Data loggers...

This gets rid of the 120/240 VAC requirement that GT micro inverters would require (and may cause your instructor some heart burn).

drees wrote: »

Enphase tare losses:

The latest units (what they've been selling for at least a year now) are specced at 30mW night-time power consumption. VA is higher than the tare indicates as the design of their inverter puts a capacitive load on the system when it's not generating power so you see about 100mA per inverter.

When the sun is shining, the inverter is completely powered from the DC side, so any power losses are accounted for as part of the efficiency rating. The older units were powered from the AC side, so tare losses were much higher.

Power/efficiency curves can be found here: http://www.gosolarcalifornia.org/equipment/inverter_tests/summaries/ They are 95% efficient through most of the curve except below 25% load (47W) where it starts dropping - by 10% they are at about 93% efficient. They will cap out at 199W AC power.

There is no way to get individual panel performance numbers from the Envoy unit without using their monitoring, so I wouldn't bother if you are on a budget.

There is an Open Source project to inexpensively build your own monitor building off the also open source Arduino box. Web site: http://openenergymonitor.org/http://openenergymonitor.org/

That should only cost about $50-75 in parts to get a single-channel power monitor and only nominally more to monitor additional sources.

a0128958 wrote: »

The WEL data logger will cost you $385 complete (before any sensors) - that includes power supply and the web site your chaarts goes to, and all data logging storage. Your sample rate will be 1 per 60 seconds.

Since the WEL is an internet appliance, there's no s/w that has to be loaded on your PC/Mac.

The WEL has two 4-20 ma analog inport ports which should well suit measurement on the DC side of a solar system.

To measure voltage you'll need to add external series resistance (precision) such that in combination with an internal 150 ohm resistor, the voltage presented at the WEL's 4-20ma port is 3 V or less (20 ma or less).

To measure current you'll need to insert a shunt into the solar's DC circuit, as Bill suggested. You'll want to size the shunt such that the voltage drop across the shunt will never exceed 3 V.

While I own a WEL I haven't used the 4-20 ma ports yet. Thus, I don't have specifics to offer as to exactly what size and precision series resistor you need to measure panel voltage, or precision shunt resistor to insert into the solar's DC circuit to measure current from. I would think, though, that pretty straight forward electrical engineering skills would guide you through the process. The key point is to never exceed greater than 3 V at the input of a WEL's 4-20 ma port.

The WEL instruction manual is here: http://www.welserver.com/support_files/WEL_User_Manual_4.0.2.pdf . Details on the 4-20ma ports are on page 31.

Good luck.

Best regards,

Bill