PV Home Solar Panels — Are They Worth It? Pt 2

In part 1 of this dual part article, I introduced a very attractive concept of the very real possibility of having your solar panels foot the bill on your monthly mortgage payments. Here in part 2 I’ll go into additional points to consider, that may give you a better understanding of the full picture and have you on your way to a higher quality of life. (I would suggest you first read part 1 if you haven’t already.)

Practical Points to Consider:

– Location is extremely important, as not all regions receive equal sunshine. As an example, an area that receives 250 sunny days a year can generate over 20% more electricity per year than an area that only receives say, 160 sunny days a year. That difference over the 20 years you’re eligible for the FiT can add up to quite an amount. See this page for a chart of all Japanese prefectures and their average number of sunny days per year over a 30 year period between 1981 to 2010. — The top prefecture (Kagawa) receives an average of about 250 sunny days per year, and last place goes to Akita with, you guessed it, about 160 per year.

Global Horizontal Irradiation

Japan’s Solar Potential. SolarGIS © 2014

Obviously, you can’t complain about less than ideal solar power generation if you insist on building your house in an area with less than ideal weather conditions.

– For countries higher in latitude, a single or mono-pitched roof sloping downward toward true south is the most conducive to solar power generation, as this provides the most surface area facing the desired direction. A mono-pitched roof is therefore a prerequisite if you hope to come anywhere near the numbers stated in part 1. Read further below for optimum solar panel tilt.

Mono-Pitched Roof Solar Panel

Up until recently the pattern has been to add solar panels onto the roof of an existing house or structure. The problem with this is that because the house and its roof were not built with the idea of generating the maximum amount of solar power — orientation, angle, roof area, etc — the power output was only a fraction of its potential.

The concept of custom-designing and building your home around the solar panels you will install, is fairly new. It’s no wonder that the “toys” most people have installed on their roofs today fall short of expectations.

– The structural design of the house must be able to support the weight of the solar panels. Due to the need for higher structural requirements (additional columns and beams), you might not have as much freedom in terms of large open interior spaces.

Other Tidbits:

– When installed so as to cover more or less the entire roof area with little or no gaps between, the solar panels act as a “second roof” or layer of insulation, adding significant efficiency to the roof as a thermal barrier. This “double roof” is known to make as much as a 3 degree difference when compared to an identical roof without the solar panels.

– The average shelf life of a standard solar panel is about 30 years, with most solar panel companies insuring up to 25 years.

– Japan is the top in terms of FiT payout at 34.5 yen per kWh, as of 2014. By contrast, Germany, a renowned leader in renewable energy production, pays the equivalent of 17 yen per kWh.

– The median nameplate capacity of residential solar panels here in Japan is 4.56 kW. Again, it’s therefore no wonder that most people are of the opinion that solar panels don’t pay.

The Optimal Angle or Pitch of Solar Panels

The sun is directly above the equator twice a year at the spring and autumn equinoxes, making the “latitude tilt” — the degrees latitude of your geographic position — the logical “best average”. However, as common sense dictates, the sun and its energy-producing radiation will be the most direct during the summer months, not the spring or autumn.

The sun's angles at various times of the year at 50 degrees latitude.

The sun’s angle at various times of the year at 40 degrees latitude.

Here we have a dilemma: put priority on the hot, sunny, summer months of fairly direct sun, in which case you tilt your solar panels at the angle which forms 90 degrees during the summer (a significantly lower angle) and hope it makes up for the rest of the year; or, stick to the latitude tilt, and let the weaker but more consistent sun work its magic throughout the year.

A third option is that of using tracking solar panels or ones whose angles can be adjusted manually. Such solar panels naturally cost more and also require more roof surface area to be effective, as they will inevitably cast shade on the adjacent panels if they are snug, rendering their superior features all but irrelevant for roofs whose entire surface area in either case is to be covered by solar panels. Tracking panels also consume a sizable amount of energy “chasing the sun”.

Seasonal manual adjustment for 50 degrees latitude.

Seasonal manual adjustment; ideal if you have the budget and space.

Needless to say, smaller houses that have limited roof surface area should go with the fixed angle solar panels. Winter snowfall will also affect the optimal tilt as a steeper angle will shed snow, while a summer tilt (a flatter angle) won’t. Snow piled on your solar panels will effectively halt your electricity generation.

Here we’ll go into what exactly then, is the optimal fixed tilt for your solar panels. Japan lies between latitudes 24 and 46 degrees north, with a 35 degrees north mean latitude. A formula that is used in the industry for areas in the 25 to 50 degree range is latitude tilt times 0.76 plus 3.1 degrees. Solar panels in Tokyo for example, would then be optimal at roughly 30 degrees.

See Optimum Tilt of Solar Panels for a more complete analysis.


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