RV Inverters: How to Size One and What It Costs You Idling

An inverter turns battery DC into household AC, and the two numbers that decide which one you buy are the continuous watt rating and the no-load draw. Neither is the number printed on the front of the box. Here is how to read the specs, using Victron’s published datasheets, and why the inverter most people buy is a size too big.

A note on the sources below: the Victron datasheets we cite are for the 230V European models, because those are the sheets Victron publishes with full derating and consumption tables. The output voltage is not the point. The efficiency, thermal derating, and idle-consumption behavior is the same topology you will find in a 120V unit, and the point is how to read those rows on any spec sheet.

VA is not watts

Inverter model numbers use VA, which is apparent power. What runs your appliance is watts, which is real power. Victron’s Phoenix Inverter datasheet lays both out side by side, and the gap is consistent:

Model	Continuous VA at 25C	Continuous W at 25C
C12/1200	1200 VA	1000 W
C12/1600	1600 VA	1300 W
C12/2000	2000 VA	1600 W
12/3000	3000 VA	2400 W
24/5000	5000 VA	4000 W

So a “1200 watt inverter” advertised on its VA number is a 1000W inverter. That is not a trick, it is a units convention, but it means the only fair comparison between two inverters is the watt row on both datasheets.

Rated power is a 25C number

This is the row people skip, and it is the row that explains why an inverter trips. Victron publishes continuous output at three temperatures. For the Phoenix C12/1200:

• 1000W at 25C
• 900W at 40C
• 600W at 65C

The 12/3000 follows the same shape: 2400W at 25C, 2200W at 40C, 1700W at 65C. Peak power is a separate and much larger figure, 2400W for the C12/1200 and 6000W for the 12/3000, but peak is a brief surge rating for motor startup, not something you run on.

Read that 65C row again. A “1200VA” inverter delivers 600W in a hot compartment, half its headline number. Victron’s units are fan cooled and rated to a 65C ambient, which is a genuinely good spec. The lesson is about placement: an inverter in a sealed basement bay behind a wall of gear, in Arizona in August, is operating somewhere on the wrong end of that curve. Ventilation buys you watts.

The idle draw nobody budgets for

An inverter converting nothing still consumes power. From the Victron datasheets, at 12V:

• Phoenix 12/3000: 20W at zero load. Search mode drops it to 8W.
• Inverter Smart 12/3000: 12W at zero load. ECO mode drops it to 1.5W.
• Inverter Smart 12/1600: 8W at zero load, 0.6W in ECO mode.

Twenty watts sounds like nothing. Left on for 24 hours it is 480Wh, which is a meaningful share of a 100Ah lithium battery before you have powered a single thing. Even the more efficient Inverter Smart at 12W is 288Wh a day.

Victron’s ECO mode is worth understanding rather than just enabling: the inverter drops to standby below a set load and wakes for a moment every 2.5 seconds to check whether anything has been switched on. It works well for occasional loads and badly for anything that draws a trickle, because a low-draw device can sit below the wake threshold.

The blunt version: switch the inverter off. It is a physical switch that saves more energy per dollar than any component upgrade on this site.

Efficiency, and what it does to your wiring

Victron lists maximum efficiency by system voltage, and the pattern is the important part:

• 12V: 92 to 93%
• 24V: 94%
• 48V: 94 to 96%

Higher system voltage is more efficient, and the reason shows up in your cables. Pulling 2400W out of a 12V system at 93% efficiency means the inverter is drawing roughly 2,580W from the battery, which is about 215 amps. That current requires very heavy cable over a very short run, and it is why large inverters are rare in 12V systems and normal in 48V ones. If you are designing from scratch and expect to run big AC loads, that decision is made at the battery bank, not at the inverter. Our lithium battery comparison covers the bank side.

Also note the input voltage window. The Inverter Smart 12V models accept 9.3V to 17V. That upper limit matters when a charge source is pushing an absorption voltage into the bus, and the lower limit is the cutoff that protects your battery from a deep discharge.

The unhelpful part: you may not need one

Inverters are oversold to RV buyers. Two honest observations.

Most 12V rigs run fine without much AC. Fridges, lights, fans, water pumps, and most charging is DC already. Every watt you push through an inverter pays a 7 to 8% conversion tax on the way out plus the idle draw on top. Running a device on 12V directly, where the DC version exists, beats running the AC version through an inverter every time. This is why we recommend powering a Starlink Mini off DC in our boondocking internet guide rather than through an inverter.

The load people buy a big inverter for is usually a microwave or an air conditioner, and neither is a good battery load. A microwave is brief enough to be plausible on a large bank. Air conditioning is not, and the honest answer for AC is usually a generator or shore power rather than an inverter big enough to attempt it. See our generator sizing tool for that decision.

If you want AC power without designing an electrical system, a portable power station is an inverter, battery, and charge controller in one box with the cabling already solved. Our power station comparison covers ten current units. It is a worse deal per watt-hour than components, and it is a much better deal in time and risk.

What to check before you buy

1. The continuous watt rating, not the VA number.
2. The derated watts at the temperature your install will actually see.
3. The zero-load draw, and whether the low-power standby mode fits how you use AC.
4. The input voltage range, against your battery chemistry and charge voltages.
5. Whether your cable run and system voltage can actually deliver the input current the inverter needs at full output.

For the panel-and-controller side of the same system, see solar panels and charge controllers. More power material lives at /rv-power/.