[aprssig] Shack backup power

Alex Carver kf4lvz at yahoo.com
Sun Jun 5 13:39:43 EDT 2011

From: "Bob Bruninga "
> >> ...With these small plug-in grid-tie inverters 
> >> in the 250 Watt class, now it is possible to run
> our shack:
> >
> > Do you have a reference for these plug-in grid-tie
> > inverters?  I can not possibly see how these
> would 
> > be legal but I want to see the manufacturer's 
> > website if they exist.
> The Enphase or is it Emphase micro grid-tie inverters are
> designed for installation right on the back of each solar
> panel and put out 240 VAC to be paralleled with all other
> panels and then fed into the power panel.  They are UL
> approved, etc.  The other ones witn a standard 115 VAC
> plug output have no such approvals.

But you've avoided my question which was to name the manufacturer of the plug-in grid-tie inverter.  I am familiar with Enphase and they are not plug-in inverters, they are hardwired and intended for use with an expandable system (buy a few now, expand later with more capacity without having to replace inverters).

If by plug-in inverter you are talking about one that has a regular receptacle output intended for use in a vehicle (such as running a laptop computer) then those are most certainly NOT grid-tie and someone will get hurt if they are plugged into the line.

> > Also, solar energy isn't wasted in the battery 
> > if your load is constantly running.  Most of the
> > daytime energy would go to the load and the 
> > excess is absorbed by the battery.  At night, 
> > the battery runs down and then gets charged up 
> > the next day.  A smart designer would make sure 
> > there was no waste.
> For remote repeater sites where the load is a constant
> 24/7/365, yes it is possible to match the power design to
> the load.  But even then the system has to be desigend
> for the worst sun in December to still meet that minimum
> load.  By DFINITION, then in the summer, when there is
> TWICE the total energy available, there is just no place for
> it to go.  The load is the same.  THe other HALF
> is all wasted.
> For a shack system, designed about one's average operating
> load, on any day when one does not fully use the shack to
> that average, then all excess solar energy is also wasted,
> because the batteries have no room for today's excess solar
> power.. (Generally).
> That's why grid-tie wins hands down.  Every penny of
> solar energy is credited to you no matter how or when you
> use it.

So just put some extra load on the system in the summer.  Summertime is hot so run a mini-fridge in the shack with the spare power and keep some cold drinks in there to ward off the hot day.  Or pump some water into a storage tank which is exactly what some larger scale systems do.  The water can be used for drinking or, in some designs, the water flows back out at night turning the pump motor into a generator to supply nighttime power.  How about using the excess energy to heat water and take some load off the main hot water heater?

You make it sound all doom and gloom and it's not.  Grid-tie isn't a one-fits-all solution.  It works in many cases but you can also make a battery system work without waste, too.  You just have to be smart about it.

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