Apologies for my ignorance, there probably is a fairly simple explanation for this, but I've never claimed to be an EE and the basic stuff we got at school is err.. long shelved in some dark part of my old brain I'm afraid =)

Anyway, the situation is as follows: a while ago I had a PV installation installed on my roof and the inverter has a basic web-page that shows me some internal statistics. The only ones I'm really interested in are AC-Power out and Total Energy over the day. However, I've got 2 -- differently sized and differently oriented -- strings of PV panels and I always wonder which one 'does best'. For fun I started logging the numbers in a .csv file so I could make some graphs out of it and a such I started having a closer look.

It turns out I don't have a power-number for each string, but I do have a Current and a Voltage number for each. I seem to remember that Power = U x I, so when I sum both multiplications I should get the output power even with some losses due to whatever process is going to convert 'random DC' to 240V AC. (The thing gets warm, so there's at least that).

Turns out, that the input-side is consistently lower than the output side, so somewhere along the road the inverter adds energy to the mix?!??! I'm willing to believe a lot of things, but this seems unlikely =)

An example:

PV Statistics snapshot

As can be seen:

  • String 1 gives me 0.60A x 259.8V = 150W
  • String 2 gives me 0.65A x 352.8V = 230W
  • AC(out) is at 1.94A x 228.6V = 443W

The input side sums to 380W, yet the output side gives me 445W according to the machine itself (it seems some rounding is going on).

Where does the 'extra' 65W come from?

Writing all this down I seem to remember there was something with a SQRT(2) conversion when figuring out peak AC voltages, but I'm not sure that applies to Power here?!?! Pout / Pin gives me about 1.17 while SQRT(2) is close to 1.41

PS: the inverter is a Sununo Plus 4K-M with Power Factor: 0.9 leading~0.9 lagging (I have pretty much no idea what the latter means but I've got a gut feeling someone is going to mention it =) And yes, it was a fairly cloudy day.

  • \$\begingroup\$ There is also a thing called power factor \$\endgroup\$
    – PlasmaHH
    Feb 12 '18 at 16:42
  • \$\begingroup\$ Average for a sinusoid is 0.632 of peak; RMS is 0.707, thus a 10% factor there. \$\endgroup\$ Feb 12 '18 at 17:01
  • \$\begingroup\$ @analogsystemsrf you're going to have to be a bit more verbose here, I'm trying to link all numbers together but I still don't see how to get from 380 to 445. \$\endgroup\$
    – deroby
    Feb 12 '18 at 18:40

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