Solar is the primary way we generate the electricity needed to charge our batteries. And the batteries power just about everything in the house. On the only exceptions are the diesel cooktop and heater, but these also need battery power to operate. So making sure the solar systems are working is important.

There are other ways to charge the batteries, but these have limitations and are not as environmentally friendly or as convenient as Solar. They are:

  • Our 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 when you have access to 110 volts ac, which is not something you typically find when out in the wild.
  • The alternator from Molly can be connected to the batteries providing a charge when the engine is running. Typically the alternator is not used, but can be turned on from a switch in the front cab. This method of battery charging is only used if th state of charge of the batteries is below 80%.

So when we don’t have access to shore power or Molly’s engine is not running, then Solar is our only practical power source.

It is all about Sizing

When we ordered the truck, we had no idea how much energy we would use, how big the batteries should be and how much Solar generation would be right. It took a few years to finally sort this out. And it took a few mornings waking up to flat batteries and not being able to make morning coffees, before it was clear how important it is to get solar and battery sizing correct. Unfortunately solar 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. We also explain why we have roof top panels and portable panels.

It all started when we ordered the truck. At the time the standard or minimum configuration was a 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. We 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. Being portable, 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 us 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 both our batteries and our solar generation. The increases were governed by the following requirements.

To have enough battery capacity and solar generation to operate normally for

  • 3 days with no sun. Most likely to happen when it is snowing hard as happened when we visited Yosemite in winter. Also applicable when parked under heavy tree cover.
  • 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 good sun and just the roof-top solar panels, assuming they have a clear view of the sky.

Our daily consumption budget (in non freezing conditions) is a very generous 2,250 watts per day. With this budget and the above requirements, a little math indicates that we therefore need at least 562 Ahr of battery capacity. 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. We purchased two 100 watt fixed panels. However we swapped these big, bulky and not really portable panels 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 of effective power for both panels, and allows us to go 5 days just on these portable panels alone. With just roof top panels and very good sunny days, we can go more than two weeks on a full battery charge. While this sounds great, the reality is that we will not be staying in the same place under sunny skies for two weeks, so this specific calculation is a little academic.

With both roof top and portable solar panels deployed and reasonable weather, we can generate as much as we consume. In fact we actually add charge back to the batteries if they are not already fully charged.

With all these convoluted calculations and some real world testing, we finally settled on what we consider is the perfect balance of battery 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.