Noise-Free Solar Inverter (DIY)

Q2 first. No. Inverters make noise because they are..... Inverting. The make what appears to be AC from DC by making high speed pulses of DC in alternating polarity. A side effect is a "tic" with each pulse, but at the high frequency of those tics, is becomes a high pitched squeal. There are inverters that raise the frequency so high that humans no longer hear it, but that increases the switching losses and thereby decreases the efficiency. Significantly. Q1. If your inverter is part of the battery charging system, you likely can't turn it off without losing the charger. If they are separate as you seem to indicate, you should be able to isolate the inverter and turn it off, I would. You can add back in a cheap simple DC meter to indicate the battery voltage.

The noise is caused by the method you wrote, but it's because of loose windings in a transformer, chokes and anything else with a winding and/or loose unsupported connections......

If the transformer(s) was completely embedded in Epoxy, and all the chokes, you would hear almost no noise. Higher quality components is another way to go.....already "potted".

BUT, the cooling may be thus affected and they may overheat.......so an extra fan may be needed to blow cool air around those components....

Is there any cooling fan installed by your friend (DIY)?

A 300 watt inverter is probably one of the cheapies as well - probably China made?

The unit may be even in the 50% efficiency range.

You also have a charger between the panel and the battery?

Yes it is one of those. I have a better one but not installed yet.

I don't have an answer to your second question

Yes I do have a charger in between

The energy lost as noise can be calculated from the dB level.

The power density at the point of dB measurement is given by:

Power = 10^((db/10)-16)w/cm².

If the noise level is 70dB this translates to 10^-9 w/cm², which is negligible and can be ignored.

You can wire your DC side so the charge controller and PV's can do their thing without having the inverter turned on. Just wire the charge controller's breaker into the battery side of your main battery/inverter disconnect.

A good inverter can go into 'sleep' mode and will only draw 5-10 watts.

Thanks to all at CR4 for sharing your expertise on this subject. I have learned greatly from you guys. The discussion will help me implement this on a larger scale.