How Many Solar Panels Do I Need

This is the first step in our Solar Power Tutorial, and most obvious initial step in starting your DIY solar panels for home use project is calculating exactly what your power needs are, and answering the question ‘how many solar panels do i need to power a house?’.

Now, if you’re going to be installing a solar power system, it helps to have some understanding of the relationship between power, voltage, currant and resistance, and the principles behind Ohm’s Law.  To help, we’ll explain that in our section on Ohm’s Law simplified.

However, you can still construct a solar power array that meets your power needs by understanding the following basics, namely:

Watts = Volts x Amps

1000 Watts x 1 hour = 1000 watts of power = 1 Kilowatt hour (Kwh)

Most domestic households have their electricity usage measured in kilowatt hours and pay a set fee for each kilowatt used.

Example: Your have an air conditioner which is rated at 2.6Kwh, which you leave on for four hours.  Your usage is 10.4Kwh.  At around 10c per Kwh, that would equate to $1.04 worth of power.

So how do we determine what our power needs are?  Well, there are all sorts of helpful calculators and spreadsheets around the internet through which we can hypothesize what our usage might be.  However, there is an easier way…look at our power bill!  Most invoices set out not only the last months usage in Kwh’s but also previous months.  Keep in mind that power usage tends to vary seasonally, and winter is often the time when households consume the most power.  Winter is also the time during which there is the least amount of sun, so it pays to use your winter power usage as a benchmark when calculating how many solar panels are required.

If you don’t have a power bill handy, all properties are fitted with a power meter.  You can take readings from your meter daily, and we recommend if you take this approach the readings be done over the space of a week (or even a fortnight) to include weekends when power use is likely to be higher.  Averaging out the value gives you a daily usage in Kwh.

As a guide, the US Energy Information Agency estimated that the average home uses 8,900 Kwh per year.

Now we know how much power we want to produce per year, and on average, per day.

Note: – Ideally, we can install sufficient panels to meet or even exceed those needs, however due to practical considerations such as available space, it may not always be viable.

Now on to our panels.  Solar panels are made up of connected cells, known as photovoltaic or PV cells.  Most panels have four rows of 9 cells, giving a total of 36, or rows of 12 cells, giving a total of 48.

Most commercially available PV cells produce 0.55 DC volts irrespective of size, however the size does effect the amps or current of the cell.  Most 6×3’ or 5×5’cells produce between 1 and 12 amps, with 4.8 being about average.

So the energy output for our single panel is likely to be:
.55 volts x 36 cells x 4.8 amps = 95 watts per panel
.55 volts x 48 cells x 4.8 amps = 127 watts per panel

Many standard size prefabricated panels are rated between 100 or 200 watts, but can cost over $1,500 to purchase.

Let’s assume that we are using the average power for a household of 8,900 Kwh per year.  That’s 8,900,000 watts per year, or roughly 24,000 watts (24Kwh) per day.

One other important thing to factor in to our calculation is that this is a solar array…hence we need sunlight to generate watts.  So the next question we need to consider is ‘how many hours of direct sunlight – or peak sunlight hours – will my array receive?’

The answer tends to be specific to your geographical area.  The average home in the US is around 6 peak sunlight hours per day, however you can check with a local supplier of professionally installed solar panels to confirm this for your area.  We should make our calculation on the basis of peak hours, even though our system may produce power during non peak hours.

So, if we need 24,000 watts per day, and only 6 hours of peak sunlight hours, we need to produce 4,000 watts per hour.  If each panel is capable of producing just under 100 watts, that means we need at least 40 panels to meet our energy needs.  If we use the larger panel size producing around 130 watts, we would need 30 panels.

30 panels probably sounds like a lot, but keep in mind the panels too can be grouped so as to be as aesthetically pleasing as possible.  Most domestic panel arrays are constructed on roof tops to make use of available space and access to sunlight.

Guess work and online calculators are well and good, but it pays to be specific to your own circumstances when determining how many solar panels do you need to meet your power requirements.  Keep in mind too that using solar cells with higher amps capacity, whilst more expensive to purchase, can reduce the number of panels you need if space is limited.

Next, we’ll be looking at choosing the right solar power system design for your home.

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