Slow Grid-Tie

[andicetexas]

I would like to add a grid tie inverter to my house, and add a panel or two every month or so when I have some extra money.  Which inverter would you recommend for the slow expansion of my system?  Are there any problems that I am not seeing to this idea?  Thanks

[Flux]

Yes I do see serious snags. Unless you use an inverter that requires batteries ( serious and unnecessary expense for grid tie) you will need a fairly high input voltage to match the grid tie inverter.

The only way you could do what you want is to add parallel circuits of the necessary voltage. Each increment would need sufficient panels to reach the inverter input volts.

You may be able to vary things a panel or two either way in a series string but there is no way to start with voltages below the inverter input.

Flux

[DamonHD]

You can usually safely parallel grid-tie inverters up (with separate PV strings, or sometimes on the same PV; check the data sheets) so far as I know, so add them incrementally, albeit in slightly larger lumps than you were hoping for.  But check with a licensed adult before taking my word for it.

For example, I have 6 panels (1.29kWp) and a 1.1kW inverter on one side of the house, and later this year hope to add another 12 panels and a (say) 2.5kW inverter on the other side.

Rgds

Damon

[Flux]

Yes you can do that if the inverter cost is the big issue, but you still need a string of panels to reach the inverter volts.

I am not sure about paralleling inverters on the same panels, I presume it would be ok on the Sunny Boy if set to the same characteristics. Not sure what would happen if both were truly self seeking mppt unless they were designed to track.

Flux

[DamonHD]

Some of them, Flux, like the Soladin 600, are designed to be paralleled on both sides AFAIK, and have a lowish (~50V?) minimum MPPT voltage.

So that could possibly be just one high-power 55V-nominal panel or maybe a string of 'only' three or four 12V nominals, which could even be a set of those favourite HF panels at a pinch, to start off.

Rgds

Damon

[zeusmorg]

 There is a newer system out there, just for grid tie. It uses one inverter per solar panel the cost is reasonable, so you could just add a panel and inverter when you could afford the expansion.

 I'm not recommending them necessarily as I have no experience with the system but it does look interesting.  

http://www.enphaseenergy.com/