Why your inverter trips the moment the fridge starts
The battery has plenty of energy, yet the system shuts down the instant the compressor kicks in. That is not a fault — it is inrush current, and it never shows up in the watts on the nameplate.
The symptom: plenty of energy, and still nothing works
The typical story goes like this. Someone buys a 1000 Wh power station to keep a 120 W fridge running through an outage. The arithmetic looks obvious: 1000 divided by 120 is more than eight hours of runtime.
The fridge gets plugged in, the station runs for a moment, then shuts down with an overload message. The charge indicator still shows a nearly full battery. The energy is there, but the appliance is dead.
The reason has nothing to do with capacity. The inverter tripped before it had a chance to use any energy at all.
Three different quantities that are easy to confuse
A backup system is described by three numbers, and each one answers a different question.
Energy in watt-hours tells you how long the system will last. That is the figure printed on the power station in big digits.
Continuous power in watts tells you how many appliances can run at the same time. A 300 W inverter will not start a 2000 W kettle even if the battery holds 10 kWh.
The starting surge tells you whether the inverter can get a motor or a compressor moving at all. It lasts a fraction of a second and uses practically no energy — but it is what decides success or failure.
Mixing up the first quantity with the third is the most common reason a purchase goes wrong.
Where inrush current comes from
At the instant of starting, a motor is not spinning. It therefore produces no back EMF — the electromotive force that limits the current during normal running. For the first fractions of a second the winding behaves almost like a plain low-resistance resistor, so the current is many times the rated value.
Starting values differ so widely that no single multiplier works for everything. A fridge compressor typically starts at anywhere from a few hundred watts to over a kilowatt, and a water pump can be several times higher — but these are orders of magnitude, not sizing data for an inverter. Look for the starting current on the nameplate. The marking LRA is the locked-rotor current: a conservative upper guide, not a draw that will necessarily occur in your installation.
Equipment without a motor — lighting, a router, chargers, a heating element — has no surge in this sense. Switch-mode power supplies and LED drivers do have their own switch-on current, because at the first instant they charge their input capacitors. It is brief, but the peak can be high. With a few dozen luminaires or several power supplies switched on at once, check the manufacturer's data and the inverter's surge tolerance — do not assume that because it lasts milliseconds it does not matter.
How to work it out for a whole house
With a single load it is simple: the inverter has to withstand its surge. With several loads the question becomes whether to add up the surges of all of them.
The answer is not clear-cut. Summing every surge usually overstates the result, but assuming a single start is not automatically safe either — you have to know what controls those appliances. The probability that the fridge and the pump start in the same second is small. A sensible assumption is one start at a time, which means:
peak = largest single surge + continuous power of the remaining loads
That assumption, however, must be made knowingly, not taken for granted. If two appliances can start together — say a circulation pump and a borehole pump driven by the same controller — you either allow for both surges at once or stagger the starts in time. The single-surge result is then a lower bound, not the answer.
What to look for in the inverter datasheet
Manufacturers usually publish two numbers: continuous power and peak power, often labelled surge or peak. The second is often double the first, but how long it can be sustained varies between models by two orders of magnitude: from a fraction of a second to well over ten seconds.
Three things are worth checking. First, how long the peak power is declared for — a figure quoted for one second and one quoted for ten seconds are not the same thing. Second, whether it applies to a resistive load or a motor load. Third, what the rating in VA is, because appliances with motors have a power factor below one and load the inverter more than the watts alone suggest.
That last difference can be surprising: a 600 W load with a power factor of 0.45 demands about 1333 VA from the inverter.
What this means when you buy
When choosing a backup system, check three things in turn, not one.
Will the energy last through the assumed outage — that is the watt-hours question. Does the continuous power cover everything meant to run at the same time — that is the watts question. Does the peak power exceed the largest surge plus the rest of the load — that is the question that decides whether the system will start at all.
A system with a large battery and a weak inverter will shut down in the first second with its energy untouched. The opposite case — a strong inverter and a small battery — will start everything and go dark after fifteen minutes. Both situations come from comparing the wrong numbers.