Your Role
•Sunny Roof or Location
•Determine Your Electric Usage
•PV Peak Output Ratings
•Energy Efficiency
•Investing in a Solar System
Horizon Energy Systems
3801 Charter Park Court
San Jose, Ca. 95136
Bruce Gordon, Owner
Contractor License #829839
C46 Solar Contractor
B General Contractor
Connect with us today!
• 408-978-0111
• info@gosolarnow.com
Charts and Information
(view on line or .PDF)
•PV Reasons to Install.pdf
•PV Solar Steps.pdf
•PV Considerations.pdf
The following information covers a variety of topics of interest to those considering installing solar electric power at their California home or businesses. Please contact us if you would like a more detailed explanation of these topics. We also offer solar classes throughout the year. Check out the MORE page for class schedules and links to solar information in other US. states.
Solar electric systems require full sun to operate properly. Most systems are installed on a roof. Ground mounting is also possible. If you plan to site the solar panels on a roof, is this roof in good shape? PV systems have 30+ year lifetimes, so if your roof has recently been replaced or is in good shape, then now is probably a good time to install a system. If the roof is in poor shape, then roof replacement is recommended before installing the solar panels.
Does your solar site get at least 5 hours of sun during the middle part of the day (between 9 AM and 4 PM)? If so, you may be a good candidate for a solar system. Summer afternoon sun is the most valuable, so if you have some morning shade or some winter shade it is not as important (financially speaking) as the valuable summer afternoon periods due to local weather conditions (fog usually burns off by the afternoon in the summer. Also there is lots of summer afternoon sun in the San Francisco Bay Area region and the time-of-use net metering billing rate schedule that PG&E offers values afternoon summer electricity generation substantially!
South or south-west oriented roofs are the very best in terms of annual solar production. West facing roofs are very good too if the roof is not too steep. Roofs with a 4 in 12 pitch or less work fine for west facing arrays. West roof pitches of 6 in 12 and steeper will significantly limit solar electric production figures over the course of a year and are thus not normally recommended. Low sloping east facing roofs are ok, but financial credits generated when the meter goes backward are much less for east arrays that perform best during the morning hours. Also summer mornings tend to be foggy which limits production for east facing solar arrays. Maximum solar rebates apply for south or west facing solar arrays, compared to smaller rebates for east facing arrays as well, according to the California Solar Initiative (CSI) which took effect 1/1/2007. The new rebate level of the California Solar Initiative takes shading into consideration, so sunny locations will qualify for the highest rebate levels. For CSI rebate calculation purposes, shadier solar installation sites get smaller rebates. This is computed via a composite "design factor" that also considers regional solar irradiance levels (e.g. sun fall), orientation, tilt and the important specific shading profile of a site. Also time of use metering creates more valuable financial credits during peak afternoon times, when the sun is more perpendicular to a west roof than an east roof - thus a low sloped west facing solar array generates plenty of valuable electricity credits.
In order to properly size a solar electric system it is essential to determine the annual electric load for the site. This is necessary since billing for grid-tied PV systems with net-metering is done on an annual cycle in California (summer credits generated when the meter is spinning backwards can be carried over to offset night time and winter electric usage at retail electric rates).
PG&E customers can call a toll-free number 24 hours a day to request billing info (800) 743-5000 (ask for “billing”). Write down a year’s history of monthly electric usage that is representative of your electric load (measured in kilo-watt hours per month or kWh per year). This data can be used to forecast your future electric demand, in order to estimate the size of a PV system needed to offset the desired amount of future electricity usage.
Is your projected future electric load the same as the recent past? Will the same number of people be using electricity at the site in the future, in the same amounts as the near past? Will an addition be added to the structure or an old refrigerator replaced, etc.? In most cases one’s future electric usage will be similar to the past and tends to creep upward over time as more electronic items are purchased. It is important that the solar investor participate in forecasting future site electric load, so that the properly sized system can be installed initially (PV systems are modular and can be expanded, however, but rebate levels could be different in the future).
Horizon Energy Systems staff can help you determine your current and future energy needs, by conducting an electric energy audit. (refer to services page)
PV Peak Output Ratings (DC vs AC) & Key Performance Factors
When investigating solar electricity it is important to understand the power output of a given system, in order to make a valid “apples to apples” comparison amongst various design options.
Solar electric modules generate DC (direct current) power. The inverter changes this DC power to AC (alternating current) power. AC is what most appliances in the home use. At Horizon Energy, we refer to the peak energy rating of the solar array in terms of AC watts, not DC watts. This is because, in our opinion, the energy that matters is the usable AC power that comes out of the system that is available to be used and not the DC power that is fed into the inverter. In the solar industry,there are specifications that refer to the capacity of a PV system in terms of peak rating in AC or DC watts. DC and AC watt ratings are not interchangeable. The difference in power between total DC watts input into the inverter vs. AC watts output from the inverter is significant.It is important, when comparing systems, to compare AC watts with AC watts or better yet compare kWh energy production output for various system configurations by carefully considering the major site factors that affect performance such as shade, solar panel orientation and the time of day of solar generation.
Solar kWh energy production over the course of a year may be difficult to objectively compare for different solar installers as the level of expertise and abilities of the solar professionals vary since some can more accurately estimate solar performance than others. It is misleading to compare different system configurations of DC watts with AC watts even though the difference in peak rating may appear small. The difference in performance may be significant and can be enough to power one or more major appliances, such as a refrigerator, for a year. So please consider comparing AC to AC or DC with DC or even better compare various system options using the forecasted kWh energy production figures (the latter can be done using the California Solar Initiative Incentive Calculator that is used for rebate purposes that documents the forecasted summer and annual kWh production figures for a particular solar array. Solar installers are required to use the same method with this CSI Incentive Calculator to compute the solar rebate for most California jurisdictions. You can request this information from your solar installer or go to the web site: http://www.csi-epbb.com to compute this yourself - note this may not be easy to calculate for the average person who may not have access to a robust shading analysis tool or who may not be familiar with the input factors used in this rebate incentive calculator such as tilt angle and azimuth).
Here’s how to properly evaluate the peak power rating of a solar array:
STC rating (Standard Test Conditions): is the factory rating, in DC watts, of a solar module. It does not take into account real world conditions. The STC rating is consistently measured under laboratory conditions and all solar module manufacturers are subject to the same set of testing methodologies. The STC rating is a solar module's peak “nameplate” rating in terms of DC power and not very useful for our purpose here.
PTC rating: This is the PVUSA Test Conditions or what we refer to as the Practical Test Conditions – this is a measure of peak DC watts as measured in the “real world”. It is the output energy of the solar module in terms of DC watts that considers the voltage loss factor that is experienced when solar modules sit in direct sunlight and heat up (the hotter the solar cell temperature the lower the solar power output, because there is a direct correlation between higher temperatures and lower voltages due to lower efficiencies at higher temperatures or conversely higher voltages at lower temperatures in the DC source circuit of the solar modules).
CEC-AC Watts: This is the California Energy Commission rating in terms of peak AC watts. This is the measure of peak energy output that a solar array can produce taking into account the efficiency of the inverter in converting DC to AC power. This is a true measure of a solar energy system's peak rating because it considers two of the major loss factors that affect system performance in the real world (heat and inverter efficiency). It is also the value the California Energy Commission uses to determine the rebate amount that a solar array qualifies for in California. Note you can get the make and model numbers of the panels and inverter and go to the CEC web site to look up their PTC numbers and the inverter efficiency for yourself if you want to. This is easy to do and the links are below. In order to claim a rebate on a PV system in California one must install only solar modules and inverters that are listed at the CEC web site and on this “approved” list.
Links to CEC:
http://www.gosolarcalifornia.org/equipment/inverter.php
http://www.gosolarcalifornia.org/equipment/pvmodule.php
Once you have found the ratings for the equipment, here’s the calculation to use:
Number of modules * PTC rating of each module * Inverter efficiency = CEC AC peak watts rating.
Divide this answer by 1000 to get the peak AC watt rating in kilowatts.
The most efficient solar arrays use the least amount of space to generate the most amount of power. With an efficient solar system you can put the solar modules in the sunniest spot for maximum long term benefit. This may be a critical factor for situations where there may be even a small amount of shade during the middle part of the day when the sun's energy is at its peak! As just a small amount of shade can significantly impact the performance of a PV system!! If there is plenty of area of ideally oriented south or southwest space to work with, that has equally good solar irradiance levels, then there may not be a significant advantage to using highly efficient solar modules. However, if there are some shading issues or space is tight then efficient solar modules may be desirable.
Another technical performance consideration deals with solar panel orientation and this applies to PV arrays with multiple orientations (e.g. angle of incidences to the sun). This is important for solar arrays with more than one orientation or tilt angle for the solar panels as this may adversely impact solar output for the same inverter. It may be best to have a particular DC source circuit configured with the same orientation, since the output of a DC source circuit is as strong as the least efficient angle of incidence to the sun for a particular set of modules in the same string. Note: a "string" commonly refers to an independent DC circuit in PV system wiring design lingo. Solar modules are configured as "strings" of many modules wired in series to increase the DC voltage going into the inverter in order to hit the maximum power point.
Energy Conservation & Efficiency
Normally each dollar invested in energy efficient appliances to offset energy wasting items, will return three dollars in avoided up front solar system costs. For example, an older refrigerator normally uses quite a bit more energy than a new energy star unit. The $600 price of a new energy efficient refrigerator will reduce the investment required for a PV system by about $1,800. One's actual savings will depend on the cost of the new refrigerator, the actual energy savings realized from this new refrigerator and the avoided cost savings realized for a smaller PV system. This same concept applies to pool pumps, lighting, insulation (for electrically heated homes) and other electric appliances that are obsolete in terms of their energy usage compared to energy efficient replacements.
Estimating electric load is one of the few necessary issues the homeowner must address when determining a PV system's size.
A small, 1.5 kW (AC rated), grid-tied, fully installed solar electric system can cost as little as $12,000 after all rebates. A more typically sized 3 kW system (completely installed, permits and all) normally runs about $21,000 (after rebates and tax credits). The cost per watt tends to drop as system size increases, depending on how loaded the inverter is (fully loaded inverters tend to yield the best cost per watt ratio as well as performing more efficiently over the course of a day). Systems can be affordably financed with home equity loans and the loan payments may be similar or even less than the monthly electric bill which makes solar a real bargain! The interest on home equity loans may be tax deductible as well.
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