Domestic Solar Energy Tutorial

solar panels

Our solar system. The solar panels are – 5 x 190 watts + 1 x 300 watts + 2 x 150 watts


In the same vein as the Complete Idiot’s Guides or solar power for Dummies.
This tutorial attempts to be a simple as possible.


I decided to write a simplified guide to domestic solar energy which would be useful to other people who are interested in installing their own system.

In this guide we are going to be talking about a solar energy system for off-grid use. Off-grid means that the house has no connection to the public electricity network and the system is totally independent. This usually only happens because the house is in a very remote location and it is too expensive to connect the house to the grid with electric pylons or connecting an electric cable. This is also known as a “Stand Alone” system.

This is where your house is connected to the normal electricity network. Normally the home-owner   has  some panels and a device called a grid-tie inverter which is used to take the power from the panels and feed it into the  home electric system. The  electricity bill from the electric company will be reduced depending on the amount of energy generated. If you generate more than you use then you can sell the electricity back to the electricity company. The price of the electricity sold back to the electricity company is normally controlled by the feed-in tariff. In many countries a guaranteed feed-in tariff has been used to encouraging the deployment of solar power in the general public.

At the moment (2012) it is much more expensive to buy electricity from the electricity company than to install a solar system and generate your own electricity unless you are in an off-grid situation. Grid Parity is the moment when home generated electrify has the same or lower price as the power bought from an electricity company. This moment will change the whole dynamics of solar energy use. Grid parity will eventually come because the price of solar panels, batteries and other items necessary for solar power will become cheaper as the technology improves.

Unless you have a very basic idea of volts amps and watts you won’t understand this guide. So here is an explanation:

These terms can be understood by making a similarity with water in a plumbing system:

The voltage or volts is equivalent to the water pressure, the current  or amps is equivalent to the flow rate, and the resistance (ohms)  is like the pipe size.

Watts is a measurement of electrical power.
Watts = Volts X Amps.
So a 24 volt system with a current of 2 amps is using 48 watts.


Solar panels are also called solar modules, photovoltaic modules or photovoltaic panels. Solar panels are normally made from thin slivers of silicon, when the panels are exposed to sunlight they generate electricity, this is know as the photovoltaic effect. The panels are normally put on the roof of the building or held up to the sun by installing them onto a mast. Sometimes the solar panels are in a fixed position and sometimes they can be made to face the sun more directly during the day by using a solar tracker. Solar panels consist of many cells which are wired together.

The output of most solar panels on domestic systems is between 100 watts and  220 watts. Most of them have a voltage rating of 12, 24 or 48 volts.


Solar batteries

Deep cycle solar batteries

In a solar system, power is generated during the day when there is sunlight. The batteries store the excess power so that the house can still have power during the night or at periods when there is no sunlight such as cloudy days. In the future there may be technological developments which will bring much better and more powerful batteries but at the moment most solar systems use lead acid batteries. The power of batteries is measured in AH or amp hours. 1 AH is power that can be attained by using a current of one amp for one hour. Batteries normally have a voltage rating of 12, 24 or 48 volts.
The life of batteries depends on how they are used:

The electricity in a battery is created by a chemical reaction between sulfuric acid and the lead plates. When the battery is used this chemical reaction begins to coat both positive and negative plates with a substance called lead sulfate this process is called sulfation (sulfation causes a yellow build-up on the  plates). Lead sulfate is a soft material, which can is reconverted back into lead and sulfuric acid, provided the discharged battery is immediately charged.

If a lead acid battery is not immediately recharged, the lead sulfate will begin to form hard crystals. A lead acid battery must never be discharged to less than 80% of its rated capacity or it will be permanently damaged.

When a battery becomes discharged and them becomes charged again it is called a cycle.  Lifetimes of 500 to 1200 cycles are typical.  An important factor in the life of batteries is the depth of discharge or DOD. The bigger the depth of charge the less time the batteries will last.

State of Charge 12V battery 24V battery DOD
100% 12.70+ 25.4+V 0%
75% 12.40 24.8V 25%
50% 12.20 24.4V 50%
25% 12.00 24V 75%
0% 11.80 22.6V 100%

A charge controller is the device which controls the supply of power to the batteries.   It does a very important job because batteries are very expensive and the charge controller has to protect them from being over-charged. Modern charge controllers have complex electronics which change the amount of charge received by the batteries at certain times.
The amount of charge in a battery is referred to as the “State of Charge” or SOC
There are 3 main charging states:

  • BULK CHARGE The first 80% of charging. Charges as fast as the battery will accept
  • ABSORPTION CHARGE The final 20% slowly decreases the amount of charge until the battery is full.
  • FLOAT CHARGE The battery is protected from over charging and kept fully charged.

There are 2 types of charge controllers:

  • MPPT Maximum Power Point Tracking. These allow a much higher input voltage. The are considered to be more efficient in many situations and the are much more expensive.
  • PWM Pulse-width modulation. They can only use the same voltage as the batteries. They are much cheaper.

INVERTER The inverter changes the amount of volts used by the batteries and converts the electricity so that it can be used by domestic appliances.  Most domestic electrical systems use either 125V or 240V
There are 2 main types of inverter:

  • Modified Sine Wave. These are normally used in cars for supplying power from the batteries. They would not be suitable for a general domestic use but could be used to power a garden shed or other low demand situation.
  • Sine Wave These are more expensive but provide a type of electricity very similar to the electricity provided by the electricity companies. They are suitable for all type of domestic appliances such as laptops, printers etc

The most important thing when choosing an inverter is the maximum power output.
The higher the maximum power output the higher the price. If you want to use an electric kettle which uses 1000 watts then your inverter must be at least 1000 watts. Most inverters can supply a bit extra for a short period of time for example to start an electric motor.

A good inverter should be equipped with a a cut of system so that the electric supply will switch off if there is insufficient power in the batteries. This is normally called LDV (Low voltage disconnect)

It is quite complicated to design a solar system because you have to predict how much power you will be using. May a good way to start is to use an on-line calculator for solar systems