Here is a series of articles explaining how a photovoltaic system (PV) works and the way best to design and cost a (PV) system for your project.
Before designing and purchasing a photovoltaic system remember conservation of energy and good home design plays an important role in keeping down the size and cost of a photovoltaic (PV) system. The use of energy efficient appliances and lighting, as well as non-electric alternatives wherever possible, can make an off-grid solar photovoltaic or wind-powered system a cost competitive alternative to the utility power grid. Generator backup for part of the day can also help reduce and offset PV installation costs.
A number of sun-hours per day, days of autonomy and the homes average energy consumption will determine the size and ultimately the cost of any PV solar electricity system.
Electricity from Sunlight (Photovoltaic)
Alternating current of course, is appropriate for most regular 240 VAC appliances, washing machines, power tools, large screen TVs and game consoles, microwave ovens, toasters…etc. Cordless telephones and answering machines, clock radios and alarms, chargers for mobile phones or ni-cad batteries all need power on a FULL-TIME basis.
When you install an off-grid renewable energy system (that is, one that’s not connected to the local utility grid) for your home, you become your own power generating company, so every kWh (kilowatt-hour) of energy you use means more solar equipment (and hence more money) is required to meet your energy consumption needs.
Preparation for any photovoltaic system begins with assessment and calculation of your energy needs. The primary tool used for this task is a “Loads List”. A loads list is simply a tally of all electrical loads that will be used in the completed system and includes everything electrical that will be used in your off-grid home, such as: lamps and lights, TV’s and game consoles, microwaves, chargers etc, must all be included on your list.
Typical Solar Power System Example
The amount of solar energy that reaches the Earth is typically about 1,000 Watts per square meter (1.0kW/m2). This amount of solar radiation varies considerably with location and time of year. Capturing this solar energy requires equipment and systems with a relatively high initial capital cost. However, over the lifetime of the solar equipment, these systems can prove to be cost-competitive, as compared to conventional energy and fuel burning technologies.
The key to successful photovoltaic system installation is to use quality components that have long lifetimes and require minimal maintenance.
Load List – Calculation for Photovoltaic Domestic System
|Load Name||Wattage||Hours “ON”||Watt-hours|
|All Lamps, General (Fluorescents)||150||6||900|
|Lights, Living Rooms (Fluorescents)||100||6||600|
|Lights, 4 Bed/ 2 Bath (Fluorescents)||240||4||960|
|Lights, Kitchen/Units (Fluorescents)||80||4||320|
|TV 40″, Living Room||140||6||840|
|TV 32″, Bed||90||4||180|
|Washing Machine (A-rated)||500||0.3||150|
|Miscellaneous (Power tools, etc.)||300||1||300|
|Home total||8,530 watt-hours|
No. of Occupants: 4
No. of Bedrooms: 4
All Lights and Lamps use Energy Efficient Compact Fluorescents
Any appliance which is used for heating and cooking such as electric water heaters, cooking stoves, central heating boilers and air conditioners are expensive loads to run on solar power and should be avoided. For cooking you should consider using alternatives such as LPG or propane gas. Use solar water heating for water and space heating and evaporative cooling as well as a passive solar design in your home construction if possible instead of compressor based air conditioning units.
Consider using energy efficient lighting. By replacing one 60 watt incandescent bulb operating 6 hours a day with one 13 watt compact fluorescent lamp. This would save about 282 watt-hours each day which would be the same extra power available each day as adding one more 48 watt PV panel. The lamp change over costs would be about £4 to £5 were as an added panel costs over £80. (Amazon prices).
Notice that these savings of 282 watt-hours is gained every day the lamp is used. On the sunny days when an extra panel would give the same amount, but the new lamp also saves 282 watt hours on sunless days when an extra panel would do little or nothing. That is why conservation in design is MORE important than the energy source itself.
Assessing Solar Power Potential at Your Location
The amount of energy you can get from a photovoltaic system at your off-grid site depends on your location and the time of the year. Generally, you can expect to receive more sunlight between the months of April through to September and have a clear exposure to the sun for most of the day, e.g. 9am to 3pm.
The measurement for the strength of the sunlight striking the earth at your location is defined as solar insolation. Using this value, you can determine how much solar power you can generate through out the year for your particular site. A solar insolation map can help size the minimum solar electric (photovoltaic system) system needed during the periods of the year with the shortest amount of sunshine for your location.
Monthly Averaged Clear Sky Insolation Incident Tarragona Area, Spain (kWh/m2/day) Reference: http://eosweb.larc.nasa.gov
Having found the solar insolation for a particular site, we need to divide the estimated daily energy usage (kilowatt-hours) above by the worst case solar insolation value from the graph and multiply it by a system inefficiency factor of 1.3 (30%) for a solar power system that is off-grid.
The estimated daily usage was 8.5 kWatt-hours per day.
Total Watts of Solar Panels Needed = [(Daily Kilowatt-hours)/(solar insolation)] x Inefficiency Factor
The worst month requiring the most solar power is December at 4.63 kWatts peak. Then we need a 4.63kWp (or 4,630 Watt) system in order to produce on average the 8.5kWh per day for our location with an average of 5.39 hours of solar insolation.
Now that we know the total wattage of the solar electric panel array need, we can select the specific solar panels that will make up this total wattage. Note that this wattage is for 100% off-grid living. If mains electricity is available or connected to the house, this value can be reduced to give a percentage amount of solar energy required to supply the house against mains electricity. For example, 25%, 50% or 80% solar, etc.
Choosing the Required Solar Panels
We already have an accurate idea of solar insolation for a particular site. We’ve done the loads list survey so we know how much electric power we require on an average day. All that remains is to specify the type and number of photovoltaic (PV) panels that will produce the required power of 4.6 kWatts.
We consume an average of 4,630 Watt-hours of power daily. Then by dividing the power consumption by the average output of a single panel will give us the number of panels required.
|Watts Required||4630W||Panels Req.|
Solar panels can vary in type, size, shape, and voltage rating. In most cases the size of a photovoltaic panel refers to the panel’s rated output wattage or electricity generating potential. Solar panels can also have different voltage ratings depending upon their size. Those with of 12 to 48 volts are generally used for off-grid applications. Maximum power (P) delivered by a panel is given as Voltage (V) x Current (I).
Individual solar panels can be combined into larger arrays by wiring them together in a fixed combination in order to achieve the required voltage or current rating. The following tables show possible array combinations for the number of panels required to provide the 4,630 watts minimum.
AKT-120-M Solar Panel
|Array Combination||Voltage (V)||Current (A)||Power (P)||Area (m2)||Weight (Kg)|
|1 x 40||17.5||274.4||4802||38.2||400|
|2 x 20||35.0||137.2||4802||38.2||400|
|4 x 10||70.0||68.6||4802||38.2||400|
|5 x 8||87.5||54.9||4802||38.2||400|
|8 x 5||140.0||34.3||4802||38.2||400|
|10 x 4||175.0||27.4||4802||38.2||400|
|20 x 2||350.0||13.7||4802||38.2||400|
|40 x 1||700.0||6.9||4802||38.2||400|
120W Array Data:
Quantity: 40 Panel Price: 215 GBP
Array Area: 38.2 m2 Array Price: 8,600 GBP
Array Weight: 400 Kg Cost per Wp: 1.79 GBP
AKT-140-M Solar Panel
|Array Combination||Voltage (V)||Current (A)||Power (P)||Area (m2)||Weight (Kg)|
|1 x 34||17.5||272.7||4772||40.3||482.8|
|2 x 37||35.0||136.3||4772||40.3||482.8|
|17 x 2||297.5||16.0||4772||40.3||482.8|
|34 x 1||595.0||8.02||4772||40.3||482.8|
140W Array Data:
Quantity: 34 Panel Price: 240 GBP
Array Area: 40.3 m2 Array Price: 8,160 GBP
Array Weight: 482.8 Kg Cost per Wp: 1.71 GBP
Sanyo HIT-200 Solar Panel
|Array Combination||Voltage (V)||Current (A)||Power (P)||Area (m2)||Weight (Kg)|
|1 x 24||55.8||86.2||4808||27.9||360|
|2 x 12||111.6||43.1||4808||27.9||360|
|3 x 8||167.4||28.7||4808||27.9||360|
|4 x 6||223.2||21.5||4808||27.9||360|
|6 x 4||334.8||14.4||4808||27.9||360|
|8 x 3||446.4||10.8||4808||27.9||360|
|12 x 2||669.6||7.2||4808||27.9||360|
|24 x 1||1339.2||3.6||4808||27.9||360|
240W Array Data:
Quantity: 20 Panel Price: 411 GBP
Array Area: 32.9 m2 Array Price: 8,200 GBP
Array Weight: 420 Kg Cost per Wp: 1.75 GBP
Looking at the tabulated data above, it is clear that the 24 Volt, Kyocera KD-240 Watt solar panel would be the panel of choice offering a better cost per peak watt (Wp) of £1.75 or £1,750 per kilo-watt peak. This gives an array of only 20 solar panels at a cost (excluding fittings, etc.) of £8,200 or £240 per square metre with an array area for roof mounting of 33 m2.
Selecting System Voltage
Depending upon the panel wattage and the array combination, there are number of voltage and current configurations available to us to supply the home with DC power.
All solar power systems with batteries as backup should include a Solar Charge Controller to prevent the batteries from overcharging and also to prevent the batteries from sending their charge back through the system to the solar panels during times of low sun, (i.e., night time).
Since a solar controller does its work in line between the solar panel array and the storage batteries, it makes sense that its selection and sizing is influenced by those components. Voltage and current are the two parameters used in solar charge controller sizing. The solar controller must be capable of accepting the maximum power produced by the solar panels while delivering the proper DC voltage and charging current to the batteries.
12 or 24 volts is the most common standard for an off-grid alternative energy home mainly because it is already a conventional standard. As we progress to higher voltages, less current (amps) is required to deliver the same amount of power (watts). Efficiencies also tend to increase with higher voltage and lower current configurations because a higher voltage array reduces voltage drop over long cable distances.
For low voltage – high current systems, long cable runs between panels or to the inside of buildings can be very expensive in cable costs as the longer the distance, the larger the diameter of the cable in mm2 it must be to reduce power losses. So by reducing the current in half by using twice the voltage we can cut the cable costs used for connecting the DC rated solar panels.
From the table for the Kyocera KD-240 watt panel, the lowest voltage configuration is that of a single panel of 24 volts, but the current supplied is the highest at 157.8 amps as all 20 panels are connected in parallel. This high current value would require very large and expensive cable runs making it unsuitable.
The lowest current rating of 7.9 amps would allow for regular lighting cable to be used but the voltage supplied by the series connected array is too high at 596 volts. Much too large for the batteries. A far better alternative would be a 2 x 10 combination of the 20 panels. This would give an output voltage of 48 volts (2 x 24) supplying a current of nearly 80 amperes (10 x 7.89).
2 Panels in 10 Strings Configuration
The nominal voltage throughout the system, the photovoltaic array, the solar charge controller, and the battery bank is now set at 48 volts (Voc = 73.8 volts max).
The next step in sizing the charge controller involves current. For systems in continuous operation, additional protection must be included. The industry standard 1.25 de-rating factor is specified to prevent the controller from becoming damaged due to excess solar panel current or power.
The maximum short circuit current of the Kyocera KD-240 panel is given as 8.55 Amps. Then the maximum current generated including the de-rating factor for a 10 string array is given as:
Array Isc = (10 x 8.55) x 1.25 = 106.8 Amperes
This value of 106.8 amps may be too high for one single charge controller to be used. Therefore the array may need to be split in half ( 2×5 ) + ( 2×5 ) and connected to two separate controllers while supplying the required system voltage to the batteries. Each solar charge controller used will therefore need to have a minimum rating of: Voc = 74 volts, Isc = 55 Amps.
Each smaller array would therefore consist of 10 solar panels each in a 2×5 combination of 2,400 watts, (2.4kWp). Two possible basic PWM solar charge controllers are the:
1. Tristar TS-60 has a nominal system voltage of 48 volts at 60 amps (150V max.), priced at: £193
2. Xantrex C60 nominal system voltage 48 volts at 60 amps (125V max.), priced at: £270
Note that other types and manufacturers are available which could also be used. Even though they may control the array and battery charging better are themselves 3 or 4 times the price of the two above.
Sizing the Photovoltaic System’s Deep Cycle Battery Bank
Unfortunately the sun does not always shine, especially at night, so some additional backup. During a nice sunny day, the solar array can produce a lot of electrical power, so you’ll need batteries to access that power after the sun goes down.
The amount of solar radiation available during the day depends upon location and time of year which affects the sun rise and sun set times. Generally during the winter season this can be as low as 5 hours or as high as 8 hours during the summer season. An average of about 6 hours a day is generally used.
Example of solar power available during a day.
Then in order to be able to use this power at any time, night or day, we will require storage batteries, specifically deep cycle storage batteries for 100% off-grid living.
Deep Cycle Batteries
Deep Cycle Batteries used in renewable energy solar systems are different from car automotive batteries. Electrical energy is captured and stored, then later consumed, in a regular cyclical manner throughout a day. For example, in a battery-based renewable energy system either solar photovoltaic system’s or wind turbine, the energy produced daily is stored in a battery bank, which is then used by to supply the house loads, lights TV’s etc, at night or on not-so-sunny days.
This repetitive process subjects the batteries to a long slow, daily charge and discharge cycle where shallow cycle car batteries on the other hand with their smaller thinner plates are designed for a quick short starting of engines.
Deep cycle batteries are only 2 volts each, so to use them in a 48-volt system, we will need 24 cells wired together in series ( 2×24 ). Multiples of 2 volt cells, ie, 4V, 6V and 12V cells are available. Also these batteries are large, very heavy and give of gases when charging, so additional equipment and space will be required to deal with such oversized batteries.
The amount of energy that will be consumed per day in the home was calculated at 8,530 watts previously using the information in the “loads list”. This is the minimum amount of storage capacity we need for one day. Next we need to determine the number of days of battery back-up that we want to have on hand. This is called “Autonomy”.
Days of Autonomy represents the number of cloudy days in a row that might occur and for which the batteries will need to supply energy to the house. A standard number of autonomy days are usually 3 days. Then the total amount of energy required for a minimum of three (3) days of storage for our 4 bedroom house that consumes 8,530 watt-hours daily is calculated at:
8,530 x 3 = 25,600 watt-hours
As the deep cycle battery is a lead-acid type, we need to divide this value by 0.8 to maintain a 20% reserve in the batteries after the discharging period of 3 days to ensure that the batteries never fully discharged to zero. This is calculated as:
25,600 ÷ 0.8 = 32,000 watt-hours
If we divide this figure by our system voltage of 48 VDC, we arrive at a lead-acid battery cell capacity of:
32,000 ÷ 48 = 667 Amp-hours
or a daily consumption of 222 Ampere-hours from the battery.
Now that we know the Amp-hour (Ah) capacity of the battery bank that will give us the storage capacity we need, we can now select a specific deep cycle battery. The required deep cycle batteries must meet both the system voltage requirements and the Ah capacity calculated above. For the 4 bed home using a 48V system, we calculated that we needed 667 Ah to produce 8,530 Whr per day with 3 days of storage.
Suitable deep cycle storage batteries identified are the:
1. Exide Solar 6 OPzV 720 which is a 2 Volt (single cell) 720 A/hr Battery, priced at: £450
2. Rolls Solar 5000 – 6 CS 17PS which is a 6 Volt (3 cell) 770 A/hr Battery, priced at: £976
3. Exide Classic – OPzS-765 which is a 2 Volt (single cell) 765 A/hr Battery, priced at: £418
The 2nd battery above, the Rolls Solar 5000 – 6 CS 17PS is a three cell 6 volt battery so only eight are required to give the 48 volts ( 6×8 ). The total battery bank price will therefore be:
1. Rolls Solar 5000 6-CS-17PS, priced at £976 will be: 8 x 976 = £7,808
Clearly, the 6 volt rated Rolls Solar 5000 – 6 CS 17PS battery would be the battery of choice being the cheapest. Also as only 8 batteries are required, less space is used and less connecting cables, links and terminals, reducing installation costs
Note that if the solar charge controller does not have a built in battery monitoring facility, a separate battery monitor will need to be purchased and installed to display the battery banks voltage, current and state of charge indication. Approximate price £150.
Off-grid DC to AC Inverter
We now have an 8.5kWhr off-grid system comprising of 20 solar panels, a Tristar CS-60 solar charge controller and a 48 volt DC battery bank using Rolls 6-CS-17PS, 6V batteries. Unfortunately, this set-up will not power the homes TV’s, computers, fridge/freezer, or washing machine, etc, as these devices require 240 VAC to operate. Then we need to transform the 48 DC voltage into 240 volts AC (alternating current) and to do this we need an inverter.
All inverters convert direct current (DC) electricity into alternating current (AC) electricity to run the AC appliances. The size of inverter needed to power the off-grid home is based on what is called the “peak load” requirements – all the AC loads that could be turned on at the same time. AC Inverters are generally most efficient when operated at or near their peak output but most of the time the inverter will be running a wide range of loads, usually not at its peak capacity.
In order to size the required inverter we could accurately calculate all the loads which will be switched ON at the same time. Such as lights, TV, computer, fridge/freezer, etc, or give a percentage guess, 5%, 10%, 30%, etc, of the total daily load.
Note also that one single off-grid inverter does not have to be used to power the whole house. Separate circuits can be installed and smaller dedicated inverters used to power the fridges, freezers, dishwashers, water pumps or other major electrical motors for example, but if these inverters are expected to run induction motors they must be suitable for surge currents many times the rating for short periods of time while these motors start.
Stand-alone off-grid inverters are available with three basic power output waveforms called: Square Wave, Modified Square Wave and Pure Sine Wave. Square wave inverters are the cheapest but they may not allow operation off, or may even destroy many electronic devices such as computers, printers, copiers, fluorescent lighting and dimmers, and flat panel screens as well as producing a background buzzing noise on radios or older TV’s. Modified square wave inverters improve on this.
Sine wave inverters may have a purchase higher cost, but they can operate almost anything that can be operated on utility power as their sine wave output exactly duplicates the sinusoidal waveform of the utility grid supply so are a better choice.
Then assuming a peak load percentage consumption of 35%, we would therefore require a sine-wave inverter running off a battery bank of 48 volts operating at about 3,000 watts, (8530W x 30%).
Suitable 48 volt, 3,000 watt pure sine wave inverters identified are the:
1. Outback FX3048T which is a 3000W, 48V Sine Wave Inverter, priced at: £1024
2. Apollo Solar TSW4048 which is a 3000W, 48V Sine Wave Inverter, priced at: £1312
The Outback FX3048T is chosen purely on price, but other types and manufacturers are available which could also be used.
Complete Off-Grid Photovoltaic System
|PV Panels||Kyocera KD-240 Watt||20||8,200||42%|
|Solar Charge Controller||Tristar TS-60||2||386||2%|
|48VDC Battery Bank||Rolls Solar 6 CS 17PS||8||7,808||39%|
|Sine Wave Inverter||Outback FX3048T||1||1,024||5%|
|Installation and Material||DC Cables, Panels, Fixings||1off||2,582||12%|
Estimated Component Price
An off-grid, photovoltaic system can be sized to provide electricity during the night or dull cloudy periods when the sun doesn’t shine. But sizing a system to cover a worst-case scenario, like several cloudy days during the winter, can result in a very large, expensive system that will rarely gets used to its full capacity.
To save on a photovoltaics installation costs we could size the system moderately, to power the lights and other essential equipment used daily, but include within the system design an internal combustion engine generator to provide the extra backup electricity needed to power larger electrical loads such as washing machines, driers, heaters, water pumps, etc, or during those periods of peak electrical demand.
As well as petrol and diesel, combustion engine type generators can be fueled with bio-diesel, ethanol, biogases or other such vegetable oil fuels to save on running costs. These generators can produce either AC electricity to power the home directly through the AC distribution panel, or to produce DC power which is stored in batteries the same way as the photovoltaic system panels. Like most internal combustion engines, backup generators tend to be loud, smoke polluting and require a fuel storage tank, so additional space is required away from the main living area and for regular maintenance.
The generator allows the photovoltaic source to be sized smaller for average daily consumption rather than for the higher peak consumption. One of the main problems with this “off-grid” setup is that if the photovoltaic panels, solar charge control or indeed the inverter became faulty or the system needed to be turned-off for whatever reason, the home would be without electricity during that period. Then any backup generator needs to be able to power all of the homes requirements plus an additional amount for motor starting surge currents.
In our off-grid 4-bed home above, we calculated that the homes power consumption would be about 8,530 watts or 8.5kW. The generator must be sized to handle this continuous kW load and any worst case starting load. Assuming an additional 10% redundancy and rounding off to the nearest kilo-watt, a single-phase 10 kilo-watt (10kW) generator would be required to power the home 100%.
Suitable diesel generators identified are the:
- SDMO XP TK9KM which is a 10.7kW, 230VAC Diesel Generator, priced at: £5299 + VAT
- Pramac S12000 which is a 11.9KVA, 230VAC Portable Petrol Generator, priced at: £1995 + VAT
- Stephill SSDK12M which is a 12KVA, 230VAC Diesel Generator, priced at: £6049 + VAT
There will be additional installation costs and cabling costs incurred to connect the generator to the distribution panel of the house or inverter panel.