Let’s continue the currently relevant topic of energy resilience during frequent power outages. In this article we’ll look at a typical case: an apartment with individual heating provided by a double-circuit gas boiler (for example, a Protherm Gepard).
A boiler needs electricity to operate — mainly to power the control board and the circulation pump. Typical power consumption is within 100–140 W (depending on the operating mode).
Goal: keep the boiler running for at least 8–10 hours, taking into account an outage schedule such as 3.5 hours ON / 8.5 hours OFF.
Let’s take the worst case: 140 W and 10 hours of autonomy.
Energy consumed by the boiler over 10 hours:
140 W × 10 h = 1400 Wh
Next, we need to account for UPS/inverter conversion losses (typically 10–15%) as well as internal consumption. Let’s assume 15% losses:
1400 × 0.15 = 210 Wh
Total required energy including losses:
1400 + 210 = 1610 Wh
Conclusion: to run the boiler for 10 hours at 140 W you need about 1.6 kWh of stored energy.
At first glance, since the boiler needs only 140 W, a compact UPS (for example 360 W) plus a battery might seem sufficient. However, with an outage pattern of 8.5 hours without power / 3.5 hours with power, the key issue is not only discharge, but also whether the system can recharge the battery during the short grid-available window.
Assume a 12V UPS can charge the battery at 20 A.
Charging power:
12 V × 20 A = 240 W
Energy that can be “put back” into the battery in 3.5 hours:
240 W × 3.5 h = 840 Wh
We can see that 840 Wh < 1610 Wh, meaning the grid-available 3.5 hours cannot compensate for the energy spent. With each outage cycle, the battery will be depleted further.
Now consider a 24V system with the same charging current of 20 A.
Charging power:
24 V × 20 A = 480 W
Charging energy over 3.5 hours:
480 W × 3.5 h = 1680 Wh
Conclusion: a 24V system at the same 20 A provides nearly the required energy recovery and can compensate for the boiler’s consumption.
Another advantage of 24V is lower discharge current, less voltage sag, lower cable losses, and more stable UPS operation.
We start with the calculated energy including losses: 1610 Wh.
Add a 40% technical reserve to account for: measurement inaccuracies (140 W may not be constant), peak operating modes, battery aging, incomplete recovery during 3.5 hours, and internal UPS consumption.
1610 × 1.4 = 2254 Wh
Convert to amp-hours for a 24V system:
2254 / 24 ≈ 94 Ah
Since 94 Ah batteries are uncommon, a practical choice is the nearest standard size: 24V 100 Ah (or 105 Ah).
For “frequent charge/discharge cycles”, the optimal choice is a lithium iron phosphate battery (LiFePO4) because it:
Important: LiFePO4 must not be charged below 0°C. In an apartment environment this is typically not an issue.
These devices are designed for 220V backup and typically provide: pure sine wave output, fast switching (usually 4–10 ms), a built-in charger, and support for an external battery.
UPS advantages for an apartment:
UPS disadvantages:
Hybrid inverters are usually more powerful and feature-rich.
Hybrid inverter advantages:
Hybrid inverter disadvantages in an apartment:
Final recommendation:
This is why it makes sense to look for a proper UPS/inverter. One possible option: High-frequency inverter 1500VA/1.5kW Kraft KRF-SAVR1500VA/1.5kW-24V
Key specifications: