When does grid parity happen in the south of Spain?

A view of grid parity from someone who has been living off grid for 10 years.
The date of writing this post is 8th July 2017.

What is Grid Parity?

Grid parity  occurs when an alternative energy source can generate power at the same  price as buying power from the electricity grid. Or to put it another way: After installing an alternative energy system how long will it take the before the cost of installation will be the same as if the house had been connected to an electric company. This is called pay back time.

The Quick Answer

In case you just want a quick answer in my opinion it would take 8 and a half years.
Below I will explain how I calculated this figure.


Our annual electricty usage is 2005 kWh per year.
This is in our  house which is not connected to the grid.
The average occupation is about 4 people.
We do not use electrity for heating, cooking or AC.
The biggest electricty use we have is the swimming pool pump.
We live a fairy normal life and we have a freezer and  fridges.
We very occasionally have to use a generator when there is a prolonged cloudy spell but the cost is minimal. Maybe 20 euros per year.

We calculate the costs of the electric company by looking at the electricty bill we have for another house. The name of the company is Iberdrola.

We use the figure of 3000 watts as the maximum amount of power that can be used at any one time. (la potencia) The figure we use for the price of electrity is 0.16 euros per  kWh. There are other expenses such as electricty tax (impuesto), equipment hire (alquiler) and VAT (IVA).

The price per year of electricity from the GRID.

The cost if we bought the electricty from the grid would be 598 euros per year.
224 for additional costs and 374 euros for the electricity.

How much our solar electrical installation would cost.
To replicate our system you would need
1250 watts of panels – 840 euros 
50 amp combined inverter charge controller 750 euros
12 x 2v Lead Acid batteries 800 amp hours. 3500 euros.

Total cost 5090 euros

Total installation cost divided by GRID cost is 8.5

So time to parity is 8.5 years

There are lots of other variables such as battery life and efficiency of the batterys which we have not considered here but 8.5 years  is a good basic figure and it feels about right..  

If I lived in a city would I cut off from the grid?
No probably not. If you have solar power  you always have to be a bit conscious of the time of day and if there is sun or not. The battery technology is the most important factor.  Batterys like the TESLA powerwall sound very promising and could provide about the bare minimum which would satisfy most people. I imagine that within 10-15 years in the future we will see a green energy revolution with excellent power storage and the  demise of the internal combustion engine.

The best thing about this situation for me is that we have been here for 10 years so our electricty is now effectively free. I just hope that our existing batterys can hold out long enough so that there are very good possibilities are available when the time comes to replace them.  

Aparently  450g  of carbon dioxide is put into the atmosphere per Kwh for electricty bought from the grid.

According to these figures we have not caused 9 metric tons of C02 to be released into the atmosphere.




Batch Water Heater – Free Hot Water

Batch Water Heater – Free Hot Water


Probably the cheapest, simplest way of getting free hot water from the sun is a solar batch heater.  All you have to do is use a tank of water placed in the sun which will act as a pre-heater for the water which normally goes into the  hot water system.

In our case we used a 150 litre immersion heater tank which had stopped working. I am sure that in many countries there would be recycling centres which would let you have old immersion heaters for free. You can even chain them together in order  to make a very large capacity pre-heating system. The only possible drawback could be that if you live in a very cold climate they could freeze up in a prolonged cold spell.  In this case it would be a good idea to disconnect them during the coldest months.

To make some sort of calculation about the energy that my batch heater could expect to absorb I asked a question on a forum. The answer is here My tank is about 50cm radius by 1 metres length so it probably has a power intake of around 400 watts.

According to my own energy calculator it would take over 11 hours  to get the water to shower temperature.

“It would take 11 hours 27 minutes to heat 150 litres of water from a starting temperature of 15 C to a final temperature of 40 C when 400 watts is applied. The energy consumed is 4.58 kWh. The cost in Spain would be  0.87 euros.”

It is not possible to give exact figures but  our current setup consists of a 50 litre black PVC pre-heat tube, the 150 litre batch heater in this post and a 150 litre thermosiphon solar heater.
One one day last week that was  enough for 5 powerful hot showers at the end of a full day of sun. After that the water ran cold.

Batch water heater creation

Hannah preparing the hot water heater

The first thing to do with an immersion heater is to strip off the insulation. I decided to only strip off the insulation which is facing the sun and leave the rest.  I cut off the mild steel covering with an angle grinder then hacked away at the foam insulation with chisels. In the photo above Shep is helping Hannah (our very pleasant Canadian volunteer) to strip the foam off.

Batch water heater in cold frame

Batch water heater in cold frame

I put the tank in an old cold frame made out of plywood. I sprayed expanding builders foam behind the tank.

Solar water heating system

Solar water heating system. 150 litre  thermosiphon, batch heater and PVC plastic tube

The window pane is from an old house before we had double glazing. It was very simple to connect the water to the tank using flexible plumbing fittings. The water that goes into the tank is pre-heated in a long piece of pvc tubing mentioned in another post

The only cost involved was the flexible plumbing tubes for just a few euros.
I estimate that this could save us over  120 euros per year.


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 drier  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.