Solar System Size Calculator
Austin developed a comprehensive Excel spreadsheet to determine the size solar system we would need. If you’re interested in using the spreadsheet, comment below. If there’s enough interest we plan to share it!
If you are looking for a quick estimation, you can use the PVWatts Calculator, which calculates the output of a household solar panel system given a specific location, month, and takes into account average weather conditions.
The objective of the spreadsheet was to determine the size of the solar array we need to power our electronics, and the size of the battery array that would sustain us if there wasn’t any sun. Our target was 24 hours at max power usage or 48 hours at reduced power usage.
The spreadsheet was split into four sections, which I’ll explain it as if you were using it to give the larger picture of how we determined the size of our solar system.
Section 1: Electrical Loads
First, you list what appliances, lights, and any other electrical draws you have in the van. Then you enter how many watts each would use at maximum power. That section is broken up into AC (i.e. 120V normal house electronics) and DC (i.e. 12V car electronics) loads.
The reason you have to split them up is because the DC-AC inverter uses a part of the power to convert the DC current to 120V so we can use regular household electronics. Typically, inverters use about 10% additional power, so powering an AC appliance through an inverter takes about 10% more energy than it would if you were powering it off the electric grid.
To find out what your loads are, buy an inline electricity usage meter for less than $20 on Amazon. This will tell you how much power your appliances use over a certain amount of time.
Section 2: Usage Schedule
In this section, the day was broken up into 24 hours. You input what fraction of every hour you would be using every appliance through a representative average day. For example, the refrigerator has a 50% duty cycle, which means it spends half of its time on and half of its time off. To represent that in the usage schedule, we indicated that the refrigerator compressor would be running for 30 minutes out of every hour.
Another example would be the toaster. It runs for about 5 minutes at 8 am. During those 5 minutes, it uses 800 watts, but since it’s only running for a short time, its total energy usage is low over the course of an entire day. Representing the electric usage in this way also allows you to visualize when your high load peak times would be. For example, early in the morning when the sun rises and you’re making breakfast, or at night when you have all your lights on.
Section 3: Solar Panel & Operating Environment Information
In this section, you input the maximum output of your solar panels (in watts), your latitude (which informs the angle of the sun), what month it is (which also informs the angle of the sun), and how many hours of daylight you get during the day in that month (which you can google for your location at any time of year). The spreadsheet uses that information to determine the bell curve for your solar panels’ output, assuming that it peaks at noon and is zero at night.
Note, this doesn’t take into account cloudy days, it assumes perfect sun.
Section 4: Solar/Battery Duty Cycle
This section shows a representation of 24 hours and when you’re charging the battery and when the batteries are discharging. It shows how much total energy you’re using during each hour, how much power the solar panels are outputting during each hour, and what the net gain or loss is during each hour of the day. This allows you to adjust your schedule, such as waiting to use a high electrical appliance until the sun is higher in the sky so the solar panels can help out a bit.
Conclusion
You can use this tool to help get an idea of how different parameters affect your electrical system. A word of caution, it will be approximate because it assumes ideal conditions. For an average month, it is going to estimate pretty high because it assumes no clouds.
Our System
We determined that for our system, the difference between a 400, 500, and 600-watt panel array wasn’t very different, but a 300-watt array wouldn’t be able to sustain our load. So we went with a 400 watt array so we could save money. We also determined that our target amp hours for a battery array was 250. This helped us determine the appropriate size of the components in our system and price it out. For our system, we decided to go with a 400-watt panel array, a 2000 watt inverter (so we could use all high load electronics simultaneously), a battery to battery charger so we can charge our battery system while driving (off of the car’s alternator), and a battery charger so we can use shore power either from campgrounds or extension cords.
Originally, when we were doing a “ballpark” budget, we estimated that our solar system would cost $1,865. We made some strategic decisions that affected the price of our system. Some increased the cost, and some decreased the cost. Now, we estimate that the solar system will cost around $1,685, which is about $180 cheaper. I’ll discuss a few of the major influences on the price below.
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Why Renogy
We decided on Renogy for two reasons. The first is because a trusted resource, Far Out Ride, recommends them as a good budget solar option. Isabelle and Antoine, who write the blog and detailed build guide, are both engineers. We really trust their recommendations and defer to them when making decisions on things we have little to no experience with (like solar brands). The other reason is because in my preliminary research, Renogy solar kits were popping up everywhere. They seemed to be well-liked, well-reviewed, and easy for beginners. I liked that they offered kits, so we would get everything that we needed in one package. I felt that based on my research that Renogy was a reputable brand.
Choosing a Solar Kit
In the 400 watt size, Renogy offers a Solar Starter Kit
Batteries
We thought we were going to get a Renogy Deep Cycle AGM 200AH battery. This one battery would be able to handle our entire electrical system needs. AGM batteries aren’t as expensive as Lithium batteries, but they aren’t as cheap as traditional lead acid batteries. When we priced it out, buying a $350 battery just didn’t make sense. We could buy three 87 AH flooded lead-acid batteries from an auto parts store, like Pep Boys, for around $240 (before sales and discounts), for a savings of $110. We plan to save even more by utilizing coupons, credit card cashback, and also by finding cheap/free batteries on craigslist to return to the store to recoup the “core charge.”
However, there are some considerations beyond cost when deciding what type of battery to get. Flooded lead-acid batteries are the highest maintenance but lowest cost and longest lasting batteries (if they’re properly maintained). Maintenance isn’t hard though, you just have to top them up with distilled water every once in a while. You can only discharge lead-acid batteries about 50%, whereas you can almost fully discharge lithium batteries without damaging them. Lithium batteries are way out of our price range, you could buy 10 lead-acid batteries for the same price of an equivalent output lithium battery.
Inverter
We had to decide between a 2000 watt inverter and a 1000 watt inverter. The biggest factor for us is that we plan to use a 100V dorm style mini fridge because we got it for free, and 12V fridges are $600-$1000+. Before we did our solar size calculation, we thought we could get away with a 1000 watt inverter. After calculating, we found that while we could use a 1000 watt inverter, running the fridge at the same time as the toaster, or trying to use an electric kettle for water, would be impossible. So to create a solar system that would fit our lives (and the way we make breakfast), we decided to pay an extra $100 to get the 2000 watt inverter from Renogy.
Our system components
- Renogy 400 Watt 12 Volt Solar Starter Kit – $660
- Renogy 2000 Watt Inverter – $300
- 3 x 87 Amp Hour Flooded Lead-Acid Batteries – $240
- Renogy 40 Amp DC-AC Battery to Battery Charger – $200
- Samlex America 12 Volt 15 Amp Battery Charger – $140
- DC Multifunction Battery Monitor with LCD display (100A Shunt) – $20
- Renogy 30 Amp MC4 Waterproof In-Line Holder with Fuse – $18
- Renogy Solar Connector Assembly Tool – $7
- Misc Wires & Fuses – $100
Total Solar System Cost = $1,685
This price is before any taxes or shipping, and before any discounts, sales, coupons, or credit card rewards.
Summary:
9/3/2020
Build Day #8
Hours Worked: 6
- Austin – 6
- Total Project Hours: 87.5
Money Spent: $0
- Total Project Spending: $16,706
Tasks Complete: 1
- Determine size of solar system
Questions Googled: Lots
- A lot about the math behind solar systems
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