for my boiler first - this text focuses specifically on the time dimension of backup power, i.e., how many minutes or hours you will actually get.
Why there is no single universal number
Manufacturers sometimes quote an approximate backup time for a specific test load (e.g., "30 minutes at 100 W"), but in practice, the boiler's consumption is variable - it differs between the idle state of the circulation pump, the burner ignition phase, and the full power phase. Therefore, it is much more accurate to calculate the actual consumption of your specific device than to rely on a single indicative number from the catalog. The runtime is not a fixed constant of the device, but the result of the relationship between battery capacity and current consumption.
Main factors affecting runtime
- Battery capacity (Ah) - the higher the capacity, the longer the theoretical runtime.
- Battery voltage (V) - in small home UPS systems it is usually 12 V.
- Actual power consumption of the appliance (W) - the sum of the boiler, circulation pump, and possibly also the control electronics and thermostatic regulation.
- Inverter efficiency - it is usually around 70-85 %, part of the energy is always lost when converting from direct current to alternating current.
- Ambient temperature - the battery delivers less energy at low temperatures (unheated boiler room, basement) than the manufacturer states at 20-25 °C.
- Age and condition of the battery - a lead-acid battery after 3-5 years of operation loses up to 20-40 % of its original capacity.
- Load characteristics - the peak load (starting the circulation pump, burner fan, ignition electrode) loads the power source significantly more than the average power consumption.
How the runtime is calculated in practice
The basic formula we use when recommending a model to customers looks like this:
Backup time (hours) = (Battery capacity [Ah] × Battery voltage [V] × Efficiency) / Appliance power consumption [W]
Efficiency is usually calculated with a coefficient of 0.7 (i.e., 70 %) for small sine wave and modified sine wave inverters, to account for losses in the inverter, cable losses, and battery aging. This is a conservative, but realistic approach - it is better to calculate with a shorter runtime in advance and be pleasantly surprised, than to rely on an optimistic number and in practice find out that the power source ran out twice as early.
Example calculation
Let's imagine a typical condensing gas boiler with a circulation pump, whose average power consumption in operation (pump + control electronics + occasional fan operation) is approximately 100 W. If we had an internal battery with a capacity of, for example, 7 Ah at 12 V, the calculation would look like this:
t = (7 × 12 × 0,7) / 100 = 58,8 / 100 = 0,588 hours ≈ 35 minutes
If instead of an internal 7 Ah battery we used an external battery with a capacity of 46 Ah (for example, in the model Avansa 500 W + battery 46Ah), the result would be completely different:
t = (46 × 12 × 0,7) / 100 = 386,4 / 100 = 3,86 hours ≈ almost 4 hours
This difference - from 35 minutes to almost 4 hours - shows why it is crucial to distinguish between sources with a small built-in battery intended for short outages (typically during a storm, until the generator starts or until the supply is restored) and solutions with an external battery of higher capacity, which are intended for longer planned or repeated outages.
Comparison of runtime at different loads
The runtime of the same power source changes significantly depending on what you connect to it. Below is an approximate table that calculates with an internal battery of average capacity (about 7-9 Ah in Avansa 300 W to 700 W models) and shows how the runtime changes with different actual consumption. These are model values for illustration of the principle - the exact parameters of a specific battery can always be found in the technical specifications of the product.
| Actual power consumption of the appliance | Approximate runtime at 7 Ah | Approximate runtime at 9 Ah |
|---|---|---|
| 50 W (circulation pump only) | ~70 minutes | ~90 minutes |
| 100 W (pump + boiler electronics) | ~35 minutes | ~45 minutes |
| 150 W (including burner fan startup) | ~23 minutes | ~30 minutes |
| 250 W (boiler with higher requirements, burner startup) | ~14 minutes | ~18 minutes |
From the table it is clear to see a simple, but often underestimated fact: doubling the consumption almost exactly halves the runtime. Therefore, it is worth measuring or finding out the actual consumption of your boiler and circulation pump before purchasing - we deal with this topic in detail in the article How to determine the consumption of your boiler and pump for backup power selection.
Effect of temperature on battery life
Most backup power sources for boilers are installed directly in the boiler room, technical room or basement - i.e. in a space that can get quite cold in winter, precisely during heating outages. Lead-acid batteries (AGM, gel and classic lead batteries) are sensitive to temperature and their actual capacity decreases at low temperatures.
At a temperature of around 20-25 °C, the manufacturer usually specifies the so-called nominal capacity. At a drop in temperature to 0 °C, the battery can deliver approximately 20-30 % less energy than its nominal value. At temperatures around -10 °C, the drop is even more pronounced and can reach up to 40-50 %. This means that the same source that lasts 40 minutes in summer may last only 20-25 minutes in a cold boiler room during January frost.
Practical consequence: if the backup power source is installed in an unheated space, it is reasonable to allow for a certain reserve - either by choosing a model with a higher capacity than would result from the calculation at room temperature, or by ensuring that the space with the UPS does not drop below freezing (for example, by placing it closer to heat distribution, not near an outside wall or garage door).
Real-life scenarios from practice
Over the years of selling and installing backup power sources, similar situations repeat themselves. We present several typical cases that we commonly encounter.
Scenario 1: Short-term outages in rural areas
A customer from an area with frequent but short outages (storms, faults on overhead lines, duration usually 5-20 minutes) only needs to bridge the boiler for this short period, so that the boiler does not have to be manually restarted and reignited after each outage. In this case, a smaller source with an internal battery is completely sufficient, for example Avansa 300 W or Avansa 500 W, since it is not expected to run for longer than half an hour. An investment in a large external battery would be unnecessary here.
Scenario 2: Cottage or building without regular monitoring
For recreational properties, where no one is present to manually restart the boiler after an outage, it is important that the power source lasts as long as possible without human intervention. Here we recommend a combination of higher power and greater capacity - for example Avansa 700 W (or Avansa 700 W) with an external battery, or directly the model Avansa 500 W + battery 46Ah, which can bridge several hours at normal boiler consumption.
Scenario 3: Boiler with higher inrush currents
Certain boilers, especially older ones with electromechanical ignition or with a more powerful flue gas fan, have a short-term peak draw of up to 300-400 W at startup, although the average operating consumption is only 80-120 W. In such a case, it is not enough to calculate only with the average power - the source must be able to handle the peak as well, otherwise the burner will overload and shut off at startup. In this case, it is appropriate to choose a power-overdimensioned model, for example Avansa 1050 W, which has sufficient reserve for short-term peaks and at the same time, when an external battery is connected, provides a longer runtime.
Scenario 4: Several devices on one source
We occasionally meet customers who want to connect not only the boiler with a circulation pump, but also the router to one backup power source, so that they have internet during an outage, or even the circulation pump of the floor heating in addition. Each additional device shortens the runtime proportionally to its power consumption - therefore it is important to calculate with the sum of all connected devices when planning, not just the boiler itself. A detailed procedure can be found in the article How to determine the boiler and pump consumption for selecting a backup power source.
How to extend the actual runtime
There are several practical steps that can be taken to increase the runtime of the backup power source without having to immediately buy a significantly larger (and more expensive) model:
- Limit connected devices to only the necessary ones - power only the boiler and circulation pump, not also other devices that are not necessary during an outage.
- Maintain the battery in good condition - regular voltage checks, topping up charging, replacement after the end of its life (usually 3-5 years with normal use). More in the article Maintenance and battery life of a backup power source.
- Place the source in a temperature-controlled space - avoid extreme cold, which reduces capacity.
- Choose a model with an external battery - if the probability of longer outages is higher, a compact internal battery simply does not suffice and an external battery of higher capacity must be connected, as in the case of the Avansa 500 W + battery 46Ah model.
- Check the actual power consumption of the boiler - instead of estimating, use a wattmeter or the data from the boiler and circulation pump label, so that the runtime calculation is based on real, not estimated, numbers.
Procedure for calculating your required runtime
When designing a suitable solution, we recommend proceeding in steps:
The first step is always to determine the actual power consumption of the appliance (boiler + pump), the second step is to decide how much time you really need to cover - whether it is a short outage during a storm (tens of minutes), or a planned shutdown or repeated longer outages (several hours). The third step is to plug in the formula and calculate the required capacity in ampere-hours, and the last step is to choose a specific model from the range - for low required capacity, a model with an internal battery is sufficient, while for higher capacity, it is appropriate to choose a version with an external battery.
Wiring diagram and its impact on runtime
The way the backup power supply is connected also affects the actual runtime - not directly through the physics of the battery, but indirectly through what else is connected to the power source. The recommended practice is to connect the UPS to a separate circuit that powers the boiler and circulation pump exclusively, not the entire electrical installation in the room.
This separate connection ensures that the battery will not unnecessarily power other appliances (lighting, sockets) during an outage, which would significantly shorten the runtime. The exact installation procedure, including recommendations for fusing and placement, can be found in the article Installation and connection of a backup power supply to a boiler.
What happens when the battery runs out
High-quality backup power supplies of the Avansa range have built-in protection against deep battery discharge. This means that the power supply will automatically disconnect before the voltage drops to a level that would permanently damage the battery (deep discharge shortens the life of lead-acid batteries much faster than normal, mild cycles). In practice, this means that the actual usable runtime is slightly shorter than would be calculated from the theoretical full capacity of the battery - and that is why our calculations include an efficiency factor of 0.7, which takes this reserve into account.
After the battery is depleted, the boiler simply turns off, just as if it had no backup power supply at all - there is no risk of damage, only the protection against operational interruption is lost. When the electricity supply is restored, the battery charges automatically and the power supply is ready for the next outage, usually within a few hours to one day, depending on the capacity and charging current.
Most common mistakes when estimating runtime
From our experience with complaints and customer inquiries, we know that the same mistakes are repeated when estimating how long the power supply will last:
- Calculating only with average power consumption without taking into account the starting current peaks when the burner starts.
- Forgetting that the internal battery of small models has limited capacity (in the order of units Ah) and is not intended for hours of operation.
- Not taking into account the temperature in the boiler room, especially in winter, when the probability of an outage (frost, snow, wind) is highest exactly when the battery is most weakened by the cold.
- Using an old, degraded battery and expecting the same runtime as with a new one - battery capacity decreases with age, even if it looks functional from the outside.
- Connecting multiple devices (router, lighting) to the same power supply without recalculating the total power consumption.
If you are also dealing with possible technical problems with the power supply, we recommend reading the article Common faults of backup power supplies for boilers, where typical symptoms of a weakening or damaged battery are discussed.
Comparison of Avansa models in terms of runtime
For orientation, we provide a simplified summary of how individual Avansa models are typically used in terms of required runtime. The exact technical parameters (power, capacity, dimensions) can always be found in the detail of the specific product; here it is a practical recommendation according to the purpose of deployment.
| Model | Typical use | Estimated runtime at normal boiler consumption |
|---|---|---|
| Avansa 300 W | small boilers, short outages | dozens of minutes |
| Avansa 500 W | standard boiler with pump | approx. 30-45 minutes |
| Avansa 700 W | boiler with higher starting currents | approx. 30-40 minutes |
| Avansa 1050 W | more power-hungry setups, reserve for peaks | approx. 25-35 minutes (without external battery) |
| Avansa 500 W + battery 46Ah | longer or repeated outages, cottages, unattended buildings | approx. 3-4 hours |
A more detailed comparison of performance parameters can be found in the article Comparison of Avansa backup power supplies by power, where the selection is based more on the required power than on runtime.
Common questions about backup power supply runtime
How exactly do I find out how many watts my boiler and pump consume?
The most reliable way is to look at the manufacturer's label on the boiler and circulation pump, where the rated power in watts is usually indicated. If the label is not readable or missing, a simple wattmeter can be used, connected between the socket and the appliance, which will show the actual current draw including peaks during startup. This topic is discussed in detail in the article How to determine the consumption of a boiler and pump for backup selection.
Will my backup power supply last the whole night during an outage?
With a standard model with an internal battery, this is usually not the case - the actual runtime is in the order of tens of minutes, not hours. If you need to cover several hours, you need to choose a model with an external battery of higher capacity, for example Avansa 500 W + battery 46Ah, or connect an even larger external battery.
Can I connect a larger external battery to the backup power supply than the one sold in the package?
For models that have a connector for an external battery, this is usually possible, but you must follow the recommended parameters for voltage and maximum charging / discharging current according to the documentation of the specific power supply. We recommend consulting a specific combination in advance to avoid overloading the charging circuit.
Why did my power supply last less than I expected according to the catalog data?
The most common reason is an older, partially degraded battery, lower temperature in the boiler room during the outage, or higher actual power consumption of the boiler during burner startup than expected. We recommend checking the age of the battery and the actual power consumption of the appliance according to the procedure above.
Does the number of outages per day affect battery life and thus future runtime?
Yes, each cycle of discharging and charging slightly wears out the battery. With very frequent outages (multiple times a day), the battery wears out faster than with occasional use, which will eventually result in shorter runtime. More information can be found in the article Battery maintenance and lifespan of a backup power supply.
Should I buy a more powerful power supply even if a smaller power is sufficient, just for longer runtime?
Not automatically - power (W) and runtime (capacity in Ah) are two different quantities. Higher power ensures that the power supply can handle the peak load during boiler startup, but the actual length of backup is mainly extended by a larger battery capacity, not by higher inverter power. Therefore, when requiring longer runtime, you should choose a model with an external battery, not automatically a model with higher wattage.
Summary
The runtime of a backup power supply during a power outage is not a fixed number, but the result of a calculation depending on the battery capacity, voltage, inverter efficiency, and especially the actual power consumption of your boiler and circulation pump. Standard models with an internal battery (such as Avansa 300 W, 500 W or 700 W) are designed to cover short outages lasting minutes to half an hour – ideal for storms or common grid faults. If you need to cover a longer period, in the order of hours, the solution is a model with an external battery of higher capacity, such as Avansa 500 W + battery 46Ah. Before purchasing, it is always worthwhile to plug in your own numbers into the formula, calculate with a reserve for battery aging and low temperatures in the boiler room, and in case of any uncertainties, also read the related articles in this Knowledge Centre – especially those on selecting power, battery capacity and correct installation.
Do you have a question on this topic?
Having trouble deciding or dealing with a specific situation in your home? Write to us – we will be happy to help.