How much power does my 3000 watt MSW inverter use when off but connected to the battery?

Welcome to the forum sdowney717,

You are correct, the filter capacitors on AC Inverter DC inputs can draw a pretty big spark when initially connected.

Many inverters document their power draw when On (no load), Search Mode (power up once a second to look for AC loads), and Off... Some are "OFF" with a switch or relay, others are "electronically" off and may still draw some current:

Just a random example:

https://www.solar-electric.com/lib/wind-sun/SureSine.pdf
And you are correct to worry about "vampire" loads (loads that daw 24x7 but you may not know they are there).

If you cannot find a specification for your inverter, you can use a DC Current Clamp DMM (digital multimeter) to see how much leakage current there may be:

https://www.amazon.com/gp/product/B00O1Q2HOQ (inexpensive, good enough for most uses)
https://www.amazon.com/gp/product/B019CY4FB4 (mid priced meter)

Note there are AC Current Clamp DMMs and AC+DC current clamp DMMs... The AC versions are very good units but don't work with DC current (may be confusing descriptions as they do measure AC+DC voltages, just not DC current).

Lets say you have a 200 AH @ 12 volt battery bank, and it sits for 6 months during the "off season". And you will accept a 10% discharge over that time:

• 200 AH * 0.10 discharge = 20 AH
• 20 AH * 1/24 hours per day * 1/180 days = 0.0046 amps = ~5 milliamp draw "acceptable to you" vampire load
• 0.0046 amps * 12 volts = 0.055 Watts

Very little current over 6 months will be an issue... And frankly, it is doubtful that a typical DC current clamp meter will even measure 5 mAmps or less accurately (the Klein can only display 60.00 amp range or 10 mA minimum).

The typical "answer" is to use a circuit breaker (or pull a fuse, when the inverter is "off") to ensure no unplanned discharging of your batter bank (inverter, other DC loads such as a radio, lights in a closet, etc. are forgotten/left on).

Other things to think about... Different batteries have self discharge... Flooded Cell Lead Acid--Need to be charged at least once a month. AGM once every 6 months. Lithium Ion--years.

Also hot batteries self discharge faster--Batteries setting at 95F in a hot RV--They will discharge 2x faster than those stored at room temperature...

If you have solar panels--They can keep up with self discharge (in general)... But some folks get bit by parking in shade/under roofs, or drifting snow in winter (solar charge controllers also have minimum current draw--And a few folks have had their cabin solar panels drifted over by snow, and the charge controller killed the bank).

-Bill