Solar Power Jargon
Watts, Volts, Amps, Amp Hours are all terms we hear when shopping for solar set-ups, so it’s worth understanding the basics of electrical measurement and what each of them mean so you can decipher the jargon and decide what you need.
You will be familiar with volts as your batteries in your vehicle will be 12v, USB points for your phone charger tend to be 5v and we all remember daring our mates to put their tongue on the contacts of the square 9v batteries. The volt is often referred to as electrical pressure and this is where the water/plumbing analogy starts when referring to these electrical terms. Think of your battery as a pump that pushes voltage around the electrical system in your vehicle.
The ampere, often shortened to “amp” or A, is the base unit of electric current in the International System of Units. Electricity flows through your wires and connectors in your vehicle and this flow is referred to as electrical current. This current is measured in Amps. This is why it is important to choose the correct wiring for your accessories and if too much current flows through the wires your fuses are designed to blow to save the wires and the accessory – these are rated in amps. A good example of this can be seen with USB sockets for phone chargers. The older sockets (or cheap ones) will be rated at 1A and you will get a very slow charge or no charge at all.
Top Tip: Thinking of adding a USB socket to your van for charging? Instead of installing a dedicated USB socket try installing a 12v classic cigarette lighter socket and then insert an adaptor to convert to USB. Technology around charging is changing all the time, as are phones and tablets, so having a simple 12v socket will allow you to change the adaptor when needed to keep you up to date.
A Watt is a unit of electricity to measure power. It measures the amount of energy or “power” released in a system. You can calculate Watts by multiplying Volts and Amps. When shopping for solar panels you will see these rated in Watts i.e. 100w Semi Flexible Solar Panel.
When sunlight hits a solar panel, voltage and current (Amps) are produced. This current, pushed by voltage, flows through wires in an electrical system to perform work when it encounters resistance, which can be measured in terms of watts.
You can see this on the display from a Victron Smart Solar Controller here. The voltage in the panel is 16.11v and if we multiply this by the current of 3.0A we get the 48w shown. It’s worth noting that these numbers are coming from a 100w panel on a cloudy day but with good light. So remember that the panel rating is the optimal rating and not always what power the panel will produce in direct sunlight.
The batteries in our vans are rated in AmpHrs so we are familiar with the term but what does it mean? Amp hours is a unit of energy, which can be consumed or generated over a period of time. Amps is a unit of power, which is an instantaneous measure.
Using the example above, this 100w solar panel is generating 3.0A – this is Amps so is an instantaneous measure. If we assume that the panel will generate this amount of power during the day (6 hours) then the total energy produced will be 18 AmpHrs. This is the difference between Power and Energy.
When designing your system you must consider cumulative charge in Amp hours fed to your battery bank, not just the instantaneous peak amperage, since your loads may want to run regardless of whether the sun is shining with full intensity on your panels or at night.
Designing your system
So now you’ve got to grips with the jargon around solar panels you can now start designing your system. It’s not as complicated as you might think, given what we have discussed above. There are several factors to consider and I promise none of them involve you having to get the calculator out. Although this can be an exact science with all the numbers and equations we talked about above – for campervans, it’s not. Factors that affect your set-up.
Type of camping you do
With a campervan being the sole form of transport that we have when away on holiday – we are unlikely to be static on a pitch for a number of days. The van will be started and moved (either to pop out for the day or to move to another site). During this time the leisure battery will be being charged by the alternator via a split charge system or via a DC to DC charger. This is important as this provides a valuable top-up to the battery during your holiday and means that you don’t have to run everything based on a net discharge calculation and net charge calculation from the solar panels.
Equally, there are some of us who after a period of ‘Off-Grid’ lifestyle may decide to stay at a campsite with facilities to empty the waste water, top up the fresh water, shower or entertain the kids etc. EHU points at these sites again off you the chance to boost your batteries via mains chargers.
If this is how you tend to holiday in the van then spending hours calculating your loads etc isn’t necessary.
What do you want to power?
It all comes down to power. What in your van needs powering? You want to start with the big hitters – Inverters and Compressor fridges. Let’s talk about inverters first, these basically suck the juice right out of your battery and quickly – so it’s best to plan to use them sparingly. If you have a laptop that needs charging then fine – run the inverter for an hour or so to charge laptop and then unplug and run the laptop on battery. You are not at the office and devices like this need not be plugged in all the time. Give the leisure battery a chance to recover some charge.
The compressor fridge is another device that is power hungry but not as much as you think. Campervan fridges are rated at around 40w, so with a 12v battery that equates to around 3.3A per hour. Now let’s say that you run the fridge for 24 hours then that equates to around 80Ah – which equates to one dead leisure battery. However, the fridges don’t behave that way – they run at that power to cool the fridge down but after that they consume much less power to maintain that temperature. So unless you plan on leaving the door open all the time the power consumption of the fridge is much less. Equally if we tie this back to our camping style – we will be travelling to our site whilst the fridge is getting down to the correct temperature so when we arrive and are reliant on the solar set-up to top up the batteries then the fridge is already cold.
Top Tip: add a simple rocker switch to the fridge power supply and you can turn it off at night when you are sleeping, this minimises the noise and power consumption during the night hours when the solar panels are not able to produce power. The fridge is well insulated and will keep a cool temperature even when turned off for a period.
Phone chargers are another popular accessory for vans and run around 2A for the decent ones. The key here is the same as the laptop – charge it then unplug it and use it’s battery as it’s designed to be used.
We all have lights in the van and increasingly these are moving over to LED – these draw hardly any current (LED light strips for example draw around 2.9W per foot) so you can dismiss them from your equations – a 6 foot light string will consume less than 1.5A. If you don’t have LED lights then rather than specifying your system to account for an array of 40w bulbs – make the change to LED and this will mean less stress on your system.
The starting point
What power do you have to start with? Or…how big are yours? I’m talking batteries of course. Starting with the largest leisure battery you can fit in the van and having that battery in the best possible health will ensure your system starts on great shape. 80Ah is often the highest the smaller vans can go with limited space in the leisure battery area but some of the home builds can cope with a bigger 110Ah or even 2 batteries linked together. Go big or go home is the basic idea.
Let’s talk Solar
So now we have thought about the type of camping we are doing and what we want to charge let’s find out what type of panel we want – what are the choices?
Rigid Frame Solar Panels
Rigid frame soar panels are popular with motorhomes and caravaners as they tend to have more roof space. With the development of flexi-panels over the years the rigid frame versions are becoming less popular as the durability on both types is now the same.
Semi-Flexible Solar Panels
A very popular choice for campervans as the semi-flexible solar panels can be bonded directly to the roof with a suitable adhesive and maintain very low profile. The new rear mounted junction box panels are popular with pop top owners as they lie completely flat with the junction box being on the back of the panel – these make a really neat and discreet addition to your van. You can see on this image how good they look.
The other advantage of semi-flexible solar panels is the flex that these panels offer – allowing your panel to match any contours on your van roof – bear in mind that they are semi-flexible and have a max bend radius – so check this out first if you want to mount to a very curved surface, you could damage the panel.
Folding or Briefcase Solar Panels
If you are not keen on attaching a panel to your van permanently then a folding or briefcase type solar panel is a great option to consider. There are some pro’s and con’s to consider. They allow you to place the panel in direct sunlight, whilst keeping your van parked in the shade and they can also be turned through the day to track the sun to ensure maximum efficiency. they are also easy to install into your van set-up as some come with built in controllers and simply require you to connect them to the battery – so ease of use is to be considered.
On the negative side, they do take up valuable storage space whilst your are travelling and you will need to factor the set-up of the panel into your camp routine. If you are wild camping then this may not be possible and then you have to accommodate the panel in the van overnight where space is even more of a premium. Also as they are detached from the van you should consider potential theft and one other factor to consider is the type of camping you do. If like me you will be using the van to pop out during the day to local attractions etc then you will have to disconnect the panel and nothing will be charging your batteries while you are parked up for the day in the local town – with the fridge on.
How big do I go?
After that preamble we get to the key question – how big a solar panel do I need?
In my opinion, the answer is simpler than you think but not truly defined but here are some choices.
- If you have a compressor fridge and an inverter then go for 120w or above – 150W if you have the space on the roof. I run a 100W solar panel and have no problems but I do use the van for everyday transport and I turn my fridge off at night.
- If you are just powering some lights and a few charging sockets then you’ll be fine with 80w – 100w
- If you want to keep your starter battery topped up when it’s parked at home then look at the 60w panels
- If you are thinking about panels smaller than 60w – don’t bother – those dash mounted cheap ones from motorists stores will do nothing for your batteries.
The choices and the technology seem a little confusing at first and my advice is to speak to someone first who can advise you on your situation but don’t get sucked into a debate over what power this accessory or that inverter uses and confuse yourself with numbers. Your camping behaviour dictates a lot about how you set up your system.
Other items you will need include a controller and there is a wealth of choice on those from cheap PWM controllers to more expensive MPPT controllers. Check out our blog here on how to choose your controller.