John Drake Services, Inc.
1427 E. 68th Street
Long Beach, CA 90805

voice line
(562) 423-4879

electricity from the sun by John Drake II    |     home
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Start with the loads you are going to operate
Most people put the cart before the horse when setting up an alternative energy system.

They start with purchasing the photovoltaic modules and then the rest of the system's

How do you pick up the right number of modules if you do not know how many you will need?

First off, I would suggest that you visit this web page and print out the following work sheet
(you can hit your browser's Back Button to return to this page):

Using the work sheet you would enter the daily power draw of the loads you will be using.
This would tell you what your total power consumption would be. It is best to have everything
work out into watts of power. Volts times Amps = Watts.

Please remember that you will be using different loads at different times of the year.
For example, you would be running lighting loads longer in the winter than in the summer.
Water pumping loads are usually operated more hours a day in the summer than in the
Unfortunately in the winter time, when you will be running the most lighting loads, is the time
of year that you will have the least amount of usable sunlight. It may sound obvious, but a
lot of people do not take this into consideration.

This means that your power consumption will vary depending on the time of year.

To get an idea how much sunlight you receive in your location please click onto the following
link (you can hit your browser's Back Button to return to this page): solar insolation chart

So for giggles, let's say that you average power consumption per day is 2000 watt hours,
that is watts used per hour times the number of hours operated in a day.

Does this mean that you will need to produce 2000 watt hours of power per day to cover
the needs of your loads?
No, it is a start.

You will need to produce more than 2000 watt hours per day - there are losses in the system
that have to be planned for.

What kind of power losses have to be figured in the plan?

First off, when electricity passes through a wire some power is lost due to the resistance of the
wire. Every connection in the system can be a point of power loss, this is why good wiring
connections are so important (also from a safety stand point). When power goes through
devices there is some losses involved - especially in d.c. to a.c. inverters.

A note on inverters.

The fan on the back of an inverter blows heat out of the housing when in use.
That heat is wasted power and the efficiency of the inverter (how many watts go in the back
from the battery verses how many watts go out the front) is a big part of your system's power

A note on batteries.

Batteries do not store electricity, they convert electrical energy into chemical energy which is
stored and then back into electrical energy when power is drawn out.
This conversion consumes power, you may have noticed that batteries heat up when being
The losses can be as little as five percent for a brand new battery to over fifteen percent for one
that is on its last leg.

All of these power losses need to be added to the total power consumption of your loads.

Putting it together.

Let's say that you daily power requirements, including losses are 2400 watt hours per day.  

The solar insolation chart shows approximate hours of full sun per day during the summer, the
winter and on average throughout the year.

If you size your power generating system for winter use, you will be producing more power in the
summer than you will need.

This is where your application will help determine the amount of generating capacity you will need.
Is this going to be a full-time year round home, a summer only cabin or a winter only cabin?
The time of year that the site will be used can make a lot of difference in the installation size.

Let's figure you are in Lincoln, Nebraska.

The high hours of full sunlight are about 5.40 hours per day, the low are 4.26 hours per day and the
average is about 4.90 hours.
If you have an off-grid home (the only power available is what you produce) you might use the average.
If so, you will come up short in the winter and have some extra power in the summer.
If you have a winter cabin, the 4.26 hours would be what you would base you generating system
size on.
A summer cabin or cottage would be about 5.4 hours per day.

So for a summer cabin we would take 2400 and divide it by 5.4 and you would come up with a requirement of
about 445 watts of photovoltaic modules on your roof.

I am not math whiz, but is sure looks pretty simple.  

If you are in an area with many days of overcast skies, you would need to figure that into your generating capacity.
In some areas of the Northern California coastline, people experience a heavy overcast until ten
or eleven in the morning most days of the year.
These folks have to have more solar panels on their roof than those people living further inland.

To be continued.