Voltage question?

[Northforker]

Why are solar panels for grid tie systems offered in 48v, 24v, 12v?????

What voltage is the most efficient for a grid tie system?

Sorry for the dumb questions, just trying to get started.

[dwaller]

From my limited knowledge, the higher the voltage, the more "efficient" you are going to be at getting that power back to either the grid you are connected to, or the batteries you are charging.

[RCinFLA]

Panels are not necessarily targeted for those lead acid battery charging.

Higher voltages are generally used when higher power requirements are needed.

As to panels, there is roughly 0.5 vdc max power point per cell and 35 mA/cm sq. There is also some standard wafer sizes (relating to I.C. processing) that come in 100 mm or 150 mm wafer diameter. Then there is a maximum reasonable sized panel dimension that can be made for handling and shipping. All this sort of sets the voltage and power parameter seen in panels.

For pure GT installation, panels are usually put in series to achieve 350-500 vdc. This minimizes the wire size needed to carry the current. The 350vdc minimum simplifies the conversion to 240 vac with its 339 volt peak sinewave.

[Solar Guppy]

I find now with the high wattage panels 190 watt and up, its more common to have 12V panels as you can get 5kw+ in a single string which is optimum for Grid Tie

[BB.]

For absolute best efficiency--you need to look at the efficiency curves for each unit... Because of capacitance of the switching devices, higher Vmp voltages for MPPT devices can be less efficient in the inverter--But because higher voltage means less current--The increase in Vmp would reduce I^2R losses in the wiring (and/or allow smaller awg/cheaper copper/and-or moving the array to a sunnier area.

You really need to look at the whole system and see what the end results will be... Pick three Vmps (low, middle, high) and work out the basic wiring costs/losses + inverter losses and see what works out best.

In the old days with PWM controllers, for best efficiency, you needed to match Vmp to the battery voltage (Vbatt+2 volts=Vmp minimum)...

Now, with MPPT controllers, the actual Vmp is not as critical. The overall efficiency differences between Vmp low or Vmp high is only a percent or two change at the system level...

-Bill

[Northforker]

Thanks to BB and Solar Guppy and RC, you have helped me understand that it depends on the number of panels per array and the toal number of arrays.

I will post a picture of my roof and the potential array scheme, then we can figure the optimal panel/voltage out.

Thanks again!