"Scout" wrote in message news:siv7tl$t50$7...@dont-email.me...
My backup system isn't nearly that large or expensive, and my normal
consumption is under 5KWH per day.
The backup supports a compact refrigerator and small freezer plus one or two
laptops for cellular Internet and antenna television. For meals I run a 2KW
inverter generator, or use the wood stove since most long outages here are
from winter ice storms. Laundry requires a 3KW generator and kick-starting
by releasing the old Maytag's belt tension with my foot on the motor. I
freeze-dry the laundry on an outdoor clothes line, typically within the same
day since winter humidity is very low.
Two fairly new 12V 105Ah marine batteries will run the inverter for at least
24 hours and maybe 48 depending on conditions. The freezer operates on 12V
DC and can go almost 48 hours on the original pair of ~10 year old
batteries, by recent test. The four marine batteries and four 100W panels
averaged $100 apiece and the sine inverter was free, a "dead" high end APC
UPS. Installation is standing the panels on fold-out legs in the yard or
driveway when I need them for an outage, otherwise a row of small flea
market panels on the roof powers the freezer. After measuring power loss I
upgraded the house solar wire to 10 AWG, a significant expense if you buy it
As best I can figure, the cost of battery depreciation if cycled daily
somewhat exceeds the cost of grid power, so I use the system only for backup
to prolong battery life. I ran the numbers for flooded, AGM and Lithium and
flooded won IF maintained, but not if neglected. I pay $0.18688 per grid KWH
and these $100 batteries would cost $0.20 per KWH if they delivered 1 KWH
each, 500 times. They probably wouldn't last that long, 500 cycles is
pushing the claimed performance of AGMs at twice the price and reportedly
half the storage life, so the high end estimate for battery cost ran around
$0.50/KWH, close to using a generator. I get nearly 10 years in storage from
used PowerSonic 12V 18A AGMs if they are topped up regularly, less than 3
years from Rhinos. They are considered dead and recycled when they won't run
the freezer the equivalent of overnight.
If I switched to daily cycling I would buy different batteries because the
flooded ones may gas if charged fast enough, meaning high enough voltage, to
fully recover from a deep discharge during winter daylight. They don't gas
if limited to the float voltage, but they also don't fully recharge in a
day. They do charge to 70~80% before reaching the gassing voltage, which is
enough for me.
Unless more trees die and fall I don't get enough sun on the roof to justify
a larger permanent installation. What I have has let me unplug from the grid
and keep operating as usual when thunderstorms threaten, and stay off it if
they last overnight. I've only needed the generator briefly a few times
during an extended winter blackout. In clear weather 400W (~330W into the
batteries) of solar power recharged the batteries into the acceptance
(declining current) range by 10AM.