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System Sizing

Introduction

This section will go over how to size your system. We will learn how to figure out how many panels and batteries you need, along with which controller and inverter will fit for your set up.

System Sizing

Step 1: Load sizing

The first step to sizing your system starts with what loads or devices you want your solar system to run. It is important to get the wattage of each item you are planning to run along with how long you plan on running them for. You will multiply the watts by the hours to get Watt-Hours. If you have more than once appliance you just add them all together to get the total Watt-Hours.

Step 2: Solar Wattage Sizing

Next you want to find out what state you are located in. This will tell you the peak solar hours that you get from your state. You then want to take the load Watt-Hours and divide it by your peak hours to get Watts. This will be the Watts you need to run those items before efficiency loses occur.

Since your system will run through a controller, there will be efficiency losses. For a PWM controller you will have around a 79% efficiency and an MPPT will be around 94%. You then want to take the Watt value from before and divide/.8 it by the efficiency to get a new wattage value. If you are using an inverter, you want to do this again by dividing the value by 90%. You now have the wattage needed to run your appliances.

Step 3: Controller Sizing

Next, you need to find a controller that can accept the wattage you need. You can check the controller specification sheet to see the wattages they can handle. For example, a 30 Amp Controller can handle 400W on 12V, so you know you can have up to 400 Watts on there.

*If you want to size it by yourself, please reference section 2.5.

Step 4: Battery Sizing

In order to size your battery, you need to double your initial Watt-Hours value in order to make it so your loads only drain the battery down to 50%. You will take that last wattage value you calculated and multiply it by 2. You then divide it by the voltage, either 12V, 24V, or 48V based on what controller you end up using to find the Amp-Hours needed.

*If you want more details, please reference section 2.6.

Step 5: Inverter Sizing

To size the inverter you need to add up all the wattages of all the items you want to run. You then need to pick an inverter with more wattage than this. Also, make sure your inverter matches your battery bank voltage as well.

*If you want more details, please reference section 2.6.

Equation Summary

  • 1.   Load Consumptions
  •  a. Load Wattages x Hours = Watt-Hours
  • 2.Panels Required
  •  a. Watt-Hours / Peak Solar Hours = Watts
  •  b. Watts/Controller efficiency = Watts
  •  c. Watts/Inverter Efficiency = Watts Final
  • 3. Battery Size
  •  a. Watt-Hours/Battery Voltage * 2 = Amp-Hours
  • 4.  Inverter Size
  •  a. Inverter Size > Load Wattages

Example

In this example we will we will take 3 loads, a TV, fridge, and Coffee maker. The TV will be 125 Watts and run for 4 hours per day. The Fridge will be 700 Watts and run on a cycle (8 hours be day). The Coffee maker will be 1500 Watts and run for 30 minutes a day.

TV Consumption: 125 Watts x 4 Hours = 500 Watt-Hours

Fridge Consumption: 700 Watts x 8 Hours = 5600 Watt-Hours

Coffee Maker: 1500 Watts x .5 Hours = 750 Watt-Hours

Total Watt-Hours = 500 + 5600 + 750 = 6850 Watt-Hours

Now that we have our consumption we can see how many panels we need. In this example we will be located in Tennessee which has 4 Peak Hours. (https://www.renogy.com/template/files/Average-Peak-Sun-hours-by-State.pdf)

6850 Watt-Hours / 4 Hours = 1712.15 Watts.

We will want to use the MPPT Controller since this is a high wattage system and want to minimize loss. We will also be using an inverter since the items are AC.

1712.15 Watts / .94 = 1821.44 Watts.

1821.44 Watts / .9 = 2023.82 Watts.

We now know we need 2023.82 Watts. In this case it is hard to find a controller to do this, so we will take a look at some kits and find a 2000 Watt Cabin Kit should do the job: https://www.renogy.com/renogy-2000-watt-24-volt-monocrystalline-solar-cabin-kit. We must not base on the title that this is a 24V system. We need to keep this in mind to size the battery and pick our inverter.

Now looking at the battery we want to divide your item consumption by the voltage, in this case 24V, and double the value.

(6850 Watt-Hours / 24 Volts) * 2 = 570.83 AH at 24V.

You can probably achieve this battery bank with 6 of the 200 AH batteries wired in series-parallel. https://www.renogy.com/upg-12v-200ah-agm-sealed-lead-acid-battery

Now to figure out how big of an inverter we need we have to add up the wattages.

700 Watts + 125 Watts + 1500 Watts = 2325 Watts.

In this case a 2500 Watt inverter or higher, and it would need to be 24 Volts. This inverter would work fine http://www.renogy-store.com/AIMS-3000-Watt-Power-Inverter-p/invt-aims3000-24v.htm.  

For wiring lengths and guage sizing please reference section 2.7.

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