﻿ Mobi Solar Calculator

# Photovoltaic Solar system calculator

If you have some experience on knowledge on the components of a Solar system and need assistance on system calculations, please go to following link

If you do not have experience with Solar Calculations, but first need guidance on the working and understanding, please read further

Let’s start with a brief revision of the major components found in a basic photovoltaic solar system solution. This should help you to understand them correctly identify and select the correct size components for your photovoltaic solar system solution .

The following diagram shows the major components in a typical basic solar power system.

A basic solar powered system:

solar power system solar panels, regulators, batteries and power inverter

It is important to note that these pictures display a very generic basic items, however the concept is applicable right through the bank of applications.

## Photovoltaic 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 at STC (standard testing conditions) of sunlight intensity 1000W/sqmetre at 25 degrees centigrade.

Different geographical locations receive different quantities of average peak sun hours per day.

As an example, in areas of the Highveld in South Africa, the annual average is around 5.6 sun hours per day. This means that an 100W solar panel based on the average figure of 5.6 sun hours per day, would produce a yearly average of around 550W.hours per day.

photovoltaic 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 between 17-22 volts (for 12V) and between 34V-44V (For 24V) but through the use of a solar regulator, this voltage is reduced to around 13 or 14 volts as required for safe battery charging.

Higher wattage panels are presented as 24V or even 36V Solar panels, i.e. 250W,300W etc

Solar panels output is affected by the cell operating temperature. Panels are rated at a nominal temperature of 25 degrees Celcius. The output of a solar panel can be expected to vary by 0.3% for every 1 degrees variation in temperature. As the temperature increases, the output decreases.

More detail on the specifications of Solar panels

## Solar Regulators

The purpose of solar regulators, or charge controllers as they are also called, is to regulate  and to transfer the energy from the solar panels and to also  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.

More modern solar regulators MPPT(Maximum Power Point Trackers) are regulators which allow panels to be connected in series to create a higher voltage e.g. 150V, and step the output down to 12V, 24V and 48V. There are one more advantage as they also track the maximum power point of operation of a solar panel.

In recent years a lot of development took place around the development of MPPT solar regulators.

Most solar regulators also include a Low Voltage Disconnect feature for direct connection of a small DC load, this 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.

Solar regulators are rated by the amount of current they are can deliver to the batteries.

Please refer to our page on the technical working of a Solar Regulator

## Power inverters

The power inverter is the main component of any independent power system which requires AC power. The power inverter will convert the DC power stored in the solar batteries into AC power to run conventional appliances.

True Sine wave inverters provide AC power that is virtually identical to, and often cleaner than, power from the grid.

DC AC Power inverters are rated by the amount of AC power they can supply continuously. Manufacturers generally also provide 3 second and 30 seconds surge figures. The surge figures give an idea of how much power can be supplied by the inverter for 3 seconds and 30 seconds before the inverter’s overload protection trips and cuts the power.

## Deep Cycle Solar Batteries

Deep cycle batteries are usually used in solar power systems and are designed to be discharged  and recharged hundreds of times, unlike conventional car batteries which are designed to provide a large amount of current for a short amount of time and not designed to be cycled.

To maximize battery life, deep cycle batteries should not be discharged beyond 50% of their capacity. i.e. 50 % capacity remaining. Discharging beyond this level will significantly reduce the life of the batteries.

Most manufacturers suggest only a 20% discharge. (DOD- Depth of Discharge)

Deep cycle batteries are rated in Ampere Hours (Ah). This rating also includes a discharge rate, usually at 20 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 120A.h is the hour rate it can deliver 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 or bank of batteries. 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.

## Sizing of a Solar system

In order for you to size your solar system correctly, you need to note the power rating of each appliance that will be drawing power from the system.

Let us take some common household appliances like lighting, a TV, and a fridge to see how one calculates the correct size solar system:

Consider a typical household who wants to operate essentials on solar for 24/7.

## Sizing of the Inverter

From table 1, you can see the peak possible power is 3850W, you inverter need to be able to deliver this power.

As a household you will probably not operate the microwave and the kettle at the same time.

So you can assume the peak at worst case at any time would be 3850W-kettle = 2350W

It will thus be logical to use a 3 Kw inverter or more preferable a 5 KW inverter.

Note: Inverters must always be bigger then your maximum peak power.

## Sizing of the solar PV array (solar panels)

Accordingly the example system will use approximately 8800W.hour per day. You need to generate more energy than you use. The effective charging hours per day vary as per location, the further north you are the more effective hours of charging per day you have. i,e.

• Cape Town: -4,5 - 5 hours
• Jhb: 5 - 5,5 hours
• Tzaneen: 5,5 - 6 hours

Effective charging hours is based on the movement of the sun, which is lower in the winter and higher in the summer and is based on average of a year. Depending on the angle of your panels you might get better performance in summer than winter or vice versa.

Let's assume we have an average of 5 hours charging per day. If we increase the 8800W.H per day to say 10000 W.h (to Allow for some extra charging), the amount of panels we will need is:

• 10 000W.h/ 5 Hours = 2000W
• Thus we need 2000W of solar panels.
• If we use 250W panels, that would mean 2000W/250W = 8 Panels.
• If we use 200W panels it would be 2000W / 200W Panels = 10 Panels.

It is usually more practical to use less bit bigger panels.

## Rating of the Batteries

It is always preferable to design a system of this magnitude at a higher voltage. We would prefer a 48V battery configuration rather than a 12 V configuration, the losses in 12V configuration can be as high as 15% more. We also do not want to cycle our batteries more than 50% - even less if we can.

Thus the batteries that we need must be in excess of 10kW.h x 2. (Times 2, to reduce the battery cycle to 50%)

So we need a 20kW.h battery bank at least. This should be the smallest bank of batteries you go for in this application. You also know the average use are about 10kW.H per day, so every day you would like additional autonomy (be able to use the system when you get lost of rain),

you need an additional 10kW.h of battery bank, i.e

• 2 day + 10kWh
• 3 day + 20kW.h etc

Let us get back to 1 day. If we decide on a 48V system we need 10 000 W.h/48V = 208 A.h

If we use 200A.h batteries, that would mean 4 * 200A.h 12V Battery(1 x row of 4 batteries in series). If you use 105A.h batteries, you will need 8 x105A.H 12V batteries (2 rows of 4 in series).

If we decide to choose a 24V system- then the same calculation is:

• 10 000W.h/24 = 416 A.h

If we use 200A.h batteries 12V, it will still be 4 batteries but rather 2 strings of 2 x200A.h in series. In this scenario a 4 x 200A.h in series in a 48V configuration would be the preference.

## Rating of the  Regulator(MPPT)

The MPPT Solar regulator is designed according to the output current rating to the battery.

We know we have 2000W of panels (independent of the configuration), if we choose a 48V battery bank then 2000W/48V = 41,6A MPPT. If we choose a 24V system, this would regulate 2000W/24 = 82A MPPT. The lost power of a 80A MPPT is double the current of a 40A MPPT, yet another reason to work with a higher battery voltage.

### The Complete Solar Power System is thus:

From the calculation presented, a system as follows will thus be practical:

• 1 x 48V 3kW Inverter.
• 8 x 250W Solar Panels.
• 4 (or 8 more preferable) x 200A.h deep cycle batteries.
• 1 x 45A MMPT Regulator.

The type and manufacturer of specific  product is not relevant here as this is a concept demonstration only.

Now you are ready to add all you information on a worksheet and see how it plays out-

Table 1- Adding appliances for a Solar Solution