Sizing Your Solar System: A Comprehensive Guide for Panels, Batteries, Controllers, and Inverters

Introduction

This blog goes over how to size your solar power 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 setup.

System 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.

A PWM controller has about 79% efficiency, while an MPPT reaches approximately 94%. Adjust the earlier Watt value by dividing it by 0.8 (for PWM) or 0.94 (for MPPT) to obtain a refined wattage. If an inverter is part of your setup, repeat this process by dividing the value by 0.9. Now you possess the essential wattage for powering 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 Solar Charge Controller Types.

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 Batteries Overview.

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 Inverters: All You Need to Know.

Equation Summary

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

Example

In this example 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 per 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 0.5 Hours = 750 Watt-Hours

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

Solar panel & Charge controller

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 (reference: Average Peak Sun Hours by State).

Required Power of Solar Panel (without considering controller and inverter loss) = 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.

Required Power of Solar Panel (considering controller and inverter loss) = 1712.15 Watts / 0.94 / 0.9 = 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 solar kit that can should do the job.

Assume we are installing a 24V solar system. We need to keep this in mind to size the battery and pick our inverter.

Battery

Now, when considering the battery size, you'll need to divide the total consumption by the system voltage, in this case, 24V, and then double the result.

Battery Capacity = (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.

Inverter

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

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

In this case, a 2500 Watt inverter or higher is required. It would need to be 24 Volts.

For details on how to calculate your solar power, see Renogy Solar Calculators.

For wiring lengths and gauge sizing, please reference Fuses and Wire Gauge.

Video Guide

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