That's what often happens in sunny climates, or those with little variability in the weather. That is still pretty close however.
BTW, just for the hell of it, what does PVWatts estimate for April for your system output?
PVWatts gives 1193 kWh with 14% system losses and premium panel but that's presumably for a new panel. Mine will be three years old in a couple of weeks. I also checked April, 2013: 1272 kWh so more of a fluke than anything for closeness of the 2013-2014 comparison. I agree that it's the uniformity of the weather in April although we did have rain and clouds both years. Still, half a kWh would not be much more than the effect of a few clouds drifting by one afternoon.
Very long term ( ~ >10+ years or so) system output is more of a function of climate. That's what estimating programs like PVWatts, etc., rely on and imply by using mostly TMY data which attempts to get a representative interpretation of long term weather (climate) characteristics.
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