How inverter load affects 12V battery draw

[GallenH]

If you have an inverter is the inverter's draw on the 12v battery constant? IOW: If you have 1000w inverter and you run a 300w appliance, is the inverter still drawing from the battery the same as if it were running a 600w appliance or a 1000w applliance? Probably a stupid question. But I ponder stupid questions.

 

[youngnretired]

The load on the 12V system will vary with the load on the inverter, plus whatever the parasitic losses are of the inverter itself. It may not be exactly a linear line with the loading, but the inverter won't be pulling 1000W if you have a 20W load on it.

 

[GallenH]

The laod on the 12V system will vary with the load on the inverter, plus whatever the parasitic losses are of the inverter itself. It may not be exactly a linear line with the loading, but the inverter won't be pulling 1000W if you have a 20W load on it.

Thanks. I'm guessing that it probably has some headroom for power surges from the appliance. I was just wondering if it got that by having an "overdraw" calculation that varied from appliance to appliance or if it was always at max output, relying on you to size for that accordingly in your inverter purchase.

 

[youngnretired]

In order for an inverter to have a constant output, it would need some equipment (like resistors) to absorb the unused output. The energy would have to go somewhere. Absorb is probably not the right word, because the current that goes to the resistors is consumed, not absorbed or saved for any future output. The rating of the inverter (ie: 1000W) is the maximum output that the inverter will power. If it was constant output, you would have a constant 1000W load on the batteries and wear them down pretty quickly.

 

[otr]

Are you wondering:
If you have a draw of 300 watts, what is the draw for the inverter to make 300 watts?
If you have a draw of 600 watts, is the inverter using the same amount of electricity to make that power or more, and is it twice as
much as for 300 watts?
And then what is the draw of the inverter to just be on with no draw?

 

[RossWilliams]

If I understand the question, there is a relatively small fixed load to operate the inverter. But the inverter draws most of its power from the batteries based on the load. So it will draw 300 watts to power a 300 watt appliance and an additional 600 watts if you turn on another appliance that draws 600 watts.

 

[booster]

Are you wondering:
If you have a draw of 300 watts, what is the draw for the inverter to make 300 watts?
If you have a draw of 600 watts, is the inverter using the same amount of electricity to make that power or more, and is it twice as
much as for 300 watts?
And then what is the draw of the inverter to just be on with no draw?

Inverters have a parasitic load that is on all the time and they also have a % efficiency in the conversion of DC to AC. The efficiency is not the same all the time, though, as it varies with the amount of load you are running vs the size of the inverter.

If you are ruuning at 20% of rated output, the efficiency could be close to 90-95% depending on the quality of the inverter, but at 95% of rated load it might be 70% or less. Most of the energy use calculators seem to use and overall of about 20% loss so 80% efficient.

At 300 watt AC load tht would take 300/.80 or 375 watts of DC.

I have only tested if the parasitic needs to be added on also, but only on small 120-180watt ones and on those it would add on. They both have low parasitic of under 5 watts measured so no big deal. Our 3000 watt inverter has about 17 watts of parasitic.