The hazards introduced by backup power systems are
numerous; electrocution, fire, asphyxiation and of
course irate neighbors.

Electrocution isn't much of a threat provided the
installation conforms in spirit to the NEC. So 
long as the backup system is properly isolated 
from the utility, via interlock or transfer switch,
power from a generator is no more dangerous than
from the electric utility.

Fire hazards are probably more significant. Energy
storage is a hazard whether it's batteries or a tank
of fuel. Rules for fuel storage are not complex, but
they can be onerous for the quantities of fuel needed
for meaningful endurance, tens of gallons or more. Early
attention to fuel storage considerations will help avoid
sticker shock when pricing out a backup system and can
radically alter design goals. Fire codes are widely
applied to fuel storage, easily inspected for and 
violations can affect insurance coverage. Neglected
fuel storage that eventually leaks is a particular
threat. 

Batteries are still not widely addressed in safety rules.
That will change as mishaps accumulate, but for now they're
less regulated than fuel. Lithium iron phosphate batteries
are generally considered safe, in that they can't be made
to fail internally in a way that starts a fire. Still, a
battery bank of capacity sufficient to provide useful backup
power is quite capable of starting a fire via faults upstream
of external loads, i.e., inside the battery bank itself. Fuses
and rapid disconnects are important.

It's important to remember that batteries are worse than fuel
in the sense they're self-contained: There is no "fire triangle"
of fuel, air and ignition. The fuel and oxidizer are already
present in the battery and can't be separated. Once ignition
occurs they're similar to "safe and sane" fireworks. 


Transportation-type batteries using lithium-cobalt chemistry
are capable of self-ignition on serious internal faults. I'd
suggest avoiding them, both because of hazard and because of
scarce resource consumption. The high power and energy density
they offer simply aren't essential to stationary backup work.
Lithium iron phosphate is good enough, cheaper and safer. 

Asphyxiation kills numerous people every year, usually via
carbon monoxide poisoning. Careful attention to generator
placement and venting are the first necessity. I'd submit
that avoiding unattended or overnight generator use is a
good second step. Most folks who gas themselves do it while
asleep. A carbon monoxide detector is nice, but prevention
is better than any alarm.

Irate neighbors are usually caused by noise problems. Here
again, avoiding nocturnal generator operation is a good 
first step. Selection of a quiet generator and placement
away and downwind from neighbors are the first requirements.
Offering to share some power might help ease friction, but
that can lead to complications of its own. 

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