Solar Power System Basics

The purpose of the information on this page to provide a basic understanding of the major components in a basic solar power system, and to help you identify and select the correct size components for your system.

The following diagram shows the major components in a typical basic solar power system.
The solar panel converts sunlight into DC electricity to charge the battery. This DC electricity is fed to the battery via a solar regulator or controller which ensures the battery is charged properly and not damaged (over charging, to fast or to high a charging current). DC appliances can be powered directly from the battery, but AC appliances require an inverter to convert the DC electricity into 120/240 Volt AC power.

Solar Panels
Solar panels are classified according to their rated power output in Watts. This rating is the amount of power the solar panel would be expected to produce in 1 peak sun hour. Different geographical locations receive different quantities of average peak sun hours per day.
As an example, in some areas the yearly average is around 5.6. The monthly figures for this area range from above 6.5 in June to below 4 in December. This means that an 80W solar panel would produce around 520W per day in June and around 320W per day in December, but based on the average figure of 5.6, it would produce a yearly average of around 450W per day. Solar panels can be wired in series or in parallel to increase voltage or current respectively. The rated terminal voltage of a solar panel is usually around 17.0 volts, but through the use of a regulator, this voltage is reduced to around 13 or 14 volts as required for battery charging. Solar panel output is affected by the cell operating temperature. Panels are rated at a nominal temperature of 25 degrees Celsius. The output of a solar panel can be expected to vary by 2.5% for every 5 degrees variation in temperature. As the temperature increases, the output decreases. With this in mind, it is worth noting that, if the panels are very cool due to cloud cover, and the sun bursts through the cloud, it is possible to exceed the rated output of the panel. Keep this in mind when sizing your solar regulator.

Solar Regulators
The purpose of solar regulators, or charge controllers as they are also called, is to regulate the current from the solar panels to prevent the batteries from overcharging. Overcharging causes gassing and loss of electrolyte resulting in damage to the batteries. A solar regulator is used to sense when the batteries are fully charged and to stop, or decrease, the amount of current flowing to the battery. Most solar regulators also include a Low Voltage Disconnect feature, which will switch off the supply to the load if the battery voltage falls below the cut-off voltage. This prevents the battery from permanent damage and reduced life expectancy. A solar regulator also prevents the battery from backfeeding into the solar panel at night and, hence, flattening the battery. Solar regulators are rated by the amount of current they can receive from the solar panels. See section below for information on correctly sizing a solar regulator.

Inverters
An inverter is a device which converts the DC power in a battery to 120/240V AC electricity. Inverters come in two basic output designs, sine wave and modified sine wave (squarewave). Most AC devices will work fine on the modified sinewave inverter, but there are some exceptions. Devices such as laser printers can be damaged when run on modified sinewave power. Motors and power supplies usually run warmer and less efficiently, and some things, like fans, amplifiers, and cheap fluorescent lights, give off an audible buzz on modified sinewave power. However, modified sinewave inverters make the conversion from DC to AC very efficiently, and they are relatively inexpensive. Sinewave inverters provide AC power that is virtually identical to, and often cleaner than, power from the grid. Inverters are generally rated by the amount of AC power they can supply continuously. Manufacturers generally also provide 5 second and 1/2 hour surge figures. The surge figures give an idea of how much power can be supplied by the inverter for 5 seconds and 1/2 an hour before the inverter's overload protection trips and cuts the power.

Solar Batteries
Deep cycle batteries that are used in solar power systems are designed to be discharged over a long period of time (e.g. 100 hours) and recharged hundreds or thousands of times, unlike conventional car batteries which are designed to provide a large amount of current for a short amount of time. To ensure long battery life, deep cycle batteries should not be discharged beyond 70% of their capacity. i.e 30 % capacity remaining. Discharging beyond this level will significantly reduce the life of the batteries. Deep cycle batteries are rated in Ampere Hours (Ah). This rating also includes a discharge rate, usually at 20 or 100 hours. This rating specifies the amount of current in Amps that the battery can supply over the specified number of hours. As an example, a battery rated at 120Ah at the 100 hour rate can supply a total of 120A over a period of 100 hours. This would equate to 1.2A per hour. Due to internal heating at higher discharge rates, the same battery could supply 110Ah at the 20 hour rate, or 5.5A per hour for 20 hours. In practice, this battery could run a 60W 12VDC TV for over 20 hours before being completely drained. There are many factors that can affect the performance and life of a battery bank. It is highly recommended that you speak with an experienced solar power system installer or solar battery provider prior to making any significant battery purchase. A solar regulator must be able to handle the maximum current that can be produced by the solar panels. Reflected sunlight and specific temperature conditions can increase the output current of a solar panel by as much as 25% above it's rated output current. The solar regulator must be sized to handle the increased current. Solar regulators often short the solar panel input when regulating. This does not damage the solar panel, but it does mean that the solar regulator must be sized to handle 125% of the solar panel's rated short circuit current. Example: A BP Solar 80W solar panel has a rated output current of 4.55 Amps and a rated short circuit current of 4.8 Amps. Minimum solar regulator size for a single BP Solar 80W panel would be: 4.8 Amps x 1.25 = 6 Amps. It is recommended that the regulator selected is even slightly larger than this figure to ensure that it is not constantly operating at 100% of its rating, particularly in regions with higher ambient temperatures. In order for you to size the system correctly, you need to note the power rating of each appliance that will be drawing power from the system.

For this example, we will calculate the power requirements for a campervan with:
2 x 15W 12VDC Fluorescent Lights
1 x 60W 12VDC Water Pump
1 x 48W 12VDC Fridge
1 x 50W 240VAC TV
1 x 600W 240VAC Microwave
(Note that a 600W microwave will consume approximately 900W of power)
Calculate total DC and AC loads:

DC Loads
Lighting - 2 x 15W = 30W DC Lights - each used 2 hours per day => 30W x 2hrs = 60Wh/day
Pump - 1 x 60W DC Pump - used 1/4 hour per day => 60W x .25hrs = 15Wh/day
Fridge - 1 x 48W Fridge - runs 8 hours per day => 48W x 8hrs = 384Wh/day
Total for DC Loads = 459Wh/day (Note: no inverter needed for the DC loads, so therefore no efficiency calculation. See AC section below)

AC Loads
Television - 1 x 50W - used 2 hours per day => 50W x 2hrs = 100Wh/day
Microwave - 1 x 900W - used 15 min per day => 900 x .25hrs = 225Wh/day
Total for AC Loads = 325Wh/day Calculating for inverter efficiency typically at 85% => 325Wh/day / .85 = 382Wh per day
Total for AC & DC Loads => 459 + 382 = 841Wh per day

In Central to Northern NSW expect a usable average of around 5 peak sun hours per day. Required solar panel input = (841Wh / 5h) * 1.4 = 235W Note: The 1.4 used in this formula is a factor we have found that can be used to simplify the calculations for basic systems.
To ensure that adequate power is produced in the winter months, use a figure of around 4.0 to 4.5 peak sun hours per day instead of 5.

Select solar panels to provide a minimum of 235W. Always best to go bigger if possible:
2 x 123W solar panels chosen which, when connected in parallel, will provide 246W or 14.47 Amps (246W / 17v= 14.47).

Note: 17v is the voltage output of the solar panel, do not use the voltage output of the regulator or controller to calculate.

The regulator or controller is selected by determining the maximum amperage. The rated short circuit current of the 123W solar panels is 8.1 Amps each (provided by the manufacturer), giving a total of 16.2 Amps. Select a solar regulator that is more than capable of handling the total short circuit current:16.2 x 1.25 = 20.25 Amps

The Steca 30Amp regulator is chosen.
Note that, as described in the notes above, you must allow 25% extra capacity or overload factor in the regulator rating as solar panels can exceed their rated output in particular cool sunny conditions. A 30A regulator will allow for an additional panel in the future as well. Hence the multiplier of 1.25 or 125% overload factor.

Select an inverter that is more than capable of supplying the maximum anticipated combined AC load required. In this example, maximum load would occur if the microwave and TV were running at the same time. Load in this case would be 900W + 50W = 950W.
Note that this calculation assumes that the inverter selected has a suitable surge rating to cope with the start-up surges of the microwave or other loads. A 1000W microwave would appear to be suitable, but a 1200W - 1500W inverter would be recommended.
1200Watt pure sine wave inverter chosen.

Note: A pure sinewave inverter is the preferred choice, but if the budget is tight, a modified sine wave unit could be used.

Select a battery, or a matched combination of batteries, that is capable of supplying the total power usage without being discharged more than 70%. In most cases it is recommended that the batteries are sized such that they have around 3 to 4 days back-up capacity. This allows for days with low sunlight and reduces the daily depth of discharge resulting in longer battery life.

With 3 days storage capacity, the battery sizing would be as follows:
Ah Required = ((Total Watthours designed for {see Sec 1 total calculated load} x # days / operating voltage of batteries) / x desired reserve in % {see Sec. 6}) x efficiency of batteries {typically 90% or a multiplier of 1.1}
Ah Required = ((841Wh x 3 days / 12V) / .7) x 1.1 = 330 Ah
(841Wh * 3 / 12V) / 0.7 * 1.1 = 330Ah.

Batteries purchased should match 330 Ah as close as possible. On a 12V system, and using 6v batteries in series, each battery can provide ½ of this rating, i.e., purchase 2 – 165 Ah batteries

Note: The appliance ratings used in the above examples may not be accurate. They have been used for example purposes only. Check the ratings on your appliances before performing any calculations. This calculation demonstrates one simplified method of calculating the solar power requirements for a campervan or similar set-up. When sizing a larger system, such as a system for a house, there are many other factors that need to be taken into account to ensure the system performs as required. These include, but are not limited to, solar panel output tolerance, battery temperatures and discharge rates, system autonomy ie. catering for days without adequate sunlight, etc. Seek the advice and assistance of a SEIA accredited designer before constructing a larger renewable energy system. ormation. Remote, Grid Interactive and