Batteries and inverters other components included in a solar system. This section will talk about the purpose of each, along with how you would go about sizing them.
The main purpose of the battery is to store energy produced by the panel. Without this component, an off-grid system will be incomplete. Batteries are generally rated by a voltage level, mostly 6V or 12V, and an Amp-Hour rating. This Amp-Hours rating is important to know the capacity of the battery. Most small systems are 12V. RV and Boats generally are 12V systems as well. A set of batteries connected together is called a battery bank.
To understand battery sizing, we must understand the capacity of the battery. Batteries being measured in Amp-Hours and Volts, need to be calculated into Watt-Hours to tell you the energy. It is important to have both information.
Imagine you have 2 x 100AH batteries, one being 6V and the other being 12V. The 6V battery will be measured at 6V x 100 AH = 600 Watt Hours. The 12V battery will be measured at 12V x 100 AH = 1200 Watt Hours. As you can see even though they have the same Amp-Hours, their energy is different.
To size a battery, we need to first understand our consumption. We can calculate consumption as Wattage of Appliance x Hours of Run Time. Once we get this Watt-Hour value, we can divide it by 12V to get the battery in terms of Amp Hours. We also prefer to double this value, as we recommend a depth-of-discharge of only 50% to help preserve the life. This means we don’t recommend draining the battery under 50%.
Let’s take an example of a 35 Watt fan running for 6 hours. We have 35 Watts x 6 Hours = 210 Watt Hours. We then do 210 Watt-Hours/12V = 17.5 Amp-Hours. We then want to double the value for DOD of 50%, so we have 17.5 Amp-Hours x 2 = 35 Amp Hours. Our battery size would be 35AH at 12V. Keep in mind this is in 12V. To see this same battery in 24V, you would do 210 Watt-Hours/24V = 8.75 AH. You can then double this to 17.5 Amp Hours. This batter would be 17.5 AH at 24V. You can utilize multiple batteries in series, parallel, or series-parallel as mentioned in Section 2.4 to get a desired battery bank size.
The most common battery type we recommend are Lead Acid Batteries. You will generally see a Sealed or Flooded Lead Acid battery. It is important to make sure that your controller is compatible with your battery type.
The purpose of the inverter is to convert DC to AC. Since batteries are DC, an inverter exists to allow you to run your AC appliances. They will come with an AC outlet to plug in things such as your computer, fridge etc. Inverters come in sizes of Watts, Volts, and can change DC to 100-120Volts, 200-240Volts, etc. It is important to make sure the voltage of your inverter matches the voltage of your battery bank.
When sizing for an inverter you need to look at 3 factors: wattage, DC voltage, and AC voltage.
Inverters will be rated by a wattage value, telling you how many watts it can run at one time. For example, imagine you had a 500 Watt Fridge and 800 Watt Air Conditioning. These two items would be 1300 Watts and would require an inverter with a higher wattage than 1300W.
The DC voltage rating on the inverter will tell you what battery bank it is compatible with. For example a 24V battery bank, will require an inverter that is compatible with 24V.
The AC voltage rating on the inverter will tell you what kind of AC appliances it will run. Most of the time a 100-120VAC(Volts AC) inverter will be ok as most household items come in that voltage. Sometimes very large loads will run on 200-240VAC so it is important to know this for special items you want to run.
Inverters come in modified and pure sine wave types. Modified sine wave inverters are usually much less expensive, but you are very limited to the amount of appliances you can use. Purse sine wave inverters are compatible with most devices, so we recommend going with these inverters. Renogy Inverter’s are pure sine wave.