# Calculate the size of the solar system

First, we don’t have electricity – we have to make it. To do this, energy sources such as solar energy must be tapped – and this energy is converted into electricity. We use monocrystalline solar panels to turn solar energy into solar power. We use the vehicle’s alternator to generate electricity from diesel. Or a power generator to convert gasoline into electricity.

The question now is which should be the primary sources of energy. Suppose electricity is to be generated exclusively via photovoltaics. In that case, the calculation is quite simple: the daily electricity consumption must be recovered, ideally not only in bright sunshine but also when it is slightly cloudy.

Do you keep driving for one to three hours every few days, or do you like staying in a nice spot for a week or two? A cut-off relay or charging booster can be useful if you drive a lot and regularly. Although this increases fuel consumption (in our case, by 1 litre per 100 km), it may make more sense than increasing the wattage of the solar system if you travel accordingly. The same applies to people who prefer the frosty north. People drawn to the Arctic Circle in winter will not get very far with solar power. The focus should be on generating electricity via an alternator and an emergency generator. A fuel cell can also make sense – but only because it is expensive and has poor efficiency.

Choosing the right RV battery

The solar array size on the RV should be synchronized with the battery. A 100Wp system on the roof and 100 Amp hour lithium battery in the motorhome? You can leave it as it is. Because if you have installed so much battery capacity, you need a lot of electricity – and will never get your consumption back with such a measly system.

Calculate battery capacity – soft factors

200Ah in lead or 100Ah in lithium? Several decision factors come together here. Do I have problems with space and payload that justify a higher investment in a lithium ion RV battery in the motorhome? Do I travel 15 or 365 days a year with my motorhome? Last but not least, there is the question of comfort: no longer having to worry about the charge level of the battery in the mobile home – an important selling point, especially for those who have already had a supply battery broken in a very short time.

Standing or driving? A lead-acid battery, in particular, needs to be fully charged regularly. There should be an absorption voltage for several hours. There are several strategies for this to succeed: some specifically drive to a socket once a week to plug in shore power overnight. Or you can calculate the size of the solar system in such a way that full batteries are guaranteed. With a lithium battery, this problem does not exist. She loves not being full.

Consume electricity – less is better

How many watts a off grid solar system should have depended on the daily power consumption. The values can vary depending on the season – a compressor refrigerator requires significantly more electricity in summer than in the cool autumn. Ideally, the heating should only run rarely in summer. The summer months are generally the lesser problem because there should be abundant electricity here. The question is whether mixed weather should not be considered when calculating the solar system.

Power consumption is an adjustment screw everyone can turn – without having to save on comfort. Being aware of where the current is going is a good starting point. Many people can save electricity here:

Is the lighting in the mobile home LED?

• Which 230V devices draw a lot of currents? Tip: everything that generates heat or cold, such as fan heaters, hair dryers, kettles
• Which 230V devices could I run on 12V? Tip: laptop, battery chargers, TV
• Does the inverter run all day? Tip: a Victron Energy remote control or inverter with AES mode will help save electricity

Small digression: Electricity jargon for dummies

The power supply in the RV is roughly the same as what is taught in 8th-grade physics class. So it’s been a long time. Here’s a little refresher:

Current abbreviations and their meaning

• Ampere (A): Amperage – how much current a device supplies or needs.
• Example 1: a 12V solar module with 100 watts of power: 100 watts: 12V = 8.33 amps
• Example 2: a 12V kettle with 200 watts of power: 200 watts: 12V = 16.67 amps
• Amp Hours (Ah): Battery capacity
• Example: A 12V battery with 100Ah delivers 20 amps for 5 hours
• Watt (W): Power that a device supplies or needs.
• Example 1: a hair dryer needs 900 watts
• Example 2: a power generator delivers 1000 watts
• Watt-hours (Wh): Watts x hours
• Example 1: An oven draws 1400 watts and bakes the cake in 3/4 hour: 1400×0.75 = 1050 Wh
• Example 2: The compressor refrigerator in the mobile home draws 50 watts and runs for about 15 minutes per hour: 50W x 0.25h x 24h = 300Wh / day
• Volt (V): Voltage of a power grid
• Example 1: the mains voltage in the mobile home is around 12V.
• Example 2: the power grid in Europe has a voltage of 230V.
• Connect solar modules in parallel or series: You can connect 12V solar modules in series (24V) or parallel (12V). Advantage of 24V or more: smaller cable cross-sections are required, and fewer problems with partial shading of the modules. Please note: the charge controller in the mobile home must also be able to withstand the higher voltage.
• PWM and MPPT charge controllers: PWM (Pulse Width Modulation) are the simpler and cheaper charge controller, and MPPT (MaximumPowerPointTracking) is the better and more expensive one. They are smarter and manage to shovel the herrings coming in via the solar array into the battery more efficiently. We prefer to sell Victron solar controllers because they are very good and reliable.
• Starter battery vs supply battery: Shockingly, cheaper starter batteries are installed (from the factory) in many mobile homes. As the name suggests, they are designed to start a vehicle – they can deliver high currents quickly. They are not intended to deliver low currents for long periods. Therefore, supply batteries are installed as mobile home batteries. If properly charged, they will last for many years.

Lead acid battery vs lithium battery

• The lead-acid battery in the RV: I’m not saying it doesn’t work. I’m just saying that the premature death rate is quite high, which is why I leave this type of battery out.
• GEL battery: Long-lasting, mostly no problems with existing chargers
• AGM battery: Be careful with the charging characteristic, which cannot be set with some EBL and charge controllers. Better for high current draw through the inverter.
• Lithium battery: expensive but hard to break – we’ve been running it for 5 years, 365 days a year. Pays off with intensive use, hardly worth it for the normal holidaymaker.
• 12V vs. 230V in the mobile home: Most mobile homes have a 12V on-board voltage – battery, 12V sockets, light, all of this should run on 12V. Household appliances as we know them (in Europe) need a voltage of 230V. Fan, laptop, coffee machine – to use them normally in the mobile home, you need a voltage converter from 12V to 230V. Everyone must consider this: Do I buy 12V devices that remain permanently in the mobile home and work there energy-efficiently? Or do I use existing 230V devices with a voltage converter? Incidentally, every conversion costs electricity and many can be avoided at low cost, for example, with a 12V power supply unit for the laptop.
• 12V vs 24V: Normal mobile homes have an onboard voltage of 12V from the alternator to the battery. Since truck alternators, on the other hand, run on 24V, it can make sense to work with this voltage in the vehicle’s electrical system. Advantages: smaller cable cross-sections. Disadvantage: for 12V devices you need a DCDC converter (24V to 12V)
• Voltage converter: Synonyms for this are inverters or inverters. These are devices that convert one voltage into another: Voltage converters convert 12V into 230V (DC voltage into AC voltage) or 24V into 12V (24V alternator > 12V vehicle electrical system).
• Starting current: Some devices have a high starting current, which can be 10 times the actual output. Above all, these are devices with compressors and motors that must first get going: compressor refrigerators and electrical tools can be relevant in the mobile home. This starting current can bring an undersized inverter to its knees.
• Calculate battery cable cross-section: The principle applies to the cable cross-section: a lot helps. The thicker the cable, the more current can flow. So electricity also flows through thin cables, but they get hot and can burn together in the worst-case scenario. And nobody wants a cable fire in the mobile home. Three factors come together for the optimal cable cross-section:
• how long is the cable, 50cm or 5 meters

How much current flows? Five amps or 50 amps?

Simply put, a cable connecting two batteries has to be damn thick – even if it’s only 50cm long. But a lot of current flows here, maybe 200 amperes depending on the battery. The solar cable of a 200Wp solar system, which extends from the roof to the solar controller, is only 4mm² and 3 meters long – because a maximum of only 10 amperes flow here.

Retrofitting a solar system for the mobile home – from the solar calculator to the installation

For example, suppose the solar power calculator has concluded that a 300Wp solar and 200Ah lead battery in the mobile home is a good idea. In that case, the next question arises: how do I get to the components, what does everything in between, and how do you install such a solar system in the first place?

Tanja has already answered some of these questions on her blog crosli.de, for example, in the instructions on how to plan and install a solar system yourself or in the essay on the best battery in a mobile home. She has also written extensively about 12V voltage converters. So here are a few additional words:

Cheap from China vs branded goods

My camper is equipped with inexpensive solar modules and batteries, and the pure sine wave inverter. With today’s level of knowledge, I would not reinstall all the components in the same way. Because first of all, the first two inverters are smoked. I was lucky that I smelled it immediately and that nothing more happened. Secondly, I know today that not everything that glitters is gold, especially when it comes to solar equipment from the Far East. With solar modules, in particular, tricks are used everywhere. Nobody can check whether 100Wp or 110Wp is installed. “Cut cells” are also used to produce these inexpensive solar panels. So actually, damaged/broken cells are re-snipped together. With such modules, performance problems CAN occur, up to and including total failure. Such examples can be found in all possible components. Quality has its price, and those who buy cheaply also need a bit of luck. Now I often notice how other campers were less fortunate – because they ended up with me. How bold the mobile home industry is sometimes – with their marketing promises and pricing. That a 200Wp solar system costs a whopping \$1900 – phew, not even the highest quality solar panels are used – double phew.