Solar is the primary way we generate the electricity needed to charge our batteries and power the house. There are other ways to charge the batteries, but these are not as effective as Solar.
- We have an inverter/charger which can be hooked up to shore power and convert 110 volts ac into 14.2 volts to charge the batteries. But this is only useful to when you have access to 110 volts ac, which is not something you typically find when out in the wild.
- Our inverter/charger can only work off 110 volts, so is useless in many parts of the world. We carry a simple battery charger for when we need to charge off 220 volts. But this is a super hassle to use, and we have to open the battery box to use it.
- The alternator from Molly is connected via a dc-to-dc battery charger which can be used when driving. This charger is sized for a very modest 20 Amps to not place too much strain on the standard alternator that comes with the FUSO truck.
So when we don’t have access to shore power or Molly is not running, then Solar is our only practical power source.
When we ordered the truck, we had no idea how much electricity we would use, how big the batteries would be and how much Solar generation would be right. It took a few years to finally sort this out. And a few days waking up to flat batteries and having to start the truck to generate electricity to make coffee to understand how important sizing is. Unfortunately solar panel sizing is one of those dark arts, that seems to be hard to understand. Rated panel watts never seem to equal actual measured panel watts, and any little thing can cause the panels to operate less efficiently.
To help demystify this dark art, here we explain how we sized both of battery capacity and the solar generation required for us.
It all started when we ordered the truck. At the time the standard or minimum configuration was single 270 Ahr AGM battery with a single 154 watt solar panel. Clearly this was not going to work, so we added a second battery and two extra roof top solar panels. So Molly was born with with three roof-top panels, each rated at 154 watts for a total of 462 watts. In reality these panels produce about 1,850 watts per day when they have good visibility to the sky. This figure can change dramatically up and down and is very dependent on the weather conditions, time of year and distance from the equator. It did not take long before we worked out we needed more solar panels. Fortunately we also worked out that we did not want to add more panels to the roof, as we usually park Molly under the cool shade of trees whenever we can, and under trees it is not possible to generate any solar power, regardless of how many panels we have. So we expanded of solar generating capacity by adding portable solar panels. Where we can move these panels around to follow the sun. We also worked out we needed more capacity than the two 270 Ahr batteries that came with Molly. It took a little longer to realize the batteries were under sized.
After calculating our expected daily usage, monitoring the solar panels to establish how effective they were and experimenting with portable solar panels, we decided to increase the capacity of our batteries and increase the capacity of our solar generation to. The increases were governed by the following requirements
- 3 days with no sun. Most likely to happen when it is snowing hard as happened when we visited Yosemite in winter.
- 5 days with good sun to the portable panels but no sun to the roof-top panels, allowing Molly to stay cool in the shade.
- at least a week with just the roof-top solar panels, assuming they have a clear view of the sky. This is for the situation where the portable solar panels might be stolen.
Our daily consumption budget (in non freezing conditions) is a very generous 2,250 watts per day. With this budget, we therefore need at least 562 Ahr of battery capacity, and we found a battery solution which provided 600 Ahr. Once we selected the battery size, we can go on to determine how much solar generation we need, at least theoretically. To survive 5 days on just portable solar panels, the calculations indicate we therefore need around 115 watts of portable solar panels. Two 100 watt solar panels will generate close to 125 watts of actual power, and this is what we initially decided on. However we swapped the big bulky and not really portable panels we initially selected for smaller foldable panels, and it turns out these were available in 120 watts version, so we up-sized a little. At 120 watts each, this is around 150 watts effective power for both panels and allows us to go a whopping 14 days just on these portable panels alone. Finally, assuming we do not add any extra panels to the roof, we would lose about 800 watts per day and allow us to run 8 days on just the roof top solar panels.
With both roof top and portable solar panels deployed and good weather, we can generate as much as we consume.
With these convoluted calculations out of the way, we finally settled on what we consider is the perfect balance of batter and solar generation capacity, at least for us. In summary:
- 600 Ahr of battery capacity
- 3x 154 watt roof-top solar panels (total of 462 watts)
- 2x 120 watts portable solar panels (total of 240 watts)
- Total Solar Generating capacity of 702 watts.
Extended Off-Grid Considerations
For extended time off-grid situations, we want enough solar generation to effectively replace whatever we use in a day. This also means we should always have near full batteries, just in case something breaks and we need to get back to civilization.
One option is to combine both the roof-top panels with the portable panels, giving 2,500 watts of effective daily generation, which is more than our typically daily consumption. But these means parking Molly in the Sun, and that is not ideal.
An alternative is to add 2 more 120 watt portable panels. This would give around 2,100 watts of solar generation on a typical day, which practically meets our consumption.
There is an added advantage, with full sun and all of this extra generation, we could (n theory) start with completely drained our lithium batteries and fully re-charge them in 5 days under normal load using all the portable and roof-top panels. Let’s hope this never happens.