Independent Power - How Many Panels?

Web

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.

Considerations

Weather

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%.

Batteries
- 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 situations.

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.

Mounting

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.

Confused?

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 exactitude.

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 and panels.

Pick your panels by lowest cost per watt, construction quality, and sensible mounting size.

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 batteries.

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.

Revised Formula

The better answer to this question is to divide the daily power requirement (times our fudge factor 1.5 to 2) by the number of hours of sunlight. If I need 5,000 watts per day that means I need 1875 to 2500 watts worth of solar panels, provided I have 4 hours a day of sunshine at my location. Pretty simple. With our fudge factor we're far more likely to achieve our goal of energy independence!

Solar panels (photovoltaics) are amazing. They make no sound, and they require no maintenance, they just collect power from the sun day in and day out. Just install and enjoy. I love 'em.