Introductory Flow
The ultimate truth of everything on this page should be taken as a first approximation of designing a PV solar system. As with all things, there are a myriad of technical and other considerations in designing a solar system: this page is some good rules of thumb. Please see Designing a Solar System/Advanced/Technical Considerations for more considerations.
Read Basic Flow and Factors
Watch the three homescale solar videos from supplemental.
You can read basic electricity and solar system vocabulary at the top of 2.9 Supplemental material for Designing a PV array.
Basic Flow
Solar Panels/Array receive sunlight -->
Transmit through wires -->
to a Combiner box (optional : useful if you have multiple strings of panels) --> Wires -->
Charge Controller (necessary if battery based) -- the brains of the situation --> (wires)
Disconnect -- a breaker (safety/control) --> (wires)
Feeds the batteries (unnecessary for grid tied) --> (wires)
Disconnect -- a breaker (safety/control) --> (wires)
Inverter (necessary for AC power) -- inverts DC --> AC for standard home use.
FACTORS in Designing a PV System
FACTORS in designing a solar system
• On Grid vs. Off Grid (Battery-Based)
• Size
• Array placement
• Components
The first question we must address is whether we will be connected to the grid as well as solar energy as this will inform much around all of the other factors.
SIZE
HOW BIG DOES MY SYSTEM NEED TO BE?
GRID TIED
In a grid tied system. All considerations are economic. Every dollar spent on grid tied photo-voltaic systems is less fossil fuels burned in the world. Of course there is ecological impact from mining for and constructing the panels but if you receive 2 Noon suns in your area most of the year, it is still more environmentally sound than burning fossil fuels.
There is an economic sweet spot for each household based on their energy usage, local utility company policies, solar window, etc. . .(roughly at about 1/2 - 2/3 of your energy consumption)
So if you consume on the order of 20 kilowatt hours per day and receive 5 noon suns then you could hit the sweet spot with a 3 kilowatt array that produce about 15 kilowatt hours (3kwatt x 5 noon suns) x 0.75 derate for energy loss = useable 12 kwhours/day.
BATTERY
For a battery based system, you need to size your array to meet your energy needs. This is done by adding up the energy loads in your household and devising a PV system that has the power needs with the hours of sunshine in your area to match the needs of your loads. Consider a hybrid system with microhydro or a generator backup for some resiliency.
My experience in Baja is that a 1.12 kilowatt solar array is plenty to run an energy conscious household with 4 L-16 batteries.
An inverter that is rated to 3000-5000 watts is sufficient to run several large draw appliances simultaneously.
ARRAY PLACEMENT
1 HARD SHADING trumps all other factors for selecting array positioning /orientation (i.e. trees, buildings, telephone pole, bird poops -- anything casting shadows on array). Bias perpendicular angle of array toward the most open window of your sky.
2 ORIENTATION & TILT
In the absence of hard shading, due south azimuth is the best.
If you have hard shadows from trees on your array at 3 oclock in the afternoon, bias your system toward perpendicular at 10 a.m.)
Most effective tilt angle for the array is latitude at the equinoxes +/- 15 degrees at the winter and summer solstice. (I change my system 2x per year on the fall and spring equinox from the winter tilt angle of 38 (23 latitude + 15) degrees to the summer tilt angle of 8 degrees (23 - 15))
Fixed tilt angles are often biased toward the winter angle (least production) for battery based systems; whereas for grid tied systems, summer tilt angle usually maximizes yearly production. (again, it is an economic game with grid tied as you never need to worry about when you are producing power; whereas with a battery system, normally you are matching your system to meet your needs.)
General Guidelines
My 1.12 kilowatt array(power rating) in Baja, Mexico (with 5-6 Noon Suns) produces more energy (power x time) through most days than I have room to store or need to burn (indeed, even when I had 8 L16 batteries, they were full by late afternoon and now with 4 old batteries, they are full by noon). And those 4-8 L-16 can easily get my refrigerator and lights through the night. I am mindful of not using other appliances in the evening.
So I try to charge all of my devices (phones, laptop, etc. . . ) and run dishwashers and washing machines during the day when i don't have enough storage (i.e battery capacity) for all the free Fuel (sunlight) coming in. At night -- I exclusively run the refrigerator and lights. There is a fine balance between how much awareness you are willing to give and how big your system needs to be.
Rules of Thumb:
• The array size is rated in kilowatts(power). Energy (kilowatt hours) is the product of power multiplied by the number of noonsuns (hours per day that your system is receiving the equivalent of the amount of energy it receives at noon).
• Calculate your energy needs by using a load list to calculate power (kilowatt) of the appliances you use multiplied by the amount of time you use them (kilowatt-hours) = your energy usage.
• Energy produced is the power of the array x noonsuns in your area minus any shading on your array.
• Energy available for use (practical) is after all the losses in your system (inverter inefficiencies, battery losses, wire losses, shading/bird poops)
• Match your battery bank to your PV array size so that your batteries are full most nights when the sun goes down. More batteries DOES NOT EQUAL better. Correctly sized systems are good.
• More panels isn't always better either, especially when the panels don't match. A panel with a different power rating tied in to a string can derate the system rather than give more energy.
• Rough guideline for system loss solely based on the electrical components is 60% (battery based) or 80% (grid tied). This is the useable energy that you get out of a system versus the produced energy; there are inherent losses in wires, inverters, batteries, etc. ..
LIVING with MY 1.12 kilowatt array in BAJA
• Use energy during the day when the sun is shining so that you minimize loss of energy due to full batteries (especially dishwasher, washing machine, sewing maching close to noon) -- Ideally use large draw appliance after the batteries are full (10 a.m.) and earlier in the day is better so that when you are done, the batteries can fully charge before the sun goes down.
• Charge any equipment with batteries (computers, phones, etc.) during the day.
• Nighttime should only run essential components off the batteries: lights, refrigerator.
• Be careful early morning of using large draw motors before the batteries have adequately recharged from the night. I usually check that my battery voltage > 26 or 2 hours of morning sun have been shining on the PV array.
Grid tie vs. Off Grid (Battery)
For Grid tie, you basically maximize the yearlong production of energy. There is no need to satisfy all of your energy needs with grid tie -- only design where it is in your economical/ecological interests to offset your energy usage with a renewable power source. The ONLY question is how many panels to buy. From an ECONOMIC perspective, it makes the most sense to calculate your energy usage and put in enough panels to cover 2/3 or 3/4 of that. This will drop you out of the top payment tiers in CA or Baja (different laws in different states) and if you use less energy than anticipated, you won't be giving away your electricity production or selling for 6 cents what it cost you double to triple that to produce). Wholesale kilowatt hours are about 6 cents per kwhour and production on a solar system is closer to 15 cents.
Generally, for grid tied in CA, it is best to do a 30 degree fixed angle mount in the largest solar window on your property.
So the components necessary in a grid tied system are an inverter that is of the same wattage as the PV array and a PV array the size that you can ECONOMICALLY and ECOLOGICALLY justify for your pocketbook and to help save the world by burning less fossil fuels.
If you would like constant reliable back up even in the case of the grid shutting down, it is more economically and ecologically responsible to buy a generator than to install batteries that will only be used a couple times per year.
VOILA
OFF-GRID
The sun only shines during the day, so for reliable power through the night and clouds, mix (hybrid) your PV with another power source: grid, wind, microhydro, battery, or generator. We will primarily discuss battery based systems. Consider a generator back up even with batteries to prolong the life of your batteries by preventing them from discharging too deep and as a way to supercharge (equalize) your batteries or simply for the days you want to run lots of power tools or whatever.
BATTERY BASED
Battery Based systems are significantly more complicated (and expensive) as you are responsible for your own back up power. (Consider a natural gas or gasoline generator). Designing the array size demands figuring out how much energy you would like to have available on a daily basis while considering weather patterns in your area and how you would like to deal with days of no sun (i.e. bigger battery bank-- consequently a bigger array as well--or a generator back up are nice options). In Baja, however, we have sunny skies almost everyday, so a battery bank that can get me through the night and maybe the occasional cloudy day is perfect. Investing in a generator that I can kick on when there is crazy weather makes more sense than doubling my battery bank (and array) for that one week a year of crazy storms (both ecologically and economically as there are negative environmental impacts from batteries as well as burning fossil fuels).
Batteries like to nightly discharge about 20% and on the 3 days of no sun can healthily discharge 50%. It is not recommended to discharge your batteries below 50% regularly or less than 20% nightly. Some battery people recommend discharging them fully at the beginning of their life and periodically to show the capacity. Maximum of 1x per month and more likely 2 x per year.