How Much Do you Want?

Solar generated power is a commodity, just like utility generated power. Almost everyone is accustomed to having unlimited electricity whenever and wherever the demand exists. But rather than solar PV being a luxury like a swimming pool, PV is a hedge against long-term rising utility costs. This changes the focus of the system cost from the installed cost per DC Watt to the lifetime cost per generated kilowatt hour. And since the amount of solar PV to install is based on several significant issues, some are your decision and some may be beyond your control, the primary factor may well be the long-term cost.


Often the most important deciding factor, the installed cost per Watt, should be pretty much the same when comparing installers. Of course when three different installation quotes are for three different systems, [different brand modules and inverter, different alignment and so on] it can be impossible to logically compare apples to apples. The tried-and-true practice of throwing out the highest and the lowest bids is still good advice when it comes to a large investment like PV. With the increase of incentives in North Texas, numerous startup “PV installers” have sprung up hoping to take advantage of the spike in demand due to the growing availability of incentives. Often their pricing is low in an effort to get new business, they may cut corners during installation in an effort to keep costs low. Also customers should be certain that the PV modules they are buying have an Underwriter Labs [UL] certification on the label. Many deeply discounted modules are becoming available in the marketplace due to the growing US demand. Non-UL listed modules are a violation of most building codes and may be of inferior quality. An excellent source for information about PV modules and their actual energy output and efficiency is Go Solar California

Roof Size and Alignment

The amount of available roof space is an important factor. A PV array rated at 4000 DC Watts in bright sunlight will take up between 350-400 square feet of space. Ideally the roof area best suited to a PV array will be facing true south. [The term azimuth refers to the compass direction] However a roofs azimuth that is facing either east or west of true south is still usable; up to 15 degrees east or west is OK. The roofs slope or tilt angle up from horizontal will have a seasonal effect on a systems performance. A low angle roof will have better production during the summer months, a good thing considering the heavy use of air conditioning in North Texas summers.


Shade is the enemy of solar PV. Traditional centralized inverter type PV systems are seriously affected by even small amounts of shade. However, new technology is allowing one or more individual shaded PV modules to perform better by making each individual PV module a separate energy production/management system; losses from shading will have almost no effect on the output of the entire PV array. This same technology can allow arrays to face several different directions and not reduce power output from un-shaded areas. And to muddy the waters more, there are a number of ways to predict the monthly/annual/lifetime output from a specific system. One of the most widely used is PV Watts. While PV Watts takes into account power loss factors based on 30 years worth of actual radiation and temperature data the calculated losses are still averages. Their estimates cannot include de-rating factors like shade on the array. Numerous other software packages create similar estimates but without site-specific shading analysis, those annual output numbers will be optimistic.

Utility Bill Offset

Generally, the amount of solar PV capacity installed can be based on reducing a percentage of your average daily usage. Average daily usage is easily determined by totaling 12 months usage (in kW hrs) and dividing by 365. However, that total, since it is an average, probably won't reflect the true demand on throughout an average year. Typical usage may be as much as one third higher or lower due to seasonal usage and other considerations.

A better strategy is to determine your base load. Simply, your base load is the number of kilowatt hours that you use during the year for everything except your heating and cooling. This includes lighting, television, microwave, refrigerator, computers and the host of other routine electrical loads that might be running off-and-on, or continuously, every day of the year. Your base load won’t change much from month to month.

The power used for your base loads can be determined by looking at your annual usage history you received from your utility www site or from your monthly bills. Generally, around the months of March and September will be the lowest bills all year because neither the heat nor air conditioner is running.

Knowing your base load will give you an even more accurate starting point for calculating a percentage of your electric usage for offsetting with PV. Keep in mind that an all electric home will have a dramatically different electric usage pattern compared to a home with gas heat over the same year and the same weather conditions. And both houses electricity usage will increase or decrease at different rates depending on the season. Most obviously geographic location will have a large but predictable effect on usage. Your usage will be unique to your personal lifestyle, needs, habits and habitat.