This post is intended to lay out what you need to plan and implement a backup electricity plan.
FUNDAMENTALS
Electricity is measured with three key variables. I find it easiest to imagine them as a river, and will use that analogy.
AMPS: The amount of current. Think of this as the width or volume of the river.
VOLTS: The force behind the current. Think of this as the speed of the water in the river.
WATTS:
A unit of measure that combines both of the above using the following
formula: Watts = Amps x Volts. They are a handy measure for comparing
total juice across systems.
Alternating
current systems, like the plugs in your house, are typically 110120
volts. A typical household circuit (i.e. all the plugs in one room) is
15 amps A/C. Therefore, a typical household circuit is about 1650
watts. Some circuits in your home, like the kitchen, may have higher
amp limits.
Direct current systems, like those used by solar
systems or your electronics, are often 12.6 volts. Most batteries will
measure their capacity in "amphours," or the number of hours that you
can pull 1 amp out of the battery at 12.6 volts. A large car battery
will have something on the order of 100 aH. The actual capacity of the
battery will vary (generally go down) in cold temperatures, with heavy
loads (hold that thought), and a few other factors. Additionally, it is
unwise to pull more than half the amps out of a battery.
It is possible to convert 110 volt AC to 12.6 volt
DC and back again. To go from 110v AC to 12.6v DC you need a device
called a converter or a power supply. Your laptop uses a power supply,
for example, to convert the wall plug power into 12.6V DC for your
computer.
To go from 12.6v DC to 110v AC, you need an
inverter. Inverters are usually measured in watts. For example, a
small 200 watt inverter intended for use in a cigarette lighter can
generate about 1.8 amps of AC power (200 watts / 110 volts).
As an example, 10 amps of 12.6V DC power is 126
watts (remember, W = A*V). If you convert that to 110v A/C through an
inverter, you should get 1.14 amps (126 watts / 110 V = 1.14 A).
Neither of these devices are 100% efficient. Expect losses due to
heat, fans, internal circuitry, wiring, etc on each of them. So, while
the real ratios are 12.6 vs 110 volts, it is fair enough to simply use a
factor of 10 or 100 when converting between the two. So, as a simple
rule of thumb, a 2000 watt generator should make 20 A/C amps. A 2000
watt inverter requires 200 D/C amps to operate.
Finally, A/C current from generators and inverters
can either be PURE or MODIFIED sine wave. Pure is better for things
like electronics that need clean electricity. It also costs more.
DETERMINE YOUR NEEDS
Before going any further you need to determine how much juice you need. You need to determine three things:
1)
Peak load. This is the maximum number of amps your devices draw at
any one time if you turn everything on. Additionally, you must consider
startup loads. A lot of motors (such as compressors in A/C units or
fridges) triple their power consumption for a few seconds while starting
up. This will be measured in amps (at a given voltage) or watts.
2) Typical load. What does the unit draw on a typical basis? This will be measured in amps (at a given voltage) or watts.
3) Average daily load (don't worry about this for generators). This will be measured in amps (at a given voltage) or watts.
The best way to determine your energy consumption
for A/C devices is to use a Killawatt meter. This handy device costs
about $20. Just plug the device into it and you can see the amps or
watts drawn; it will also keep track of total watts drawn over time so
you can let it run for a few days to determine average daily load. It
may not properly capture peak loads, however, so be careful there.
The manufacturer's notes may also give you some idea
of your device. You can also make some assumptions based on similar
devices. Finally, the Energy Star rating and cost per year estimate
gives an idea of the average daily load.
SELECT A GENERATOR
If
buying a generator, this is relatively easy. Most generators will
advertise their peak power, as it is the bigger number. For example,
the popular Honda EU2000i can surge to 2000 watts at 120 volts (aka 16.6
AC amps), but it is only rated to run at 1600 watts (aka 13.3 AC amps)
for long durations. The surge can help the generator handle peak loads.
If buying a generator I suggest you pick one of the following four:

Harbor Freight Storm Cat. You can get this for about $80 if you get a
coupon off ebay. It generates 800 watts continuous (6.6 amps), which is
enough for a window air conditioner, a fridge, or some electronics (not
all at once). It is a two stroke engine so you'll need to mix gas and
oil like you do for a chainsaw or weed whacker. This is a
marginalquality device but it is a great price and for most people will
meet emergency or camping needs just fine. I personally have one. For
the price you could easily put away a spare.
 Honda EU2000 or Yamaha 2000i. Both are great machines, gold standards for generatorsat a commensurate price.

Hyundai's 2K generator. Similar to the Honda or Yamaha, but cheaper
and with slightly lower quality. Just like the cars  do you want a
Honda Civic or a Hyundai Elantra?
Some people may be able to get by with a 1000 watt
generator. You will save some money, weight and gas. Look at your load
requirements carefully and remember the difference between peak and
continuous rating.
SELECT A SOLAR/BATTERY SYSTEM
Many
people want to try solar. It is important to be realistic about what
you can get out of solar. The math  which we'll explore shortly  is
cruel.
Let's look at a basic solar system and the components:

PANELS: These are measured in watts. Most panels intended for 12.6V
batteries actually put out 18V or so which allows them to charge the
batteries. Every 100 watts of solar panels thus produces about 5.5 amps
of 12.6 DC power. Panels cost about $1 per watt, but the price is
higher for smaller panels.
 CHARGE CONTROLLERS: These are measured in amps, usually 12.6V
DC. You need a controller which can handle the input of your panels.
For example, if you have three 100 watt panels, you need at least a 15A
controller. Controllers can be PWM (cheaper, less efficient, requires
thicker gauge wires from the panels) or MPPT (1020% more efficient, can
use thinner wire). Price for a good controller is about $3/amp (PWM)
or $10/amp (MPPT).
 BATTERY: Your charge controller juices up batteries. There are
many options for batteries but the simplest is AGM. Battery capacity is
measures in "amp hours" and known as "C." The optimal charging rate
for an AGM battery is about C/8. The optimal discharge rate for an AGM
battery is about C/20. Using our hypothetical example with 15 amps of
solar coming in from the panels, we'd be best off with a battery around
120 aH (8*15 amps) in size. AGM batteries cost around $2/aH.
 INVERTER: If you want to convert your stored 12.6 volt energy in
the battery to A/C so you can plug in your TV or fridge, you will need
an inverter. These are measured in watts, as we've discussed. The
optimal discharge rate for our 120 aH battery is thus 6 amps  at 12.6
volts DC. That would correlate to 0.6 amps A/C  or about 66 watts.
Youch! That wouldn't run much. Break out your KillAWatt and see
what 0.6 amps will power... Much like generators, inverters need to be
sized for peak loads.
The other complication is that we don't get 24 hours
a day of sun. Thus we need to collect enough during the day to make up
for our energy consumption at night. Most places in the US get about
4.5 hours per day of optimal sun on average. This can vary up to 6 in
the SW or down to 3.5 in the far north. Thus, our 300W of panels making
15 amps of juice will generate a total of 67.5 aH (15 amps x 4.5 hours)
each day. That is where factor three above (total daily consumption)
comes in.
SOLAR EXAMPLE: RUN A FRIDGE
So,
let's examine what would be necessary to run, say, a fridge. I slapped
a KillaWatt on my fridge and saw that it pulls about 200 watts (~2 AC
amps) when running, with a surge up to around 400 (~4 AC amps) on
startup. Daily consumption was about 1500 watts (~15 AC amps) because
it is not on all the time.
 This means I would need at least a 400W inverter
to handle the peak load. This would suck up to 40 12.6v DC amps at once
out of a battery. Peak loads above 40A are problematic; voltage loss
(from resistance in wires) at 12.6v are high as the amps ramp up. You
will need thick wires to run this. 40A is the most that typical
automotive fuses will handle for a reason.
 The sustained load would be about 200W, or 20 12.6v DC amps.
This implies that my battery needs to be 400 aH in size (20A*20 optimal
discharge factor). We could likely get away with a smaller battery,
but exceeding the C/20 recommendation reduces the efficiency of the
battery and thus we'd get fewer amps than the rating out of a smaller
unit.
 Daily consumption would be 1500W, or about 150 12.6v DC amps.
That means in the 4.5 hours a day we get good sun, I'd need to collect
at least 33 amps every hour to charge up my batteries enough to pull out
those 1500 watts over the course of the rest of the day. If you
recall, our optimal charging rate is C/8, so I'd need a battery of at
least 264 aH, which shouldn't be a problem based on our discharge
requirements which mandate a 400 aH bank. Additionally, it is bad to
pull more than half the amps out of a battery. So, my battery needs to
be at least 300 aH in size (150A per day x 2), again, not a factor based
on the discharge requirement above.
We can estimate the cost of such a system:
 Solar Panels sufficient to generage 30 amps: 500 or 600 watts in panels should do this. ~$550.

30A Charge Controller: $120 (PWM Morningstar) or $355 (MPPT
Morningstar). If you opt for PWM the panels must be close to the
controller (around 10 feet).
 400 aH battery bank: $800.
 400W inverter: <$100
 Misc wires, breakers, fuses, etc: ~$100200
 Transfer switch which will flip our system from solar to shore power if the power comes on or the battery gets low: ~$50
 Low Voltage Disconnect to turn off the inverter if our battery gets low: ~$50
The
total is around $2K, more or less. This is about the same price as the
nice generators but it runs a LOT less stuff. Your 1600 watt generator
gives you 1600 watts every hour. The 400 aH battery bank gives you
about 2500 amps  all day, with much lower peak and sustained loads.
SOLAR PAYBACK
It
takes about 14 years to payback a moderate solar system as described
above, based on typical electricity costs. The good news is that
certain residential systems qualify for a 30% rebate in the form of a
tax credit. Additionally, if you pay a surcharge for electricity at
peak hours (usually midday) you may get back your investment sooner.
CONCLUSION
Solar is
attractive as it is silent, renewable, and can save you money in the
long run. The downsides are that it takes a ton of panels to run even
modest appliances, the system is not particularly mobile, and it is far
more expensive wattforwatt than a generator and a few cans of gas. As
you can see, it costs $2K to run a fridge  and that assumes no cloudy
days! Nothing beats dead dinosaurs for compact energy sources.
You will need
to decide what meets your needs best. Personally, I have the harbor
freight generator to run a fridge or window A/C plus 250 watts of panels
going into a 110 aH battery which gives me enough to run some 12V
lights, fans, and electronics indefinitely. I have interest in
upgrading to a Honda or Yamaha as well as a 12KW solar system but those
are relatively low priorities.
Remember,
if you are unclear of the details, you should experiment with a
subscale system to learn how to safely make connections, fuse things
properly, etc and/or consult a professional. I shorted a 12 aH battery
and generated quite a little fire with melted wiring; I'm glad I learned
my mistake with a small battery and limited consequences before messing
around with my much larger system!
I
now have a safe, quiet solar system that works for me every day, along
with a generator for larger loads like the window A/C. It is a good
combination. We use them for camping now but they would be equally
useful in an emergency situation.
I
hope this was helpful. I have researched the topics substantially and
hope you can apply this information to your own scenario.