To do determine this, first you need to find out how many amps you are using on average each hour (Ah), and how many that totals in day. Once you figure out how many amps per day you use, then you can see what size panel/s it will take to recharge your battery bank. Sounds easy, but not exactly due to limitations in roof space, compartment space for batteries, system design and budget.

Before you get started make sure all the appliances, lighting, and anything electrical used is as efficient as possible. This may include changing the lighting to LED, replacing the Fridge to a more efficient one, charging the laptop during sunlight hours and unplugged at night while in use. List each item, the number of Amps each item uses, and how many hours per day on average those items are used for.

Now that you have added up the number of amps used in a day, you can look at how much storage is required. A good quality AGM battery can discharge up to 50% of its rated capacity, so if your daily consumption for example is 30 Amps, then a 100Ah capacity battery will be the minimum in this example. From a system design perspective, the minimum is not what we would call ideal, but we are not going to go into days of autonomy and sustainable off-grid living in this write up.

Now we have a battery sized up for the system, it is time to decide how much solar is needed. A quick way to figure this out is decide the minimum hours required to charge our battery bank (2-3 hours per day is a general rule). In this example to get us back to full we need 30A / 2 Hours = 15A (per hour), then multiply that number by the voltage of the Battery e.g., 15A x 12.8V = 192W. So, at a minimum we can put a 195W solar panel on the roof.

Now these are not “real world” performance calculations as our panels will be sitting flat which is far from the optimal position, controller efficiencies and general losses are also not factored in.

Now we have the Battery and Panel sized up, we need to use a Solar Regulator to make sure the battery is getting the right amount of voltage and charge to the battery. There are 2 types of Solar Regulators PWM (Pulse Width Modulation) and MPPT (Maximum Power Point Tracking). We won’t dive into the technical differences here but a PWM is a cheaper regulator that just drops the voltage of the panel and delivers the right amount of current based on the charge mode it is in at the time, where the MPPT is more expensive and uses the full voltage curve of the panel to maximise the amount of current being delivered to the battery – this essentially boosts the current required to achieve these maximums rather than throwing away that much need voltage like the cheaper PWM Regulators do (expect to lose up to 25% of the panels rated Wattage if using PWM).

Because we only need to deliver 15A of charge from the Solar Panel, a 15A Solar regulator will be enough to do the job as per our example. However, because we are always adding new devices, and the weather is so unpredictable you may want to think about future proofing your system. So, if you do have room to add another panel in the future, make sure the solar regulator you put in now is big enough to handle the increase in solar energy.

These are just the basics when sizing a system and becomes overly complex when all the components are designed in, especially with safety in mind. So, If you are thinking about installing solar, or looking to use your caravan off-grid for days on end, then come and talk to one of the team at RV Repairs so we can design a realistic, sustainable system for your needs.