Battery backup (UPS) for a gas boiler: how to choose it, how long it lasts and how to install it
Why a gas boiler shuts down in a power cut, what kind of UPS it needs (pure sine wave, power rating, battery), how to work out how many hours you get, common installation problems and when a portable power station is the better choice.
In short
- Almost every modern gas boiler or furnace stops in a power cut: the pump, fan and ignition need electricity.
- Get a UPS designed for boilers, with pure sine wave output and an external deep-cycle battery.
- Quick math: a boiler typically draws 80–150 W. A 12 V, 100 Ah battery runs it for roughly 4–6 hours of continuous operation.
- Have an electrician install it: earthing is essential, and some boilers need neutral and earth bonded correctly to detect the flame.
Every winter, during the first long power cut, many people discover that their gas boiler stops with the lights, even though gas is still flowing. A dedicated UPS, installed correctly, keeps the heating on for hours or even a day, at a cost that’s small compared with frozen pipes or a cold house with small children. This guide is written mainly for wall-hung gas boilers common in Europe, but the same principles apply to gas furnaces.
Why the boiler stops
A modern wall-hung boiler needs electricity for:
- the circulation pump, which pushes hot water through the radiators;
- the fan, which removes combustion gases (on fan-assisted boilers);
- ignition, sensors and the control board.
Without power it won’t start. That’s a good thing: a boiler burning with no pump and no flue fan would be dangerous.
Your options
| Option | Pros | Cons |
|---|---|---|
| Boiler UPS + battery | Automatic switchover, install once, cheap per hour of runtime | Needs an electrician, battery replaced every few years |
| Portable power station | No installation, also runs the fridge, phones, lights | Pricier per kWh, runtime limited by capacity |
| Generator | Unlimited runtime while fuel lasts | Noise, outdoors only, carbon monoxide risk, needs a proper connection by an electrician |
For ordinary outages of a few hours, a dedicated UPS is the simplest answer. For multi-day outages, a UPS plus generator or a large power station covers more. For generators, see the separate guide.
Choosing the UPS
- Find the boiler’s consumption. Look for the electrical rating (in W) on the label inside the casing or in the manual. It’s usually 80 to 150 W.
- Choose a UPS rated 2–3 times higher. The pump has a start-up surge, and a UPS run at its limit overheats and wears out. For a 100 W boiler, 300–600 W is about right.
- Pure sine wave. Non-negotiable for a boiler.
- External battery. Boiler UPS units don’t have a built-in battery: you choose the capacity.
How many hours you get
The rough calculation:
hours ≈ (voltage × capacity × usable share × efficiency) ÷ boiler consumption
For a 12 V, 100 Ah AGM battery (1,200 Wh), using about half of it to keep it healthy, with a UPS efficiency of around 85%, and a 100 W boiler:
12 × 100 × 0.5 × 0.85 ÷ 100 ≈ 5 hours of continuous running.
In practice the boiler doesn’t run nonstop: the burner switches off when the house is warm and the pump may pause, so real runtime is often longer. Two identical batteries in parallel double it. With LiFePO4 you can use 80–90% of the capacity, not just half.
Installation
- Licensed electrician. The UPS goes between the boiler’s socket or circuit and the boiler. Ideally the boiler has its own circuit and dedicated outlet.
- Earthing is mandatory on both the input and output of the UPS.
- Neutral and earth: if the boiler shows a flame error on UPS power, this is usually why. The electrician fixes it following the manufacturers’ instructions.
- The battery: in a dry, ventilated place, not directly under the UPS, with short, thick cables and correct polarity.
- Test after installation: switch off the boiler’s breaker and check that it carries on running on battery.
Maintenance
- Test the system once a month, especially in the heating season
- Check the battery terminals for corrosion
- Replace AGM or gel batteries every 3–5 years, or when runtime drops noticeably
- Write the installation date on the battery
A boiler on UPS doesn’t replace the rest of your power outage preparation: you still need light, water and a power bank for your phone.
Recommended equipment
What to look for when buying. Links go to partner shops; if you buy through them we may earn a small commission at no extra cost to you.Details.
Pure sine wave UPS for heating systems, 300–600 W
Enough for most wall-hung gas boilers. Used with an external battery.
- Pure sine wave output
- Rated at least 2–3 times the boiler's consumption
- Adjustable battery charger and fast automatic switchover
Deep-cycle AGM or gel battery, 12 V 100 Ah
The battery itself: its capacity decides how many hours the boiler runs.
- Deep-cycle (for long discharges), not a car battery
- Sealed AGM or gel
- Recent manufacture date
LiFePO4 portable power station, 1,000 Wh or more
The no-installation option: runs the boiler and other things at home, and goes anywhere else you need it.
- Pure sine wave AC output
- UPS/EPS function with fast switchover
- LiFePO4 battery
Frequently asked questions
How much power does a gas boiler use?
The electrical rating is on the boiler's data plate and in the manual, usually 80 to 150 W while running: circulation pump, flue fan and electronics. Condensing boilers with modulating pumps often draw less. Don't confuse it with the heat output (24 kW, for example), which is something else entirely.
Why does it need a pure sine wave?
The boiler's electronics and pump motor are designed for mains power. Cheap 'modified sine' inverters can cause errors, noise, an overheating pump or even damage. UPS units sold for boilers are almost all pure sine wave, but check the spec sheet.
Which battery should I use?
A deep-cycle battery (AGM or gel) built for long discharges, not a car starter battery. For more runtime you can connect several in parallel, but they must be identical in type, capacity and age. LiFePO4 batteries last far longer, but check that the UPS supports them.
The boiler shows a flame error on UPS power. What now?
It's a known issue: many boilers sense the flame by ionization, which only works properly if neutral and earth are connected as they are on the grid. Some UPS units have a floating output. An electrician can fix it following the UPS and boiler instructions, for example with a neutral-earth bond on the output or an isolation transformer. Don't improvise.
Sources and further reading
Preper is for general education and does not replace instructions from your local authorities or advice from a doctor. In an emergency, call your local emergency number.