[RE-wrenches] 2KW PCS import/export limit on a 25KW transformer; Oncor in Texas

[Tyrone Houck]

Here in Oregon we have the Public Utility Commission that acts as a
regulatory agency to keep Utilities on similar standards. Their standard
residential limit to PV export is 25kW. Occasionally we will have a
customer with a smaller transformer and, though I would typically not go
beyond the transformer rating, I leave it to them to say no during the net
metering application process. I heard second hand from one of pacific
power's estimators that if the customer wants to avoid the cost of a
transformer upgrade to direct them to run heavy loads for a couple weeks
and complain about lights flickering, surging, etc. And pac power will be
more inclined to cover the cost of upgrading to a 25kW transformer.
Recently Pac Power tried to make policy that the PUC limit of 25kW meant
that the AC rating of inverters couldn't be higher than 25kW regardless of
PV export capability. Our local lobbying agency ORSSIA challenged that at
the PUC meeting and forced them to change that policy on the basis that
they weren't allowed to limit the customer's ability to back up their
loads. Not sure if there are any unified standards among utilities in
Texas. It sounds as if the customer may need to consider the cost of
upgrading the transformer...but I would definitely keep trying to reason
with the utility to avoid them cementing a possibly mislead notion as
policy. I've found that being able to explain it well to the right person
can be the most effective approach(and possibly alleviate the utility's
concerns as well). Best of luck!




On Wed, Nov 26, 2025, 7:53 AM Zeke Yewdall via RE-wrenches <
re-wrenches at lists.re-wrenches.org> wrote:

> I do not know how they do it in Texas, but in Colorado, it's pretty
> typical to run a 200A (38kW continuous) service from a 10kVA transformer.
> Or even several houses with 100 to 200A services from a single 10 or 15kVA
> transformer.  If all even a few of the houses start drawing anywhere
> near their full amperage for a long period of the time (grow operations
> were the common reason for this) the transformer explodes.  They are
> apparently relying on the very low typical load factor for residential
> services. Many of these are older services installed in the 70's to 90's,
> but some are newer ones and still not a 25kVA transformer for a 200A
> service that would seem more reasonable.  A grid tied PV system will often
> load the transformer much higher than typical residential loads because of
> the long hours of operation.
>
> I've had two ESS/PV systems recently where they required a transformer
> upgrade before they would allow me to backfeed at full system capacity.
> Both were all in one inverters, not separate PV and ESS inverters.  In one
> case I had 10.135kW CEC rating of inverter capacity on a 10kVA transformer,
> but in the other case they calculated that I could not go over 6.6kVA of
> export on a 10kVA transformer for some reason -- this was on a brand new
> service, new construction, so I was quite surprised by this limit.  2kW
> seems insanely low for a 25kVA transformer to me.  In both of my cases in
> Colorado, they are upgrading the transformers to 25kVA at very little cost
> to the customer -- $300, which I figure is 10% or less the actual cost --
> so we opted for the transformer upgrade rather than limiting export.
> Sounds like Oncor is charging actual cost for the transformer upgrade?
>
> --
> Zeke Yewdall
> PV Engineer
> NABCEP #031508-89
> zeke at darkforestsolar.com
> 303-523-3592
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