Sizing RV Solar – Solar Panels, Battery Bank, & Inverter
I’ve gotten a lot of questions about how to size the various components needed for a complete RV solar system. Sizing RV solar components can be difficult if you don’t know the steps required. Back when I assembled our solar system I found it very hard to find the information needed to accurately size our RV solar components. Because of this, our RV solar setup isn’t as perfect as it could be. It is undersized in a couple areas, and oversized in another.
However, I’ve learned a lot since then and – with Sweetie’s help – I’m here to explain to you step-by-step how to size your RV solar. So hopefully after reading this post you will be able to avoid the mistakes I made and succeed in accurately sizing your RV solar components.
Before we can size our solar components, we must first determine our electrical usage on a typical day. So grab a piece of paper and a pencil and create a table. This electrical usage table will have a number of rows equal to the number of electrical components we use on a typical day.
The first column is the name of each electrical device. The second column is the amperage draw of the appliance from a 12 volt power source. The third column is the hours per day the appliance will be used. And the final column is the total amp-hours (Ah) used by the appliance each day. Fill out the first column listing each electrical device you will use in a typical day. At this point your table should look something like the following picture.
Example electrical usage table
The second column is amperage draw of each electrical device. There are a few ways to determine a device’s amperage draw. The first way is to look up the manufacturer’s specifications. Also, many times this information is on the side or bottom of the appliance. Or you could purchase an electrical usage monitor that will tell you the exact amperage draw of the device while in use. Remember, in our table we are using amperage from a 12 volt (V) source. So for a 120 V AC appliance the manufacturer’s listed amperage draw will need to be multiplied by 10 for our table. 12 V DC electronics can have their amperage draw listed as is.
The third column is the number of hours each electrical device will be used in a given day. And the final column is the amperage draw multiplied by the hours used each day. Now your table should look like the following example. Now we’ve got some usable numbers to work with to size our various solar components.
Example electrical usage table filled out
Solar Panels – The Power Plant
The job of the solar panels is to charge the battery bank with the electrical energy they generate to replenish what is used in a 24 hour period in the 7 or so hours of useful daylight sun. So to determine the watts of solar panels required we must first turn to our electrical usage table and add up the total amp-hours used by our electrical devices in a typical day.
Total Amp-hours used in typical day
In our example our electrical devices use, all together, 206 Ah. Therefore, the solar panels need to generate 206 Ah in the mere 7 hours of daylight sun. Now, the number of useful daylight sun hours will vary depending on where you are, how cloudy it is, the latitude of your position, what season it is, if there is tree cover, etc. But for now we are going to assume we get 7 hours of useful daylight each day and take all those sun-reducing variables into account later.
So take the total amp-hour usage of your electrical devices and divide it by 7 hours. The number we find then is the amount of amps the solar panels need to generate in full sun. The example shows 29.4 amps (A).
Amps the solar panels must generate in full sun
Solar panels are rated in watts, but we know how many amps we require. So to determine the wattage of solar panels required we will use the formula Watts = Volts x Amps (W = V x A). For amps we will plug in the number we just found, 29.4 A. For volts we will plug in the typical voltage rating for solar panels used in RV’s, 18 volts. 18 volts times 29.4 amps gives us our required wattage output for our solar panels, 530 W.
Absolute minimum wattage of solar panels without MPPT charge controller
This number we just found, 530 W, in our example is basically the absolute minimum wattage of solar panels required. We haven’t taken into account the various sun-reducing factors, if you have an MPPT charge controller or not, or what type of battery bank you have. These factors all weigh in to the final solar panel wattage requirement.
First lets take into account various sun-reducing factors like where you plan to travel to, how cloudy it may be, the latitude, the season, possible tree cover, etc. Unfortunately this part isn’t an exact science and will be different for everyone. All we know for sure is you wont get 7 hours of full sun every day. So to take all these sun-reducing variables into account we’ll apply a Factor of Safety (FoS). This number you’ll have to determine yourself based on what sun-reducing factors you think you’ll encounter. But for us, we’ll use 30% Factor of Safety. So we will multiply the wattage we found earlier, 530 W, by 1.3.
Your FoS may be different, 30% is used as an example
Now we need to take into account whether or not you’ll be using an MPPT charge controller. I highly suggest you do because an MPPT charge controller can increase the amperage sent to the batteries by 30%! This means your batteries will charge 30% faster using an MPPT charge controller than with a PWM charge controller. So, if you are using an MPPT charge controller you can go ahead decrease the total wattage of solar panels 30% and divide by 1.3.
1.3 FoS and MPPT charge controller factored in
The last factor that needs to be taken into consideration is what type of battery chemistry you’ll use for your battery bank. Lead acid batteries only have an 85% charge efficiency. What this means is that from ever 1 amp sent to a lead acid battery bank only 0.85 amps are stored for use. Therefore, we’ll need to multiply the wattage of solar panels a lead acid user will require by 1.15.
85% charge efficiency of lead acid taken into account
We use lithium iron phosphate batteries however, which have a charge efficiency of nearly 100%. Therefore, every amp sent to them is stored for use. We also use an MPPT charge controller. So using a Factor of Safety of 30% our final wattage of solar panels required is 530 W.
Battery Bank – Reserve Power
The job of the battery bank is to provide power to our electrical devices during times when the sun isn’t shining such as at night and during cloudy days. Now if you plan on living off the cord like we do you will depend on your battery bank to get you through a string of cloudy days. If you don’t though, you’ll just need your batteries to get you through the night. With this in mind ask yourself “How long do I need my batteries to last during periods of little to no sun?” Since we literally live off our batteries, we would like our battery bank to last through 3 straight cloudy days.
Even though solar panels can still make about 25% during cloudy days we are going to assume they make absolutely no power. This will build a Factor of Safety into our calculations. Now, looking back on the electrical usage table, we are going to remove the cooking appliances from it because we don’t use those on cloudy days. We just cook with gas instead to conserve battery power.
Electrical devices the batteries have to power during cloudy days
This is the list of electrical devices the battery bank needs to power during periods of little or no sun. Just as before, we need to add up the amp-hour usage of these devices to find the total. In this example, it is 156 Ah of total electrical usage in a 24 hour period.
Total Ah of electrical usage during cloudy days
Now, for this example, we want our batteries to last through 3 full days of little to no sun. Therefore, we need to multiply the total amp-hour usage of our electrical devices per day by 3. This gives us the total usable amp-hour battery bank capacity required.
Total usable battery bank capacity required to get through 3 cloudy days
At this point, battery bank type comes into play again. Sorry lead acid users, you’re about to get hit again! Since the allowable depth of discharge of a typical lead acid battery is only 50% (AGM can be as high as 80%) we have to double the usable amp-hour capacity required to find the rated amp-hour capacity required.
Rated capacity of lead acid battery bank required
Lithium batteries however have an allowable depth of discharge of 80% or more. So if you are going to use lithium batteries then divide the usable amp-hour capacity required by 0.8 to determine the ratedamp-hour capacity required.
Rated capacity of lithium battery bank required
We use lithium iron phosphate batteries. Therefore, in this example 585 Ah total rated lithium battery bank capacity is what we require.
Inverter – Powering Household Electronics
The inverter is the final component we need to size for our RV solar system. The job of the inverter is to convert 12 V DC electricity from the battery bank to 120 V AC electricity. This is then sent to the RV’s AC panel to power the various 120 V AC appliances and outlets throughout the RV. To size the inverter, look back at the electrical usage table and remove from it all the 12 V DC electronics. The inverter does not power 12 V DC electronics, the battery bank does.
Now determine all of the listed 120 V AC electronics that you want to run simultaneously. For our example we will want to run our laptops and electrical cooking devices at the same time. So we will add up their total combined amperage draw.
120 V AC electronics we want to power simultaneously
Now, like solar panels, inverters are rated in watts but we have amps. So again we will use the formula W = V x A to determine the power in watts our inverter needs to handle. For our example we will plug in 113 for amps, and 12 for the voltage since the numbers in our electrical usage table are all from a 12 volt power source. 113 A x 12 V = 1356 W
Total watts of power inverter needs to supply
Now when it comes to inverters, you really don’t want to push the limit on the maximum rated wattage output of the inverter. So it is a good idea to oversize by quite a bit. In our example our inverter needs to be able to supply 1356 watts. So we could go with a 1500 W inverter, but in my opinion this is pushing it too close. For our example I would recommend going with a 2000 W inverter for a fair amount of breathing room.
And there you have it! Following the steps in our example here you’ll be able to accurately size the wattage of solar panels, Ah rated battery bank capacity, and wattage of inverter you’ll require to be a happy camper!
The electrical usage example shown in this post is very close to what Jenni and I use in a typical day. So the solar panel, battery bank, and inverter sized in the examples is about what we should have. Unfortunately, our actual solar array is 480 W – slightly smaller than the 530 W sized in the example. Our actual battery bank rated capacity is only 400 Ah – well under the 585 Ah required rated capacity found in the example. And our actual inverter is 3000 W – way overkill from the 2000 W inverter size found in the example.
If I knew what I know now about RV solar systems I could have more accurately sized our RV solar system. Hopefully though with this information you’ll be able to accurately size yours! If you have any questions that I didn’t answer in this post or if you don’t quite understand something feel free to comment below!