You'll probably be using the mill as a battery charger / inverter load driver. So it will be driving a charging current into a battery and/or having part of that current stolen by the inverter - at the battery voltage.
Further, the current you want is the AVERAGE current, because in this type of application that's what matters: Average current multiplied by time tells you how much material is moved between the plates and the solution in the battery, and thus how much charge you added to it.
That means you DON'T want a wattmeter, even if you could find one. A wattmeter does a continuous multiplication of instantaneous voltage by instantaneous current and averages THAT. RMS-reading meters perform a similar operation by taking the square Root of the Mean (average) of the Square of the voltage or current, which is an approximation that assumens a resistive load. This is convenient for looking at how much power is delivered to a RESISTIVE load - which is not what you have. In a charging application, if your voltage is momentarily raised by a high current portion of the cycle working against the battery's internal resistance, the extra energy is wasted heating the battery - no extra molecules are converted. (In a resistive load, a boost in voltage means a {square-law} boost in power delivered to the loa.)
You're really after the charging current times your battery bank's operating voltage. You're expressing it as watts to make it convenient for comparing to other things - like a similar mill on a different voltage battery bank, or for calculating how much power you have for running your house after correcting for battery and inverter efficiencies.
Similarly, for figuring how much power you'll get you really don't care about the power wasted heating the wiring from the genny to the battery, or in the diodes, or heating the genny's own coils. You just care about the power delivered to the battery and inverter.
So when you're measuring "genny watts" you want to measure the current using an average-reading ammeter (which means an analog meter movement unless you're sure you have a really GOOD digital meter), with the genny hooked to a mostly-charged battery via the normal diodes - then multiply that by the battery voltage, and measure power at a number of shaft speeds or wind speeds.
That's assuming you're going to use a simple connection. If you're going to use a delta-wye switch you'll want to do one set of measurements in delta, another in wye.
If you're going to use a peak-power-point voltage-converting regulator it's a whole 'nother can of worms.
A thing to remember is that the battery's load on the generator makes the generator put a load on the blades, limiting their speed. This is good: The force on the blades is proportional to the current in the windings, so it means you're actually getting some power from the blades. B-) And the load helps to limit and regulate the speed of the mill: The generated voltage - induced in the coils, not the voltage you see at the coil terminals - is proportional to the shaft speed. If the coils, transmission lines, and guts of the battery were all superconductors and the diodes' voltage/current function were an ideal right angle rather than an offset exponential, the mill would spin freely up to cutin speed and then hold that speed, fighting harder against stronger winds up to any wind speed. What actually happens is that it speeds up somewhat with higher winds: The generated voltage equals the sum of the battery voltage and all these current-dependent voltage drops. So as the current goes up the mill speeds up somewhat from the cutin speed.
You need to keep that in mind when testing your genny:
- Load it the way it will really be loaded: Battery, diodes, and only enough extra resistance to model the wire to the tower. (Include the resistance of your ammeter in that - or use some of your model wiring as a "shunt" resistance: measuring current by measuring the voltage drop with a voltmeter.)
- When interpreting the power vs. shaft speed curve, bear in mind that the mill will turn freely up to cutin, then fight (and slow) further shaft speedups as the output current increases.
Making sense now?
