is a standalone power system more reliable than "the grid"? SOLAR.KnowledgePublications.com

is a standalone power system more reliable than "the grid"?
25 aug 2000
dear seekers of truth:

let's consider this question: is a standalone power system more reliable 
than "the grid"? this is a separate question from "is it cheaper?" or
"is it more environmental?" or "does it give us more of a personal sense
of control or independence?"

the answer depends on the grid. in the backwoods, with no grid, the
question is moot. my lawyer inquired about installing a windmill at his
vacation home on st. kitts after some frustration with grid power there:
after a week with no power at all, he went down to the power plant near
the docks and was told "da generator is broke, mon. we need to go get
a new part wit da boat, but da crew dey all dronk now." i suggested he
power the island with a forest of air 303s, but he seems to have decided
against that. a personal standalone power system might be more reliable
on st. kitts. the combination of a standalone power system and the grid
could be more reliable than either one.

what's the answer in general, on the average, in the us? about 1% of the
trace inverters sold come back in the first year, and about 2% come back
over the next 5 years. ignoring the first year, ie infant mortality and
people who change their minds about models or brands, the failure rate is
0.02/(5x8765h) = 0.456 x 10^-6/h, ie 1/2.2 million hours, only once every
249 years, incredible for a power-handling electronic gizmo with some 500
parts, when computer makers are struggling to make simpler power supplies
last more than 5 years, on the average. then again, that estimate comes
from trace, who may well be biased. then again, conditions vary... some
inverters live in the rain on fishing boats, with others in air-conditioned
living rooms, and still others lying unused in their original boxes. 

with no redundancy, the system failure rate is the sum of the failure rates
of all of the parts. each additional part raises the system failure rate
and lowers the reliability or raises the unreliability, ie the downtime
measured in minutes per year (or seconds, in the case of phone companies.)

i called our local utility (peco) and asked what the average downtime is for
a peco customer. the operator asked "is that a survey or research question?"
i said yes, and she said "we have no manpower to answer survey or research 
questions." click. it's less than a half-hour per year, where i live.

most power companies have an advantage over homepower systems. the equipment
needed to serve a house is fairly simple and reliable (poles and wires and
transformers), and power companies can design with higher level redundancy
and economies of scale (eg being able to feed power in either direction
from 2 sources along a distribution line) and spares (eg a spare generator.)
few home power systems have any redundancy. 

what else besides the inverter needs to work in a home power system? pv 
panels, lightning arrestors, batteries, panel and battery wiring, a charge
controller...

suppose 8 pv panels with 20 year guarantees have 1/40 year failure rates,
and 8 batteries have 1/20 year failure rates, and a charge controller and
so on add 1/20 years. these "inverter accessories" raise the system failure
rate to 74.6x10^-6/h, ie once every 13.4k hours or 18 months. with a 2-day
repair time, this raises the average unavailability to 100/(1+13.4kh/48h)
= 0.35%, for an average downtime of 31.3 hours per year.

how does this compare with the downtime experienced by alt.energy.homepower
readers, ignoring too little sun, but including planned downtime, system
updates, maintenance, and so on? be honest now...

nick