Inverter sizing gets treated as a single number, but it is really three decisions: what continuous load you need to run, what surge the same equipment demands on start-up, and what the DC side, cable, fuse and battery bank, can actually deliver to support that. Get any one of the three wrong and the inverter either trips on the first kettle or drags the battery voltage down under load.
This guide sets out how to work through the sizing properly rather than picking a number that sounds safely large.
The continuous rating is what the inverter can supply indefinitely, and it is the figure to size against your normal running loads, a fridge, chargers, a coffee machine, cabin sockets. The surge rating, sometimes called peak, is what the unit can deliver for a few seconds and is only relevant to starting motors and compressors.
Motors and compressor-driven appliances, fridges, microwaves with a transformer, some washing machines, draw several times their running current for the first moment of start-up. A microwave rated 1000W might draw 1800 to 2000W for under a second on start. An inverter sized only to the continuous figure on the appliance label will trip repeatedly even though it never sees that load in normal operation.
An AC load is drawn from the battery at roughly ten times the current on the low-voltage side compared with the 230V side, once inverter efficiency is accounted for. A 3000W inverter running near its continuous rating can draw 250 to 300 amps DC. That current needs cable sized for both the amperage and the run length, because voltage drop over even a short run at that current is significant and shows up as the inverter cutting out under load despite the battery reading fine at rest.
As a working guide, inverters over 2000W need cable sized in the region of 50 to 70mm² for short runs, and the manufacturer's cable and terminal torque specification should be followed exactly rather than approximated. Undersized cable is the single most common cause of inverters shutting down under load on boats we are called to.
A DC fuse or class T fuse sized to the cable, not to the inverter's maximum draw, sits as close to the battery as practically possible. Its job is to protect the cable from a fault current, not to protect the inverter, so it needs rating against the cable's ampacity and the installation run.
A fuse mounted a metre or more from the battery leaves that length of cable unprotected against a short. On a lithium bank capable of very high fault currents, that unprotected length is a genuine fire risk, and it is one of the most common shortcuts we find on inspection.
A bigger inverter than the boat needs is not simply safety margin. Larger inverters draw more idle or standby current, which matters on a boat left on the mooring for days between visits, and they demand heavier cable, larger fuses and often a bigger battery bank to support the surge current without the bank voltage sagging.
Sizing against the actual simultaneous load, with a sensible margin for future additions, generally produces a better result than buying the largest unit that will fit the locker.
The battery bank has to sustain the surge current without its terminal voltage collapsing, which is a function of internal resistance as much as amp-hour capacity. Lithium banks generally cope with high surge current far better than AGM of similar capacity, which is one reason inverter upgrades and lithium conversions are often specified together.
If the inverter is added to an existing AGM bank sized for lighter loads, the bank itself, not just the cabling, may need reviewing before the inverter will perform as expected.
Check the microwave's actual power draw label, not just the cooking wattage, and size the inverter's continuous rating with margin above it, then confirm the surge rating clears the transformer inrush on start-up.
It is possible but adds complexity in wiring and load management. Most boats are better served by one correctly sized unit with a documented load list.
Yes. Efficiency losses, typically 10 to 15 percent, become heat and increase the DC current drawn for a given AC output, which feeds directly into cable and fuse sizing.
Soft-start devices on compressors can reduce surge draw meaningfully and sometimes allow a smaller inverter, but they need to be matched to the specific compressor rather than fitted as a generic fix.
That is a classic surge issue, the continuous rating is adequate but the inverter's surge capacity or the DC cable's ability to deliver that instantaneous current is not, and both are worth checking.
We size inverters against your actual load list and the boat's DC system, then supply, cable and fuse it correctly. Free on-board assessment.