Before we can estimate the required size for your autonomous solar panels, we first need to calculate our total energy consumption per day. To do this, we have prepared a practical guide with the simple steps described below:

## 1. Calculate our total energy consumption

First, we need to calculate the energy consumed by our household electric appliances. Assuming that we have 5 energy efficient light bulbs of 15W each, (you may note that a 15W energy efficient light bulb produces the same illumination as a 60w conventional light bulb), and a 15-inche TV set. A 15 w light bulb will consume 15 wats in one hour of operation. Assuming an average time of 4 hours per day, we have 4hrs x 15watts equals 60Wh (watt-hours). For using 5 identical light bulbs about 4 hrs a day (each) we will consume 300 Wh (5 bulbs x 4 hours x 15W).

A 15-inch TV set is estimated to consume around 40 watts per hour. Consequently, for an average operation of 5 hrs per day, our TV will consume 200 wh (5 hours x 40 watts).

In a similar way, we can make a rough calculation of the energy consumed by each of our electrical appliances and calculate the total sum; i.e estimate the total energy consumed by our household electric appliances. Note that the wattage of each appliance is usually written on the appliance itself, or on the power supply plug or transformer. For our example, our 5 light bulbs and 15-inch TV set will consume on average 500 wh per day.

## 2. Calculate the size of our photovoltaic PV panels

To estimate the right size for your solar panels, try dividing your total energy consumption (in our example daily average of 500 Wh) with the total productive hours of solar radiation per day. This divider refers to the number of hours during the day that your solar PV panels produce electricity and is directly related to your location-specific latitude and climatic conditions and of course to the time of season. For the months of November to February, divide by 3 and for the months of March to October divide by 5 (note that summer months have longer radiation periods – up to 7 hrs per day). Thus, in our example, we will need to install a solar panel system at around 100Wp of installed capacity assuming an average productive radiation period of 5 hours per day (500Wh/5hrs = 100Wp).

## 3. Calculate the size of our battery (12 volts)

In a similar way to estimate the right size for our battery to support our system we need to multiply our consumption by a factor of 2 at least (so that we refrain from totally discharging our battery everyday which might lead to reducing our battery’s useful life). Therefore, in our example we will need a battery of at least 1000Wh (2 x 500Wh). Since battery capacity is usually denoted in ampere hours (Ah), we need to divide our Wh capacity figure with our battery’s voltage. Thus, for the 12V battery of our example we divide by 12 giving a battery of 83.33 Ah (1000Wh / 12V = 83.33Ah).

## 4. Allow for safety margin

In order to increase our capacity adequacy for both Photovoltaic PV panels and battery, we need to increase capacity by a factor of at least 25%. This is important to cover for expected fluctuations and waste in our system’s performance. Thus, we will need a Photovoltaic system of 125Wp and a battery of at least 104.16Ah (say 105 Ah). We need to note that the bigger our battery size, the better with respect to performance and useful life. So we may choose our battery size to be equal or above 105 Ah, always subject to our budget.

This result will be adequate to cover our energy consumption for a day, provided we will have 5 hours of productive sunlight. If we need to compensate for fluctuating weather conditions, e.g for 2 full cloudy days, we will need to multiply (increase) our capacity levels by 2.

## 5. Calculate the size of our inverter

For electrical appliance of 12Volts, we can connect them directly to our 12V battery (or to a charging regulator if available). If we have appliances requiring 180 or 230 Volts, then we need to install an inverter 180V or 230V respectively. The size of the inverter will depend on the total wattage of our electrical appliances that will operate simultaneously (note that devices that operate on a motor, such as refrigerators, may increase their starting charging load by a factor up to 10 times larger than their respective nominal values. To compensate for this we need to install a larger, scaled inverter.

## 6. What is the cost of solar panels?

Solar panel prices may vary depending on brand and performance criteria. In general solar panels cost around $1.10 – $1.37 per Wp of installed capacity (prices as at 2012-2013). In our used example, the cost of 125Wp solar panels will come at about $170.

From recently taken quotations, the cost of solar panels for a home installation of installed capacity 7.2 Kwp is estimated at about $1,400 per Kwp including solar panels, inverter, cabling and installation. For a thorough update on solar panels cost see how much solar panels cost