Inverter efficiency and standby draw
An inverter switched on but powering nothing still draws its idle current. On our NPS range that's 0.6A for the 700W, 1.1A for the 2200W and 1.5A for the 4500W, at 12V, which compounds to 14 to 36Ah a day, often the largest single load in a very small system. The fix costs nothing: turn the inverter off when it isn't working, or fit the remote switch panel so switching it off is convenient enough to actually happen.
Efficiency isn't one number. The 90 percent on the spec sheet is measured where the inverter runs best, around half to full load. At the light loads that make up most of real life, the inverter's own draw is a big share of the total. A 30W television through the 2200W NPS costs ~2.5A for the television and 1.1A for the inverter just being on: 3.6A from the battery, of which the television is 2.5, about 70 percent. This is the case against buying an inverter far bigger than you need, and the sizing guide makes the same point from the surge side: a unit matched to its usual load spends its life in the efficient part of its curve. But you need to also balance the lower efficiency of powering small loads with all the benefits of the increased headroom.
It's worth doing an overnight check once with a battery monitor: switch everything "off" and watch what still flows. The inverter's idle draw plus the small electronics that never sleep, TV standby, chargers left plugged in, detectors, add up, and in winter that phantom total can exceed a small system's whole solar generation. It's the classic diagnosis behind "the battery just dies overnight and nothing was on". Every phantom amp you find is an amp of solar you no longer need to generate.
The habits that follow: use the remote switch so the inverter is off between sessions; run DC-native appliances on DC rather than through the inverter if they are available and affordable, a 12V compressor fridge beats a mains fridge on an inverter every time, because it skips the conversion loss and the need to leave the inverter on all night; and group AC use into sessions rather than leaving the inverter on all day with nothing running. None of this is going without. It's just not paying the inverter's overhead for nothing.
More in our Inverters FAQ and Batteries FAQ, and the campervan page covers the whole van system.
Common questions
How much battery does an idle inverter use overnight?
At the 1.1A idle draw of a 2200W NPS, an inverter left on from dinner to breakfast, 14 hours, costs ~15Ah, and 26Ah over a full day. On a 100Ah bank that's a real slice of the usable capacity, spent converting nothing. The 700W idles at 0.6A, the 4500W at 1.5A.
Should I run my fridge through the inverter?
If a 12V compressor version exists that is affordable to you, run it on DC. You skip the conversion loss and you don't have to leave the inverter on 24 hours a day just for it. Mains-only appliances are what the inverter is for; DC-native loads should stay on DC.
Does a bigger inverter waste more power at idle?
Yes. Idle draw rises with size: 0.6A for the 700W NPS, 1.1A for the 2200W, 1.5A for the 4500W. That's the running cost of buying triple what you need for the future, paid every hour the inverter is on.