On Tue, 31 Jul 2018 12:22:14 +0100 (BST)
"Dave Liquorice"  wrote:

> On Mon, 30 Jul 2018 18:47:14 +0100, Ahem A Rivet's Shot wrote:
>
> > At domestic scale they make the householder immune to brown out and
> > outages short enough not to drain them, proven repeatedly by everyone
> > who has domestic power storage
>
> er, AIUI the vast majority of domestic solar PV systems shut down
> when the mains goes. They do *not* provide any autonomy.

  I was talking about batteries not solar panels.

> > So for off-grid winter usage you need a system that produces enough
> > energy for your needs at 20% of rated capacity,
>
> So for 20 kWHr/day you need at *least* a 100kW rated system.

 Erm try 100kWHr/day system - which assuming shortest days of 6
hours is more like a 17Kw system. If I were going for fully off-grid I'd
probably make it a 20Kw peak system.

> > The usual estimate for off-grid usage is at least five days storage.
>
> 1 kW @ 48 V = 20 A (ish). Total battery capacity 20 * (5 * 24) =
> 2,400 AHr.

 100 kWHr of storage or about 2000 A/Hr. *IF* you want to be able to
run completely off-grid. Which is not what I was suggesting. The only
reason I ran through those numbers was to counter the suggestion that you
needed enough battery storage to last the whole winter - four or five
megawatt hours.

> >>> A few hours storage (say ten to twenty kilowatt hours) and a few
> >>> kilowatts peak of panel (just enough that the total collected
> annually
> >>> is close to the total used annually)
>
> "a few kilowatts peak"?  Do the maths ... see above ... 20 kWHr/day
> at 20% means a 100kW peak array.

 See above for the corrected calculation.

 More to the point I was suggesting that a system as described in
the bit you quoted with a few kilowatts peak power generation and ten to
twenty kilowatt hours of storage will reduce grid usage and grid dependency
substantially.

 Let's take a look - based on 4kW peak generation, 20 kWHr storage
and daily use.

 In summer with 12-18 hours of daylight it will be a rare day that
doesn't see a more energy collected (48-72 kWHr per day peak) than used
so there will be almost no energy drawn from the grid and quite a lot
dumped, or given or sold to the grid.

 In the depths of winter the system will produce less energy per day
than is used and something like 3/4 of the energy must come from the grid.
But that 20kWHr of storage means that it can all come from off-peak which
helps. All in all with such a setup you could expect to use less than 1/8th
of the grid energy that you would use without probably more like 1/10th.

 In summer or winter with the batteries fully charged a full day of
power cut won't cause a problem - the last time I saw a power cut of more
than a few hours was more than 20 years ago - that one was three days or so
around Christmas which was something of a nightmare.

 It then boils down to a simple money question - does 87.5-90% of the
electricity bill over the lifetime of the system exceed the cost of the
system. Also how much do you value immunity from power cuts, surges,
brownouts etc - which of course partly depends on how common they are.

 If you can sell that summer energy and buy off-peak in winter the
cost savings are even greater than the energy savings - it may even be
profitable.

 It's the old story - the last 10% of the job takes 90% of the
cost/effort. In this case being on the right side of the 90/10 law is
pretty good.

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