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Rural solar has a gotcha

A good friend of mine copied me in on an email conversation between himself, a Tesla Powerwall dealer and the local lines/power company -- and it's an eye-opener.

I guess most of us think that a solar generation system on the roof would be a great investment, even moreso if you live out of town where the reliability of mains power may not be to the same standard as in a suburban environment.

Well a mate of mine paid for a bunch of solar panels and a Tesla Powerwall to be installed at his rural house. He figured he'd sure-up his energy situation and save a bit of money long-term.

Everthing went just fine, until they tried to activate the system.

The configuration was supposed to deliver an infeed to the power grid at times when solar generation exceeded the amount of energy the house was drawing.

This is pretty standard where households remain grid-connected, using solar to simply reduce their power bill rather than going totall off-grid. Although the power companies pay only a fraction of the amount they charge per KWH, this back-feed provides a small amount of extra return on the capital invested.

So the switch was thrown to get the sun's rays charging that powerwall and powering the household when possible.

Fail.

It seems that the power management system associated with the Tesla Powerwall wasn't happy.

No charging was taking place. The solar array sat there in the sun, the battery remained empty and the house was drawing energy only from the grid.

Right now you're probably thinking -- ah, there was a fault with the solar array, the power management system or the battery.

You'd be wrong.

It appears that the problem is in the lines from the nearest transformer to my friend's house.

As is not uncommon in a rural situation, the impedance between the house and the nearest transformer was too high. This usually happens either because the 240V cabling is too long for the gauge of wire used or (increasingly commonly), extra houses have been added to an existing transformer causing it to become unable to provide full voltage under the load it then sometimes sees.

Now interestingly enough in this case it's not the fact that the voltage can drop very low that is the issue... it's more likely the fact that trying to push infeed energy back into the grid causes the voltage to rise excessively -- to a point that the system shuts down for fear of damaging itself. There appears to simply be too much resistance in the lines.

After logging the voltage, the PowerWall vendor discovered that the voltage was varying from around 200V up to 245 throughout the day.

Since New Zealand has a "nominal" mains voltage of 230V, anything over 240V is considered abnormal -- at least by the Powerwall control circuitry.

However... and here's the kicker...

The Powerwall dealer says that their product is working to spec (operating at up to 240V I guess) and the power company says its *delivery* voltages are also within spec.

The power co does suggest that replacing the wiring from the transformer to the house (presumably with a thicker conductor) would fix the problem -- but it would cost my friend around $40K if this was done.

When I was asked "what should I do" I told my friend to spend that $40K going totally off-grid.

Get a wind generator to supplement a little more solar and add more battery then kick the power company to the curb.

I don't think he's made a decision yet but it does show that sometimes using solar to try and reduce your power bill is not as straight-forward as you might think.

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