Estimating the Size and Cost
of a
Solar Photovoltaic (PV) System

Solar PV systems vary in complexity. Some systems are referred to as "stand-alone" or "off-grid", which means they are the only source of power to a home. Off grid systems can be designed to run with or without battery backup depending on the usage. Off-grid home power systems usually have some form of storage to store the energy generated during the day for use at night. Typically this would be a battery bank.

Off-grid systems are very cost-effective when compared to alternatives such as grid utility connections.

Grid-connected systems where the amount of energy generated exceeds the energy used and is exported (sold) back to the electricity provider. Energy can also be drawn from the electricity grid when the energy from the PV system is insufficient to power the load of the building. Naturally, you have to have an agreement with your electricity provider to be able to export excess energy to the electricity grid.

Grid Connected systems are very cost-effective when compared to alternatives such as grid utility connections.

Regardless of the system chosen each will require specific components besides the solar PV modules.

To generate alternating current (AC) an inverter is required. Battery storage requires special batteries (AGM or GEL) depending upon the application and a battery charge controller.

The final cost of any solar PV system depends upon the size of the array, the battery bank size, and other additional components, such as inverters, PV mounting frames etc.

Estimating the size and cost of the Solar Photovoltaic (PV) Modules

Step 1

Determine load, available sunlight, pv array and battery bank size

1.a. Determine the Load - The best way to approach this is to anticipate your daily load requirements.

Before you can install an array (one or more solar pv panels) you do need to estimate all the different loads used in your house on a typical day. The above table shows how to calculate the watt-hours (Wh).

Of course it is very difficult to estimate each and every single load. Some appliances draw power even when they are turned off, e.g, electric clock's, stereo's TVs etc. Because of this it is always recommended that the estimated daily load is multiplied by "fudge factor" of 1.5.

1.a. Determine the available sunlight.

The amount of available sunlight should be determined from an average day in the worst month of the year. This is known as the "isolation value". Using the worst month of the year will ensure that the system will operate year-round. In the UK, depending on where you live, the value ranges from 1.5 to 4.0 with the lowest value in the north of the UK and highest in the south. The isolation value can be interpreted as the kilowatt-hours per day of sunlight energy that falls on each square meter of solar panels at a tilt latitude.

1.b. Determine Battery Bank Size

We recommend that batteries are shallow cycled and never deep cycled - requiring lots of charging and discharging. Daily discharge should be about 20% of the capacity of the battery and deep cycling can be saved for those crucial days .

A 20% discharge figure suggests that the capacity of the battery bank system should be about five times the daily load. This also suggests that your battery bank storage should be able to provide power for five days. To determine the Ampere-Hour rating of the batteries, multiply the daily load by 5 and then divide by the battery voltage.

Step 2

Estimate the cost of the PV array based upon the cost per watt.

Estimate the Battery Bank Cost - low prices can be achieved per amp-hour (Ah)

Estimate the Inverter cost. An inverter is always need for AC output. For off-grid systems the inverter should be sized to provide 125% of the maximum loads you wish to run simultaneously at any one time. For example, from table 1 we can see that running a computer, computer monitor, 4 lights and a radio amounts to 1,315 Watts. Choose an inverter rated for a continuous power output of 2,000 watts. For grid-connected pv systems the maximum continuous input rating of the inverter should be 10% or more higher that the PV array size. NOTE: the input rating of the inverter must never be lower than the pv array rating.

Estimate the balance of system components. We use a factor of 25% to cover the balance of system components.

WORKSHEET - ESTIMATING THE COST OF YOUR SOLAR PHOTOVOLTAIC SYSTEM

STEP 1. Determine the Load, sunlight, PV array size and battery bank

1.a Determine the energy required in Watt-Hours (Wh) per day. Multiply the number of watts the load will consume by the hours per day the load will operate.
Multiply result by 1.5.

Total Wh per day required:_____________________________(Wh)

1.b. Determine the hours per day of available sunlight at the installation site:

Total available sunlight:_________________________(Hrs/Day)

1.c. Determine the PV array needed.
Divide the energy needed (1.a.) by the number of available sun hours per day (1.b.)

Total array size required:____________________________Watt

1.d. Determine the size of the battery bank (is used). Multiply the load (1.a.) by 5 (result is watt-hours, Wh). Then divide by the battery voltage to determine the ampere-hour (Ah) rating of the battery bank.

Total battery bank required:_____________________________(Ah)

STEP 2. Calculate the cost of the PV system for the site

a. Multiply the size of the array (1.c.) by £ per watt

Cost estimate for PV array: £_______________________________

b. If a battery bank is used, multiply the size of the battery bank (1.d.) by £ per amp-hour:

Cost Estimate for PV array: £______________________________

c. If an inverter is used, multiply the size of the array (1.c.) by £ per watt

Cost estimate for inverter: £______________________________

Subtotal:£______________________________

d. Multiply the subtotal by 25% to cover system losses etc.

Cost estimate for balance of system components: £____________________________

Total Estimated PV System Cost: £____________________________

If you have completed the above worksheet the chances are that the costs seem high. Always remember that it pays to be energy efficient so taking care to ensure that your home is energy efficient will pay big dividends and it will make your solar pv system much cheaper.