50 litre solar hot water heater for only 71 euros.

Black platic tubes

32mm tubes for solar hot water

 The temperature of our cold water supply is very cold maybe 5C. When this water is mixed into the solar hot water tank it cools the rest of the water down considerably.

We got ripped off for 1200 euros by a company called http://tutiendasolar.com/ (We paid them for a solar hot water system which they were never able to deliver.) So I did not have an appetite to spend any more money.

I realised  that if we could increase the temperature of the water supply it would increase the efficiency of the hot water system.

I bought 100 metres of black 32mm PVC agricultural tubing which can withstand 8 atmopheres for 57 euros. The two brass connectors cost 7 euros each so that brings the total to 71 euros.

To calculate the amount of water in the tube you use this formula.

Length of pipe x Pi x Radius of pipe

In our case:

100m x 3.142 x 1.6cm = 502.72 cc

That is 50 litres.

According to my calculations:
If the water in the tube could get to 38C it would be enough for an  8 minute shower at 6 litres per minute.

Here are some other calculations:

To compare the cost of heating the water in this tube with an electric water heater we need this calculation:
We are heating 50 litres of water by 35C (from 5C to 40C)

The formula I have is imperial:
weight of water in pounds x  temp increase in deg Farenheit /3413 = KWH used by an electric heater.

The metric figures are:
55 kilos = 110 pounds
35C = 95 farenheit

110 x 95/3413 = 3.061822 KWH

The average price of electrity in Spain is  0.19 per KWH
3.061822 x 0.19 = 0.58

Each time the tubes heat enough water to have an 8 minute shower it theoretically saves 58 euro cents.
The supplementary water system will pay for itself after 122 showers.
Let’s assume that there are always 5 people here and they have showers on average 5 times per week. That is  25 showers per week.

This system should pay for itself in about 5 weeks.

According to carbonfund.org the carbon emissions of electricity are :-
0.0005925 metric tons CO2 per kWh

If there are 5 people here having 5 showers per week that is 1300 showers per year
If each shower uses 3.061822 KWH that is 3980 KWH saved per year (or 756 euros)

This means that we theoretically  save 2.35 metric tons of CO2 being emitted per year.

Maybe you can find an error with my figures but it seems that solar water heating makes sense.

See another solar water: How to make a batch water heater











How to Design a Domestic Solar Photovoltaic System


solar panels

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



If you are thinking of installing a solar energy  system in your off grid home it is very important to understand the basics of system design. It is best not to leave all the decisions to a salesperson.  It is very important to make the right decisions about battery size because old batteries cannot be mixed with new ones. So it it will not be possible to buy some supplementary batteries if you don’t have enough.

1. Calculate how much power you use

You have to find out how many kilowatt hours you use. A kilowatt hour or kWh is 1000 watts used for 1 hour.For example  If you have a 2000 watt fire on for 2 hours it will have consumed 4 kWh.
If you have an on-grid house the  electricity bill from the electricity company states the amount of kWh that you have used. This could be a way of finding out how much electricity you use although it is very likely that you would be much more generous in your use of electricity  if you are on-grid.  Another way to find out how much energy you use is to use devices to measure electricity use. If these are not possible you will have to calculate your daily kWh by making a list of all electrical devices  and calculating their energy use. If you want to know exactly how many watts each device uses it is possible to buy a plug-in  Electricity Usage Monitor. You plug the device into it and it tells you the amount of watts being used.

To calculate the daily kWh  use this formula.
(hours used X watts) divided by 1000 = kWh

 2. Find out how much sunlight you get.

To be continued…..
I also wrote a calculator to help you decide what you need.

Our Domestic Solar Power System – Specifications

Domestic solar power system

Domestic solar power system


These are the specifications of our system. It is not a system which should be copied because it is a system which was inherited and then added to. If I was designing it from scratch I would make different decisions. The most important thing when designing a solar system is to buy enough battery power because it is very difficult to mix old batteries with new batteries. You may notice in the photo above that there are 3 charge controllers with 3 sets of panels. This does not seem to be a problem.

For a lot of the time it serves 7 people with a swimming pool pump, fridge, freezer etc. If the sun is shining I can boil water in a kettle. 

See also my solar power tutorial

System Voltage: 24V

Solar Panels:

  • 2 X 125 watt panels. Connected to a Leo Atersa charge controller
  • 3 X 190 watt panels. Connected to Victron MPPT  charge controller
  • 1 X 300 watt panel + 2 X 190 watt panels Connected to Solener PWM charge controller

Batteries: 24 volt system

12 of the following battery cells

Classic 5 OPzS 380 2v Solar Liquid Cell
Voltage: 2V
Capacity: 380Ah @ c120

This should provide 9.1 Kwh (1000 watts for 9 hours)

Inverter: Victron 24 volt 2000 watts. LVC 28.8 volts
For the washing machine: Atersa 750 watt.

Battery charger Tudor 30amps.

Honda 2500 watts
Honda 450 + 900 watts dual.


Leo charge controller

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


Solar Drier made from chimney tube

I had spend a lot of time looking at very complex home-made solar food driers on internet. Most of them consist of a black area which heats up the air. The hot air dry then flows over the food to extract the moisture.

I realised that a very quick free version can be achieved just using a back tube and a metal colander. The air heats up in the tube and then passes through the holes in the colander.

This drierĀ  can dry out a couple of apples in about 6 hours. It is a bit unstable and can easilly be knocked over. A good variation would be to use several black tubes taped together with a big cardboard box on top.