Is it enough to back up only one pump out of two?

That depends on the alternating-duty scheme and the flow requirements. The engineer must confirm that one pump covers the required flow and that the automation works correctly in that mode.

Why is it important to know the pump’s run hours?

Run hours show how much the pump actually runs per day and give a basis for the energy calculation. Without this data the estimate will be conservative and may overstate the required battery capacity.

Backup Power for Pumps and Building Water Supply

Author: Svitlo-Lux editorial team. Published 2026-10-23, updated 2026-10-23. Review (engineering): approved (Project owner).

Without electricity the pumping station stops, and water disappears first on the upper floors. Backing up a pumping station is harder than it seems: pumps run in cycles, have an inrush current, often alternate with each other, and their daily energy depends on water consumption that nobody has measured.

What exactly stops working

Depending on the building’s scheme, a power loss removes pressure in the risers and stops booster pumps, the pressure booster station or hot water circulation pumps. Residents of lower floors may have water by inertia for several hours, residents of upper floors almost immediately. So the question “how many hours of backup runtime” is better formulated for water through the number of floors and the reserve of water in the network and tanks.

Types of building pump equipment and backup specifics
Unit	How it works	What matters for backup
Cold water booster station	Pumps switch on by pressure or flow, often in turns	The start of each motor; whether there is a soft starter or a frequency converter
Pressure booster set with tank	Cyclic switching, the tank smooths consumption	Fewer running hours but a sharp start; tank size matters
Hot water circulation pumps	Run continuously, low power	Small power, long operation; a comfort matter
Drainage and sewage pumps	Switch on by level	Rarely run, but their stopping can cause a flood in the basement

Duty cycles: why daily energy is unknown

A pump does not run continuously. How often it switches on depends on how much water residents use, and during an outage that use changes: people stock up on water, and during a long outage the evening and morning loads differ sharply. So the pump’s duty factor in a long outage is hard to predict, and it is best measured with a run-hour meter or an hour counter, if one is provided.

Practical advice: ask the maintenance company to record the pump’s run-hour meter readings over a week or to observe the cycles. Even a rough estimate is better than an invented one. If there is no data, the calculator uses conservative values and shows that they are unconfirmed.

Illustrative example (hypothetical inputs)

Suppose there are two pump units with a shaft power of 3 kW each, running in turns, so at any moment one is leading. Assume a motor efficiency of 0.85, cosφ of 0.8, a start that is three times the running apparent power, a starting safety factor of 1.25 and a backup window of 8 hours at a duty factor of 0.4. Then the running apparent power of one pump is about 4.4 kVA, of both about 8.8 kVA, and the peak for the scenario “one starts while the other runs” is about 17.6 kVA, or roughly 22 kVA with the margin. The electrical power of one pump is about 3.5 kW, and the energy over the window is about 11.3 kWh.

Next, using the methodology’s formulas: with a 20 percent reserve and an inverter efficiency of 0.92, the usable energy approaches 14.7 kWh, and the nominal capacity, with a permitted depth of discharge of 0.8, a temperature loss of 10 percent and a 20 percent ageing margin, exceeds 25 kWh. This is only an illustration of how the calculation proceeds. The real number depends on your data and requires an engineering calculation.

A water reserve as part of the solution

Sometimes it is wiser not to buy more batteries but to change the mode. A storage tank at an upper level, a tank or a gravity scheme reduces the number of pump starts. This is a hydraulics question and goes beyond electrical backup, but it affects the size of the backup. Ask the design engineer whether electrical backup can be combined with a buffer tank.

How many floors the station serves and what the minimum required flow is.
Whether there is a tank that supplies water without the pump running, and for how long.
Whether the operating mode can be reduced during an outage, for example by supplying water on a schedule.
Whether there is a soft starter or frequency converter that lowers the start.

Phases and pump protection

Pressure booster pumps are usually three-phase. The backup source must provide the appropriate voltage and withstand the start without faults, otherwise the motor protection will trip the pump exactly when it is needed. In addition, the supply must be coordinated with the alternating-duty scheme so that after changeover the station does not start all pumps at once. Protection settings are made by a specialist; this page does not describe them.

Typical mistakes

Adding up the rated power of all pumps, although usually one or two are running.
Ignoring the start and sizing the inverter for the running mode.
Taking daily energy from assumptions about an “average resident” instead of run-hour readings.
Backing up the pumps but forgetting drainage and fire pumps, if there are any.
Forgetting the automation and pressure sensors, which also need power.

What to ask the engineer for

Before approving the water supply solution

Pump datasheets: shaft power, efficiency, starting method.
The alternating-duty and cascade scheme and how it behaves after power is restored.
An estimate of the start and peak with the assumptions stated.
An estimate of the water reserve and the supply mode during an outage.
A description of what powers the pumping station’s automation: controllers, sensors, communications.

Frequently asked questions

Is it enough to back up only one pump out of two?: That depends on the alternating-duty scheme and the flow requirements. The engineer must confirm that one pump covers the required flow and that the automation works correctly in that mode.
Why is it important to know the pump’s run hours?: Run hours show how much the pump actually runs per day and give a basis for the energy calculation. Without this data the estimate will be conservative and may overstate the required battery capacity.

Battery capacity calculator Preliminary backup power calculation

This material is for general information only and is not a power supply design, legal or financial advice. Calculations are indicative; installation and protection selection must be done by qualified specialists only. The platform does not accept or hold funds.