Solar/Inverter Question

BFL13 wrote:
I don't understand how Wh work with inverters. I changed my Trimetric to read watts instead of amps and the initial draw with the small MW was 700 watts but that rose to 900 watts with the voltage getting lower. The Tri only does AH so I never found out what it all came to in Wh.

The inverter tries to maintain the 120v load, so it draws more DC amps or watts to try to keep up as battery voltage falls. So that skews whatever is going on. I don't think it makes it easier to use Wh instead of AH!

I still have not tried that with a DC load and no inverter in the mix, to see what it says in watts instead of amps as voltage falls. Someday maybe. :)

Don't worry about measuring too accurately for design purposes. It's best to add a fudge factor anyway as sometimes, you might warm up an extra cup of coffee in the microwave other times not. Plus there are inefficiencies when converting power. You lose some charging the batteries and then you lose some more pulling it out with the inverter.

For these calculations, W-Hr is simply voltage times amps. Then at the end add 15-20% for losses and you should be in pretty good shape. If your inverter is on all the time, you probably want to look up what it consumes in the resting state but good ones are in the milowatt range.

Lead Acid batteries do have issues when you get into large demands.
- Yes, as the battery voltage goes down, it will take more amps to generate the watts demanded but if you have a 20% fudge factor, it shouldn't be a big issue.
- Probably the bigger issue if you start pulling 1000w or more is the Puerket (spelling?) effect. Battery amp-hrs are rated based on how many amps are being drawn relative to size. The higher the amp draw, the lower the amp-hrs available. If you are pulling 1000amps to turn over a big diesel engine, the same battery will have a fraction of the available amp-hours. Not an issue if the diesel starts in 2-3 seconds. If you compare to a basic trailer just running some lights and maybe a furnace fan, it may be pulling under 10 amps. If you have a 200amp-hr battery, that's a 20hour rate (a common spec for batteries) or If you are trying to pull 100amps over a long period of time, that would be a 2 hour rate and the available amp-hr will be much lower. Of course, it would be unusual to pull 100amps for 2 hours straight but even 20-40amps would have an effect.