We are really excited about the prospect of solar panels generating 100% of our electrical needs. Being self-sufficient is one of our goals in life. We are currently designing a 1600+ Watt Solar Panel Array to be mounted on the roof of our skoolie. As a bonus I will be designing a rooftop solar tracking system to keep the panels pointed directly at the sun, when we’re parked. We’ve gone back and forth with the actual wattage we think we will need, but we want to live comfortably, and as such, we understand we will need A LOT of solar to power our appliances and recharge our battery bank each day. It may seem like a lot of time, effort, and money, but for us, it makes sense as it’s our goal is to live as sustainable as possible in our Skoolie.
12v or 24v Panels? And does it really matter?
To really answer this, we need to first consider Voltage and Amps in the context of a garden hose. Bear with me, this is the best method I’ve seen. Think of Amps as a unit of volume, like a gallon of water; while Voltage is a measure of pressure, like pounds-per-square inch (psi).
Let’s say our battery bank is 1000AH and is fully charged at 14.5v. For our purposes, the fact that the battery is fully charged at 14.5v tells me that I’ll need to charge these batteries at a voltage higher than 14.5v. It’s not enough to match the batteries full charge voltage, instead we need a higher voltage, acting like a higher pressure, to squeeze that last few % of Amp Hours into the battery bank.
On average, 12v solar panels are capable of generating 18v during direct sunlight, peak hours. If 12v solar panels actually generated only 12v, there would not be enough voltage to even charge a standard 12v battery. While the 18v generated by 12v solar panels is great for a charging voltage; it is only available for a few hours each day.
This is where 24v solar panels, or 12v wired in series, come into play. With 24v solar panels, 24v becomes the minimum generated voltage. During direct sunlight, peak hours, these panels are capable of generating ~36v! This all becomes more important as we talk about Solar Charge Controllers below.
We’ve, for now, settled on the SolarWorld SW 285 Watt Mono Solar Panel. It sells for ~$1.10 / watt, a decent price for a very reputable brand. It measures roughly 5’x3′ and (6) will take up roughly a 1/3 of our rooftop as a 1,710 watt array. The panels max power point is 32v @ 9amps. These panels will be perfect for wiring in parallel to an MPPT Solar Charge Controller.
- 285w x 6 = 1,710 watts
- 9A x 6 = 54 Amps
PWM vs MPPT Solar Chargers
Solar Charge Controllers are specialized 12v battery chargers that regulate the voltage coming from the solar panels, going to the batteries. Often, these are very specialized deep-cycle battery chargers. There are two main types of controllers: PWM and MPPT.
PWM (Pulse Width Modulation) Chargers slowly lower the applied voltage to the batteries as they come closer and closer to a full charge. PWM chargers have their place, in that they cause less stress on the battery, while obtaining a fuller charge. Often included with “solar kits” directed at new buyers. These chargers are the lower to mid-range quality chargers.
MPPT (Maximum Power Point Tracking) Chargers are the most recent and best style of solar charge controller. MPPT Chargers are able to convert excess voltage into charging amperage! Thus, by converting the excess voltage into amps, the charge voltage can be kept at an optimal level, while reducing the total time to charge. MPPT Charges are up to 30% more efficient. MPPT charges can typically run 2x the price of their PWM counterparts, however the pros outweigh the cons for us.
Remember how 24v panels lowest produced voltage is 24v? Combine that with the fact that MPPT chargers can convert voltage into amperage, and you have a recipe for success. Instead of waiting for the sun to be high overhead, we can harness the morning sun more efficiently with 24v panels and an MPPT charger.
We know we will be using a MPPT Charger to reap the benefits of higher voltage panels. However we haven’t settled on a specific charger; though, personally we are considering the Outback FM-80, and the Morningstar TriStar 60 Amp, they are of the most versatile MPPT chargers currently available.
Make sure to check out the post about 12 Volt Batteries to learn about the different options available for your off grid needs.