This easy to build and not to expensive system will provide
backup power during power outages to power your most essential
loads. The energy is from the sun, so you normally won’t need a
noisy and fuel hungry generator.
The advantages of a solar system over a gasoline generator are:
no need to safely store gasoline, no noise, no worries about
gasoline going bad over time, no dangerous fumes to deal with,
supplies power 24 hours a day while the generator must be on
whenever power is needed.
<overview pic
<Need a simple overview diagram of the system.
Objectives:
Provide power when utility power goes down
Continue working for extended outages
Provide meaningful power
The system consists of the following components:
Solar panels to collect solar energy
Batteries to store the collected solar energy
A charge controller to provide for efficient charging of the batteries
and prevent over charging
USB and 12 volt outlets for charging phones etc.
An inverter to power 120 Volt AC loads
A battery maintainer to maintain the batteries at full charge between
power outages.
While this system will not
supply all of the juice that would be needed to run your whole house, it
is typical of what you might use for a small off grid cabin or RV – so, it
will make a large improvement in your comfort level for a long power
outage. And, it will never run out of gasoline!
What will it power?
If the discharge of the two golf cart batteries is limited to 80% of their
capacity (for good battery life) they will provide about 2100 watt-hours
or 175 amp-hours at 12 volts.
On a good sun day, the 200 watts of solar panels will provide about 1000
watt-hours or 80 amp-hrous of charge to the battery. Additional panels
can easily be added to about double this if needed.
These are some examples of how many of those 2100 watt-hrs certain loads
will use...
10 hours of 8 watt LED (800 lumen – 60 watt incandescent equivalent) --
80 watt-hours
Charge a cell phone from empty -- 15 watt-hours
Charge a laptop computer from empty -- 70 watt-hours
4 hours of 24 inch LED TV -- 200 watt-hrs
4 hours of 20 inch RV 12 volt TV/DVD – 40 watt-hrs
4 hours 20 inch box fan – 200 watt-hrs
10 minute run on small (600 watt) microwave -- 130 watt-hours
30 minutes on induction hot plate at 700 watts -- 350 watt-hrs
An efficient Energy Star fridge for a day -- 900 watt-hrs (less with
steps described below)
2 hours running an efficient gas forced air furnace at 50% duty cycle –
150 watt-hrs
2 hours running 9K BTU EER 12 window AC unit at 50% duty cycle –400
watt-hrs
Note that energy used by the furnace or AC will depend on outdoor
temperatures, insulation levels, size of the area you are heating or
cooling, and efficiency of the furnace or AC.
Just add up the watt-hours of the things you will want to do each day
from the list above to get your daily total. You will be able to use
about 2000 watt-hrs on the first day and about 1000 watt-hours on
following days as long as you have descent sun. If this falls short of
you needs, see suggestions below for increasing system capacity.
So, the system will provide good lighting and keep cell phones and laptops
charged and allow you to keep up on the news on an efficient TV all with
power to spare. Efficient, Energystar, moderate size Fridges are a
possible with descent sun especially if you take steps to reduce fridge
consumption (see below). Furnaces or window AC’s might be run for limited
times, but probably not all day. Cooking is better accomplished with
something like a propane or butane camp stove, as electric cooking devices
use a lot of juice.
Bottom line is that having this system will make life a lot better during
a power outage than you would be without it.
Things that use too much electricity for this system:
Anything that requires 220 VAC to run (electric dryers, some well pumps,
electric ranges, electric clothes dryers, Central Air Conditioners…) Full
size (1100 watt) microwaves actually draw about 1700 watts, so, a somewhat
larger inverter than is used in this system would be needed to run one.
Even with a larger inverter, a full size microwave would use about 300
watt-hrs for 10 minutes of use – a heavy load that is probably best
avoided.
Components
Batteries
This project uses two 6 volt deep cycle golf cart batteries hooked in
series to provide 12 volts with an energy storage capacity of about 215
amp-hrs, or about 2.6 Kilowatt-hours of stored energy. These batteries
are flooded lead acid, which is the oldest, but cheapest way to store
energy for this sort of application. Only about 80% of the capacity
should be used to avoid damaging the batteries with overly deep
discharges, so the usable capacity is about 175 amp-hrs or 2.1 KWH.
The flooded lead acid batteries require some maintenance – they must be
checked for low water a couple times a year. They must also be used in a
space that is large enough to let hydrogen generated in the charging
process to dissipate on the air. They also self discharge over time, so
some type of battery maintainer to keep them fully charged is also needed
to insure that the system is ready when you need it. The battery
maintainer also prevents battery degradation due to undercharging.
Alternate battery choices would be Absorbed Gas Mat (AGM) or Lithium.
AGM batteries offer lower maintenance (no need to add water), and no need
to vent. but cost about twice as much as flooded lead acid. Li batteries
offer lower weight, no venting requirement and no checking of water level,
and longer cycle life, but are about ten times more expensive per amp-hr
stored. Cycle life is not a big factor for this kind of application as
there will be few charge/discharge cycles.
The golf 6 volt, 215 amp-hr, GC2 cart batteries cost about $90 each from
Costco or Sam’s Club. With good care, they will last about eight years..
Inverter
The inverter allows 120 volt AC appliances to be run from the
batteries. We chose a fairly large 1500 watt sine wave inverter which is
large enough to run most household appliances including most refrigerators
and furnaces. We chose a “pure sinewave” over “modified sinewave” in
order to insure that even finicky AC appliances would run without
problems.
There are many inverters available with a wide range of power outputs to
choose from. I would take a good look at the ratings from people who have
used them over time and buy one that people have found to be reliable, as
inverters tend to be one of the more trouble prone components of the
system. If you don’t anticipate running larger AC appliances, then a
smaller inverter would be fine.
Solar Panels and Charge Controller
We used a Renogy system kit consisting of two 100 watt, 12 volt PV
panels, and the 40 amp Renogy Rover MPPT charge controller.
Charge controllers come in PWM and MPPT varieties. The MPPT ones cost
more, but do a more efficient job of matching the PV panels to the
batteries, especially with partial shading of the panels. If you can get
the MPPT feature for a small price premium, then it is probably worth
spending the extra money for it.
We could have gone with a 20 amp charge controller for the two 100 watt
panels we have, but decided to spend a bit more for the 40 amp version,
which will let us add two more 100 watt panels in the future if desired
without changing to a larger charge controller.
The only ongoing cost of the system is battery replacement, which works
out to about $22 per year for an 8 year life.
Possible Smaller or Larger Systems
You may be happy with being able to run some LED lights, charge your cell
phones and laptop, watch a small (efficient) TV to catch up on the news
and not much more. In this case, a smaller, cheaper and less bulky system
might be fine. Here is one possibility for such a system:
Such a system comes out a bit over $300 and still provides a very useful
backup power capability for a power outage.
For a larger system that would run larger AC appliances for longer
periods of time, here is one option.
Add two more golf cart batteries (total of 4), which would bring the
usable stored energy up to about 4 KWH. Hook the batteries in series to
form a 24 volt string. The same Renogy Rover charge controller could be
used to charge the 24 volt battery, and in this configuration it will
support up to six 100 watt panels for a total of 600 watts. This would
add about $700 to the base system described above.
Another option for a larger system would be to use the same four golf
cart batteries as described just above, but use larger 300 watt PV panels
that are commonly used for home PV systems. This takes a different kind
of Charge Controller that can handle the higher voltage of the larger PV
panels. For example: … <---------------
Running a Refrigerator
A refrigerator is one of the things you would really like to be able to
keep running during a power outage. Unfortunately, it is also one of the
larger electrical loads in the house. Energy Star refrigerators in the
average household size use about 1 KWH per day. This system stores about
2 KWH when the batteries are full, and the 200 watts of solar in a good
solar location will produce an average of a bit over 1 KWH per day. So,
the system will likely keep up with the fridge demand for a day or so, but
will fall behind with only 200 watts for the longer haul. It is possible
to add another 200 watts of solar panels to the system without changing
any other components, and this would make the odds of keeping up with an
efficient fridge over the longer haul much better. The section on
alternate systems suggests a couple ways to add even more power.
You can reduce the energy use of your fridge by 1) not opening it
anymore than you have to, 2) turning the fridge thermostat up to as high a
setting as will keep the food OK, 3) cleaning the condenser coils (usually
under the fridge), and 4) adding some insulation over the fridge – maybe
just that down comforter you are not using. Most fridges have warmer
elements along the door seals to reduce condensation – if these can be
turned off, that will also help. If you want to know how your fridge
stacks up, get a Kill-A-Watt
meter and measure its consumption for a few days. The Kill-A-Watt
meter is inexpensive and has many other good uses, so it’s a good
investment.
If you have advance notice that a power outage is likely (e.g. hurricane
coming or utility announcing a planned outage to prevent forest fires),
then you could freeze some blocks of ice in containers in advance of the
outage. The frozen blocks can then be moved from the freezer to the
fridge section as needed to keep the fridge section cooler. With the
fridge section and freezer section thermostats set to higher temperatures,
this should result in less run time.
Note that non-energy star fridges and larger fridges can use well over 2
KWH per day, and the solar backup system described here will not be able
to keep up with their demands.
Other Ways to Charge Batteries
If the solar panels are not providing enough energy to meet the demand
(perhaps due to cloudy weather), then these other methods might be used to
charge batteries:
Generator: Some generators provide a 12 volt
DC outlet that can be used to charge your deep cycle batteries directly.
The disadvantages of this is that the maximum charge current is usually
low, and the generator will not do a multi-stage charge, which is easier
on the battery. It is generally better to plug a 120 VAC battery charger
into the outlet on the generator and hook the deep cycle battery to the
charger – this will provide better regulated and faster charging of the
deep cycle battery. The advantage of using the solar along with a
generator is that you only to run the noisy generator long enough to
charge the deep cycle batteries, and on many days you won’t have to run it
at all.
Hybrid Car: If you have a hybrid car (eg a
Prius) you may be able to charge the batteries by tapping into the system
that charges the 12 volt accessory battery that most hybrids use. In this
case, the main traction battery will charge the 12 volt battery when
needed, and when the traction battery needs charging, the hybrids internal
combustion engine will start up to charge the traction battery. Accounts
on the internet say this all happens smoothly, automatically, and
efficiently, but you will want to investigate it carefully as expensive
equipment and warranties are involved.
Building the System
The system is setup with the batteries, inverter and charge controller
mounted on a wheeled support that resembles a hand truck. The wheels
allow it to be stored out of the way until needed and then wheeled into
the house during the outage to allow for convenient plug in of loads to
it. The whole setup weighs about 140 lbs, so the wheels are pretty much
a must for moving it around easily.
Between power outages, the stand can sit in a corner of the garage with
the battery maintainer plugged into a nearby outlet. The only between
outage maintenance is to check the battery water level a couple times a
year.
The two PV panels are separate so that they can be placed out in the
yard with the wires running into the base unit through a door or window.
The two PV panels are hinged together into a single unit that is easy to
carry. A prop holds the PV panels at the desired tilt angle. The panels
can be adjusted a few times during the day to point directly at the sun,
which will increase the amount of energy collected.
The pictures show the construction, which is quite simple, but does the job
well. A bit more than half a sheet of 4 by 8 plywood is used to make the
stand. I used Medium Density Overlay plywood, which is very stable and
durable, but any good quality ¾ inch plywood would be fine
