Theory/Installation Guide/Size Array:

Lesson 1: How to Size Your Solar Array

Before you can start buying your components and build your PV system, it is important to do a little bit of research before you start. This will save you money and you won’t end up with an array that doesn’t fit your needs.

The very first thing you want to find out (and the big question of Lesson 1) is:
How many Watts of solar modules do I need?

Determining how many modules (and what kind) you need is called ‘sizing’ an array. Obviously, the size of your system totally depends on how much green electricity you want to get out of it. You don’t want to end up spending $2,000 on a phone charger… So lets do some calculations!

solar resource
Solar Resource of the United States by the DOE. Click to enlarge.


Daily Demand

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).

Daily Yield

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.

  • A quick estimate is possible by using a simple solar resource map. Find your location on the map below and read off how many kWh/m2/day you get on average. New York: 4.3 kWh/m2/day. This is equal to 4.3 hours of full sunlight (1,000 Watts/m2). Note that this number is for ‘latitude tilt’. Latitude tilt is a well-known term in the solar industry. It refers to a rule-of-thumb: by mounting your panels at the latitude tilt angle, due South, you get the highest annual yield. Lets assume we will mount our panels at this optimum angle. Now, assuming you lose ~20% from inefficiencies in your system (batteries, wiring losses, other electrical components), multiply the number with 0.8. So for New York, per Watt of solar modules, the array will generate 4.3 * 0.8 = 3.44 Wh. Do you see that this would be sufficient for recharging my iPhone once every two days? Now, of course the energy yield will be lower during winter days and therefore we need a more accurate assessment, which can be done using PVWatts. You only need to know your zipcode, how easy is that?!?!
  • A more accurate method to estimate your daily yield is by using PVWatts. Here, you can get an accurate estimate for your average daily insolation throughout the year. You can go to the PVWatts website, fill in your zipcode and click ‘send to PVWatts’.

A window will open where you have to fill in the following information:

  • DC Rating: Fill in ‘1 kW’ for now, even though your system might not be of that size.
  • DC to AC Derate factor: Fill in ‘0.8’ (this factor accounts for inefficiencies in your system (charging/discharging the battery, wiring and array losses).
  • Array Tilt: If you plan to mount your system at a certain angle, fill in the angle that the array makes with a horizontal surface. Fill in ‘0’ for Array Tilt if your system is horizontal (like ours).

Other fields you can leave as default. Click 'Calculate' when you're done to find out the yield.

PVWatts main input screen: fill in the information and get an accurate yield estimate for your array.

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!

System configuration

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.

  • Small systems (< 20 W) typically power phones and small batteries at ~5 Volts.
  • Medium systems (20-250 W) can be used to charge larger 12 Volt batteries that run garage lights, a fan or other 12 Volt electrical devices. This is the ideal education set-up.
  • Large systems (>250 W) store the electricity in any battery pack voltage configuration and these systems typically have inverters installed that convert the DC feed from the battery and array to electricity at 110 V AC output. Any normal 110 V plug from a computer, TV or laptop can be plugged in here.

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.

Series Circuit PV Parallel Circuit PV

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.

< Install Guide Home ••• Lesson 2: Buying your Equipment >

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