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