Lesson 1: How to Size Your Solar Array
First of all, figure out how much energy (Wh, or maybe kWh) you need each day. Do you want to charge a battery in an electrical device like a phone? Google "[your phone’s name]" and “battery capacity” and look for a number with the unit mWh. If you only find mAh instead of mWh, simply multiply the battery’s voltage with its mAh capacity:
mWh = Battery voltage (V) * Battery capacity (mAh)
The iPhone has a 3.7 V battery with 1,400 mAh, so that makes 5,180 mWh = 5.18 Wh. In order to find out what your daily demand is, you need to take into account how often you have to charge your device. If you charge your phone once every two days (like I do with my iPhone), you can allow your solar system to charge the battery in two full days. Your daily demand would therefore be only half of one iPhone’s charge: 5.18 Wh * 0.5 = 2.6 Wh. Would you for instance like to run a couple of 12 W lightbulbs for 5 hours each night, your daily demand would be 60 Wh (5*12).
Now, after you’ve figured out your daily demand, you need to translate this into an array size. Of course, your daily generated electricity depends on how much light your panels receive. The easiest way to figure this out is by making a crude estimate yourself or to let a software program do it for you more accurately.
PVWatts will then display your results like this: you can see that the irradiation received by a horizontal surface in the winter, is much lower than in the summer. No wonder why we plan our Solar Journey in June/July! You can play around a little with the array tilt. You will see that the higher your tilt (put in 60 degrees), the higher your yield during the winter months. Naturally, you sacrifice some of the yield in the summer months, when the sun is at its highest point.
Size your array!
Remember that for your input you put 1 kW: from the Results, PVWatts tells us that we would generate a minimum of 38 kWh in the whole month of December (which is about 1.2 kWh per day). So every Watt will bring us a usable 1.2 Wh per day in that month. With the 2.6 Wh I need each day to charge my iPhone, I would need about 2-3 Watts of solar modules. You should size the system depending on your needs. In this case, we took the worst solar month because we need to charge our electrical devices in the winter as well. If you plan to run a fan to circulate air in your house, your demand advantageously peaks in summer when the solar resource is highest. In that case, you can pick June instead of December as your reference month, resulting in a smaller array, a fatter wallet and thus more money is left over to donate to Solar Journey USA!
The configuration of your array depends a lot on the size of your system and the voltage that your load requires. The list below gives you an indication of what voltage level your system will probably operate in depending on the array size you just calculated.
You want to configure your solar module array to operate at a voltage similar to that of your battery pack. This will allow the charge controller to operate at its highest efficiency. For small systems, get a 5 V battery and try to find one or more small 5-6 Volts modules. For medium and large systems, you can play around with the voltages a little bit: if you connect two solar panels in series, you add up the voltage. For example, two of our Ascent Solar panels in series would make 32.9 V + 32.9 V = 65.8 V. Connecting them in parallel will result in added currents. Similarly, if you are considering a 48 Volt battery bank like the Brooklyn stand-alone solar EV charger, you can series-couple four 12 V panels to get up to the desired battery voltage.
Now that you’ve calculated how many Watts of solar panels you need, you can start designing your array and spend some dollars! In the next section, we will give guidance on what equipment to buy and what online stores are great places to get your goods.
Comments, Questions? Let us know and we will help.