Saturday 29 September 2012

Solar PV and Solar Water Heating

Given my long held desire to live off-grid the site I purchased in Crete was suitably remote. One and a half kilometres from an electricity source and about a kilometre from a mains water supply. The sunshine record for this area of Southern Crete is excellent:

Local solar water heater
and, given that I had a quote for connecting to the mains electricity of 30 euros per metre run plus a minimum of 600 euros for a transformer (given the distance), the decision to elect for a solar photovoltaic (PV) system was easy. The decision for solar water heating was equally easy since it is the standard method of water heating adopted throughout Crete. As an additional input for water heating I also made an early decision to have a wood fired range cooker (2 large hot plates, a roasting/baking oven and a simmering oven) for winter cooking with a back boiler to supplement the hot water supply. Cooking in summer will be by lpg cylinder gas hob and outdoor wood oven.

Since a major design objective for the house was to rely on solar passive heating and cooling using high thermal mass and earth sheltering (supplemented with wood burning stoves as and when necessary), see earlier posts, there was no requirement to use electricity for any heating or cooling.  This was particularly appropriate since solar energy is not suitable for such tasks.

Power from the sun
Estimated consumption

All the literature I absorbed both in print (see reading list at end) and on the web were agreed that a major effort should be made when embarking on designing a domestic PV system to very carefully consider ones power requirements with a view to minimising the demand and hence the size of the system.

Going though the process of calculating the daily kilowatt hours requirement by checking the power rating of appliances and really considering the hours they would be required to run in order to arrive at a daily total proved to be a very iterative and enlightening experience. After many iterations it was clear that to keep the system within reasonable bounds high efficiency (A+++) appliances must be used together with low energy lighting. This, coupled with a soul searching analysis of ones real needs, resulted in defining a much simpler lifestyle, not encumbered by the perceived need for everything that the current all embracing consumer society would want us to possess.

In our case this process resulted in a demand figure of up to 5.85 kilowatts of energy per day with a peak load at any one time of approx 1700 watts.


Solar energy is a combination of hours of sunlight at any particular spot and the intensity of that sunlight. Solar energy will therefore vary throughout the year and the position of the earth relative to the sun.

Local solar farm
This combination of hours of sunlight and intensity of sunlight is known as insolation and the results are expressed as an average irradiance as watts per square metre or as kilo-watt hours per square metre spread over the period of a day.

NASA's network of geostationary weather satellites has been monotoring the solar irradiance across the earth's surface for many years anf publically available tables exist providing estimated irradiation for most regions where solar PV is a realistic possibility. Several web sites (see below) offer a solar PV system sizing tool where it is only necessary to input location details and consumption details and answer a few simple questions for the calculations to be made.

Other factors

Not every day is the same and isolation figures will vary depending on the weather. For this reason the batteries used to store the solar electricity generated must be sized to be able to store enough excess energy to cover a few cloudy days.

Solar panels Bahrija Oasis Malta

Another factor affecting the efficiency of power generation is the direction the panels are pointing and the angle of tilt of the panels. In the case of fixed arrays maximum efficiency is achieved by pointing the panels directly to the south (in the northern hemisphere) and varying the angle of tilt at different seasons of the year.

Example Calculation

There appear to be a multitude of methods for designing PV systems all largely similar in that they take basic consumption data and use irradiance data for particular site. They differ on how allowances are made for the inefficiencies of the various components but ultimately arrive at broadly similar results. This example uses my interpretation of the method outlined by Michael Boxwell in his excellent "Solar Electricity Handbook".

Solar array sizing

Assuming a consumption of 5.85kWhrs of energy per day with a max load at any one time of 1700 watts.

Allow for 25% inefficiency due to cables, inverters etc therefore need 7.31kWhrs per day.

Using irradiation tables estimate the lowest level of irradiation for winter months, say 3, therefore the solar array needs to deliver 2.44kW.

But normally looking at only 75% efficiency for array panels so therefore need 3.05kW output from the solar panel array.

Given the size of the system this should be at least at 24v, if not 48v dc.

Battery sizing

From above need 7.31kWhrs/day to meet estimated consumption requirements.

Further assume a maximum 3 day holdover for bad weather therefore need 21.93kWhrs of storage at say 24 volts.

That equals 913.75 AH @ 24 volts.

But need to assume a maximum discharge of 50%

Therefore need 1827.5 AH @ 24 volts of battery storage.

Other components

A solar regulator is required that is capable of handling the total short circuit current of the solar array panels plus a 25% contingency. If power rating of PV array exceeds the rating of readily available charge controllers connect multiple units in parallel.

The inverter dc to ac must be capable of supplying the maximum AC load placed upon it at any one time - 1700 watts in this case. It is worth oversizing components such as the inverter at the outset to allow for ready expansion to meet additional future power requirement needs.


When I purchased my off-grid plot I had only undertaken very basic research into how to address the lack of electricity on site. My observation at the site meant that I was able to identify a location for the PV array and solar water heater that ensured the maximum sunlight possible throughout the day and a quick look at local weather records suggested that there should be no lack of sunshine throughout the year to power the system.
Local solar farm

Since then I have been heartened to note that no fewer than 4 solar farms have been established within a few kilometres of the plot vindicating my decision to go off-grid.

I am still finalising the design of the system to be used on the plot and am currently considering whether an additional wind powered system could provide sufficient input during the winter months to enable me to reduce the size of the array since the monthly average irradiation data for the November to February period, at 3kWhrs per square metre, is half that of the remaining months.

Books consulted:

"Solar Electricity Handbook" - Michael Boxwell
"Photovoltaic Design and Installation for Dummies" - Ryan Mayfield
"The Off-Grid Energy Handbook" - Alan and Gill Bridgewater
"The Renewable Energy Handbook" - William H. Kemp

PV Sizing Websites:

Solar Water Heating: (Solar Hot Water, A Primer)