Surge Power and Motor Loads: Why a Battery System Can Trip on Startup
- Continuous power is what an inverter can supply indefinitely; surge power is a short burst, usually for seconds.
- Motors such as pumps and compressors draw a large start-up current.
- A start-up surge above the inverter's short-term rating can cause a trip or shutdown.
- A bigger battery adds runtime, not surge capability.
- Ask installers about peak ratings and how they are specified.
A battery can hold plenty of energy and still fail to start a motor. The reason is surge power: the brief burst of current a motor draws when it starts, which can be several times what it uses once running.
Continuous power versus surge power
An inverter has a continuous rating, the output it can sustain, and a higher short-duration rating for peaks. A guide from the inverter maker Sol-Ark describes surge power as a short burst, lasting a few seconds, that covers motor start-up, and says outage trips usually occur when the start-up surge exceeds the short-duration rating, not because the steady load is too high (Sol-Ark: sizing a hybrid inverter for surge power).
This is a US manufacturer's guide, and some examples refer to large American air conditioners, so treat the numbers as illustrative. The principle applies to any inverter.
Why motors draw so much at start
A motor that is stationary has to overcome inertia, and until it spins up, it draws a high current, called inrush or locked-rotor current. The same guide cites the US Department of Energy as saying motor starting current can be two to three times running current, with examples of roughly three times for a large air conditioner and two to three times for a well pump.
In the UK, the loads most likely to matter are compressors and pumps: fridge freezers, boiler or heating circulation pumps, heat pump compressors, and in some homes, borehole pumps. Most refrigerators draw modest running power, so their surges are small in absolute terms, but they add to other loads at the same moment.
What happens during a power cut
When the grid is connected, it absorbs start-up spikes without anyone noticing. When the house is running on the battery, the inverter must supply all of it. The Sol-Ark guide says that if surge capacity is exceeded, the inverter may reduce voltage, drop frequency or shut down, which it describes as protecting the power electronics rather than a failure.
It also makes a point worth remembering: adding batteries extends runtime but does not increase surge capability. If your priority is running a motor during an outage, look at the inverter's peak rating, not the battery's kWh. Our capacity guides cover energy; the power side is on the datasheet.
Good to know
Heat pumps with variable-speed compressors typically start more gently than fixed-speed ones, but check the specifics with the heat pump maker if you plan to back one up.
How to check before you buy
Work through these steps with your installer, especially if you want backup for circuits that include a motor.
Ask for the inverter's backup output in two numbers: continuous and peak, and for how long the peak can be held. Ask which circuits the backup supplies and what is on them. And ask whether a soft starter or variable-speed drive on the motor is possible; the guide notes these can reduce inrush by 30 to 60%, though the benefit varies by equipment.
- List the motors on the backed-up circuits, such as pumps and compressors.
- Find their running power on the label and ask the installer to confirm the start-up allowance.
- Leave margin rather than sizing to the exact peak rating.
- Do not assume the battery's headline kW covers the surge.
Common questions
Will a bigger battery fix tripping?
Usually not. Tripping is about the instantaneous power the inverter can supply, not stored energy. Check the peak rating.
Does this apply when the grid is on?
Normally not, because the grid supplies the start-up current. The issue arises when the house runs on the battery.
Can I back up a heat pump?
It depends on the heat pump and the system. Ask the installer to check the surge requirement and the circuit, and ask for it in writing.
How long can an inverter hold its peak output?
It varies by product and is usually a matter of seconds. The datasheet or the maker should state it.
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