With ever-increasing energy bills, more people are considering the option of investing in a photovoltaic (PV) solar panel system in order to generate their own power.

 

Photovoltaic technology is fast-improving in viability, leading to a rise in interest which in turn has seen a number of companies offering solar solutions. Among these, however, are a few 'cowboy companies' eager to take your money for expensive PV systems that won't deliver sufficient power, nor the return-on-investment in the time they promise. It's Wild West territory out there – so be careful!

Yet solar technology isn't as complex as some would have you believe, and there's no need to be at the complete mercy of the specialists – in fact, you can do the basic calculations for yourself.

I'm often asked: "How many PV solar panels do I need to generate sufficient electricity for my home?" The answer is surprisingly simple. You don't need to attach fancy meters to the power cables in your house, or ask an electrician to produce a daunting 20-page report. All the information you need is in your electricity bill!

Think about it - the electricity company has done the measuring for you, in their bill, which is recorded as 'kilowatts hours' (kWh). The bill is a sum of two components: firstly, the standing charge, i.e. the regular amount you pay for a contracted supply to your house; secondly, the consumption charge or kW hour rate, which in Portugal it is around 17 cents including IVA.

To cover all the electricity needs of your house, a PV system must obviously generate the same amount of kilowatts as you consume. You don't even need to know what a kilowatt actually is, just that it equates to money - i.e. that if you consume 20 kilowatt hours in a day it will cost 17 cents x 20 = €3.40.

Electricity consumption has peaks and troughs according to usage patterns, so it's useful to take averages. My method is to add all the year's bills and divide the total by 12 for an average monthly cost. From this deduct the standing charge and you will get an approximation of your energy costs for the month. Divide this by 30 (average days per month) to arrive at how much you typically spend per day. Then divide this by the current EDP cost per kilowatt 0.17 cents (NB: based on peak hour rate), and you arrive at your approximate average daily consumption.

 

Here's an example in figures -

Annual bill = €3000

Average monthly bill is 3000 ÷ 12 = €250

After deducting standing charge (e.g. in this instance 20%) = €200

Divide by 30 days = €6.67 per day

Kilowatt hours used per day is 6.67 ÷ 0.17 = 39kWh

 

So, to cover its energy requirements, this house needs a PV system capable of generating minimum 39kWh per day.

Because PV systems use sunlight (or bright daylight at least), there are a limited number of hours per day to generate 39kWh. In the case of Faro, the average number is sunshine hours per day across the year is 8.3 hours

So if we divided 39 by 8.3 we get a figure of 4.7kW, which is a rough guide to the size of PV system for this property. In reality there are several factors which further limit the efficiency of PV panels, such as solar radiation intensity, ambient temperature, pitch/orientation of the panels; plus some energy loss through the cables, inverters and other systems.

To determine how many PV panels are actually required needs some further simple calculation. Each solar panel has a 'watts peak' rating (1 watt = 1000th kilowatt). I commonly use panels (polycrystalline type) rated at 240 watts peak, which is 0.240 of a kilowatt. So if we divide 4.7 by 0.240 we get a figure of 19.58. Because we can't have half a solar panel (!) we must round this figure up, to arrive at 20 panels.

In conclusion, we need a minimum of twenty 240 watt panels to generate 39kWh per day. Each panel is about 2mx1m so this number of panels requires about 40m2 of roof space.

This is still a little simplistic, as it assumes 100% efficiency from the panels, plus constant sunshine (solar radiation) over the 8.3 hour period, but it gives you a good idea of the minimum number of panels you'd realistically need to install.

Having said all this, it's important to bear in mind that the above guidelines are for a PV system tied-in with EDP, whereby you are still connected to the national grid. If you want to go completely off-grid, i.e. totally independent and self-supporting, you'll need a considerable number of high-performance batteries to store the power you're generating – a significant additional cost.

Whatever the PV system, you'll still need the services of a reputable solar installation company to offer advice and technical expertise specific to your property. But, by roughly assessing your own needs as shown, you'll be better-empowered to enter into a dialogue with them, to understand what they are offering – and not to be taken hostage by the cowboys!