Independent Power - How Many Solar Panels?
How much power do you need?
The quick answer to this question is to divide the daily power requirement by the number of hours of sunlight. If I need 5,000 watts per day that means I need 1250 watts worth of solar panels, provided I have 4 hours a day of sunshine at my location. Pretty simple. The problem however is that like most general assessments there are details that throw a monkey wrench into this simplified view.
So, if your serious about setting up a solar
power system you can live with, let's dig a little deeper
and see if we can get a clearer view of the issues. If we're
going to do this thing, let's try to get it right. It never
hurts to understand the basic issues involved.
- Anything that blocks the sun from landing on your panels is an issue. The affects
of clouds, bad weather, and site location problems like living in a deep valley
where the sunshine is more limited needs to factored into the equation.
- Recovery Time
- After a bad weather period when the batteries are low, you need extra power from
the sun to bring the batteries back up to full. This can take several days, depending
on how much power you are collecting and how big your battery system is..
- Charge Controller
- There are many different charge controller technologies in common use today, but
the best is usually called "Power Point" charge control which extracts the maximum
power from your panels to the batteries. In a future article I explain how that
works, but for now remember that the charge controller is important. We need every
watt we can get from our panels.
- Charge Losses
- When batteries are charged some power is wasted in the process. This charge loss
varies but can be estimated at 10-15%.
- A large high quality set of batteries is required in order
for us to have the power we need to get through periods of bad weather. When I
kept records at my location for 3 years, I found that a four day stretch of bad
weather was extremely rare. So my current planning is for four days of no solar
input. However, with a generator for backup you can get by with fewer batteries,
but the idea is to eventually not need the generator except for real emergency
- Seasonal Changes
- The sun is up higher and shines longer in the summer than the winter. The path the
sun takes through the sky is longer during the summer, and the angle of sunlight
on the panels varies considerably throughout the year. Another consideration is snow
fall. Snow can block the panels from sunlight.
- Tracking solar panel mounts can reduce
the number of panels you need, or help you get more
power from the panels you have. Pricing has dropped
and they are now a more realistic purchase than
they once were, in my opinion. Site location will
determine the extent of the benefit. Here we have
difficulty finding a good location for tracking
mounts, particularly since I need several for the
power I want to collect.
- Be aware of trees and buildings that
could block the panels. Think about both winter and summer
solstice when the sun is at it's extremes of elevation.
What works in the summer might not work in the winter
when solar energy is most difficult to collect.
- In our case we have large fixed arrays set to the
minimum angle (optimum for winter). The panels shed the snow quickly
after a storm, and we get the best results in the winter when
collecting power is harder. True, we get less power in the summer
but since we have a lot more sun for a longer period we have what we need.
No moving parts, no maintenance. It just works.
- Power Use Estimates Wrong
- A common problem is that people underestimate power usage. Try to be accurate as you
can be, but remember that even when you are being careful, lights are left on,
you might have forgotten something, etc Always over estimate your power requirements.
Don't be. The real answer to how many panels
is to add a realistic fudge factor, so that you can estimate
the daily power. Estimation is all you can do in a case like
this where there are so many variables. None of us has time
to analyze these things to the degree required for scientific
A sensible fudge factor would be 50 to 100%.
If you need 5,000 watts per day, then base your assessment
on 7,500 to 10,000 watts per day. This will give you recovery
time for bad weather and enough to compensate for errors
in estimating power consumption, etc.
What Size Panels?
There are a lot of options regarding the size
of solar panels physically. High efficiency panels are considerably
smaller, but in some cases are not as resistant to physical
abuse. I have quite a few Unisolar US-64(s), which are a
large panel for the wattage rating, however they can withstand
amazing abuse and still keep working. I was initially concerned
that solar panels might not handle hail and other physical
stress as well as the US64(s). I love them still, but if
I had it to do over I might have picked from the current
crop of 225-285 Watt solar panels which are lower in cost,
(now solar panel cost is less than $2/watt) and are much
more efficient. Increased electrical efficiency and smaller
physical size becomes important if you need a lot of power
Pick your panels by lowest cost per
watt, construction quality, and sensible mounting
I'm going to be writing more about power
point tracking (PPT) regulators soon, but what I
want to mention right here is that you are going
to be wiring the panels in series and they will
most likely be running at a high voltage and using
the PPT regulator to get the proper voltage for
optimum charging. The battery voltage varies over
a wide range, and so does the power output of the
panels. The proper regulator is essential in bridging
the gap and providing an optimum solar power system
by maximizing the charge rate and protecting your
Newer high efficiency panels
from reputable companies can take a lot of punishment. So
it may be that worry about ruggedness is a non-issue.
If you have more space for panels, and you find panels
that might be a little less efficient but designed for abuse,
I would make that choice.
The newer, higher efficiency panels I recommend
for most installations.
The better answer to this question is to divide