Select Solar's Frequently Asked Questions

How do I choose a solar panel?

There are several things to consider in choosing a panel:
How much power (wattage) do you need?
Does the panel need to fit in a particular space?
What is your budget?
What accessories does it come with and how easy is it to mount?
Does it need to be portable, lightweight, flexible, or is it a permanent installation?

How much power (wattage) do you need?

The wattage of solar panel you need is perhaps the most important thing to get right. If you underestimate the amount of power you need you could be very disappointed in your panel, but if you overestimate you might end up spending more than you need to. The panel needs to be sized according to how much power you are using, not according to the size of battery you are charging. The battery is a store of power – to maintain that store at a constant level the panel needs to put in to the battery the same amount of power as is going out of the battery. As a general rule, if you have a caravan you'll need a panel between 20-60W, whereas most motorhomes are fitted with panels of 80W and above. For laptop charging you need at least 25W to provide a useful trickle charge. See 'Sizing your system' below for more details.

Does the panel need to fit in a particular space?

One of the reasons we like to supply a range of different manufacturers panels is to provide lots of different sizes. You can also add panels together to get the overall size you need. Remember that for the equivalent wattage a crystalline panel will be smaller than an amorphous panel. For large orders we can have panels made to your specifications.

What is your budget?

Put simply, the more powerful the panel, the more it will cost. Rigid panels cost less per watt than flexible and folding panels. There are also differences between equivalent panels that may reflect the quality of the junction box and frame, packaging and instructions, cables and other accessories that might be included, and how good a deal we can get from the manufacturers!

What accessories does it come with, is it easy is it to mount?

Think about what accessories you'll need and how confident/competent you are with the wiring, and whether you would be better getting a kit. Some panels come with cables already fitted to the junction box (all flexible and rollable panels, smaller Kyocera panels), or with plug-and-go connectors (Sunsei range). However many will require you to fit cables and sometimes blocking diodes, both of which we can supply. Consider also whether you need additional cables like a 12V cigarette lighter socket or extension cables, and if these are available for your chosen panel. Finally, how will you mount the panel? Sunsei panels come with integrated mounting feet, but most rigid panels will have an aluminium frame that needs to be raised off up by about 10mm to allow the air to circulate underneath. We can supply simple mounting brackets, but if you require something more complex (a pole mount, or angled mount) please contact us.

Does it need to be portable or is it a permanent installation?

Generally speaking if weight and portability is your main concern then you need a folding or rollable panel. However, if you're not trekking/canoeing/cycling or doing some other extreme sport that requires you to carry all your kit, then you might like to look at our kits for Caravans. These are based on a rigid panel, so are less costly, but include a carry bag and stand. For permanent installations on motorhomes, sheds, boats etc it generally makes sense to use a rigid framed module. However, if you need to walk on the panel, or need it to fit on a curved area, you might consider either a rollable Powerfilm panel or a Solara semi-flexible kit.

Sizing a system

In sizing a system, the aim is to balance the power going in from the solar panel with the power going out of the battery over a period of days or weeks (depending on how it is being used). A 10W panel will give 10W (0.6A @ 16.5V) over an hour under standard test conditions (1000W/m sq and 25oC). In the UK allow around 4 hours equivalent sunshine in summer and 1 in winter. In Winter a 10W panel will give 10W over a whole day, whereas in summer it will give 40W. These are fairly conservative figures – some companies use up to 6 hours in summer. You can do the same calculations with the Amps. Some simple steps for sizing a 12V system:
1. Find the Wattage of your appliances. List all the 12V electrical appliances you´ll use in a typical day, and find out how many Watts they each consume. Usually this is on the appliance or in its handbook. If you can only find a figure for Amps, simply multiply this by 12, to convert it to Watts.
2. Calculate your daily total Watt-hour requirement. Estimate how many hours you would use each appliance for over a typical week, then divide by 7 for a daily rate. Multiply each appliance´s wattage by the hours you’ll use it for in a day. Then add all the totals together to get the final daily total Watt-hours you require
3. Next calculate your panel size. Simply divide the daily total Watt-hours you require by the hours of usable light you expect in an average day. This will give you your minimum panel wattage. In the UK, allow 1 hour of light in winter, rising to 4 hours by mid-summer.
4. Then your battery size… Multiply your daily Watt-hour requirement by 7 to create a weekly requirement, and divide this by 12 to convert back to Amp Hours, which batteries are rated in. Multiply by two to give the correct battery size.
5. And finally, your charge controller. Size your charge controller according to the Amps produced by your panel. Calculate the Amps produced by dividing the panel wattage by 16.5.
6. A worked example. In one week you want to run a 65W television for 4 hours, and an 8W light for 5 hours. Your daily Watt-hour requirement for the TV is 65 x (4/7) = 37Wh; and for the light you require 8 x (5/7) = 6Wh. Your total daily requirement is thus 43W. You only intend to use the system in summer, so you need a panel that is 43/4 = 11W or more. Your battery size needs to be (43 x 7 x 2)/12 = 50Ah. And you need a charge controller suitable for a solar input of at least 11/16.5 = 0.7A

Charge controllers

The charge controller (or regulator) connects between the panel and the battery. The charge controller functions
1. to protect the batteries from overcharging;
2. to protect the panel from power going back into it from the batteries at night (assuming no blocking diode fitted); and
CHOOSING A CONTROLLER: Charge controllers are rated for a certain solar input in Amps. This should always be higher than the rated Amps of the solar panel. If you only know the panel wattage then multiply the Amps x 12 to give you a conservative wattage equivalent.
WHEN TO USE A CONTROLLER: As a rule of thumb, use a controller when the rated Amps of the panel is more than 1% of the battery capacity.
TYPES OF CONTROLLER: There are two basic types of controller: on/off and shunt (also known as Pulse Width Modulated). On/off as its name suggests, switches of the power from the panel to the battery when the voltage reaches a certain level, and switches back on when it drops. This means that there are periods of time when there is no power going from the panel to the battery. Shunt controllers allow the battery voltage to increase, and then maintain it at a steady but high level.
SELF-CONSUMPTION: Beware, all controllers use up some of the power produced by the solar panel. This is normally a tiny amount, but adds up over time and can be important if there isn´t much slack in your system. The Sunsport and ASC controllers are useful in such a situation as they do not consume anything unless the panel is generating power.
MOUNTING: Charge controllers should be mounted somewhere dry. Some are more watertight than others.

Solar panels

POWER: The amount of power that a solar panel provides is indicated by the wattage (W). The higher the wattage, the more powerful the panel. Amps (current) and volts are also important because:
W = A x V Solar panels suitable for 12V battery charging generally have voltages rated at around 16-17V. The lower the voltage, the higher the Amps, and the faster current flowing out of the panel, and the faster a battery will charge.
PANEL RATINGS: Panel ratings are performed under test conditions of 1000W/m sq and 25oC. Although all 10W panels will give 10W per hour under these test conditions, they won’t all perform the same under other conditions.
PANEL OUTPUT:A 10W panel will give 10W over an hour under standard test conditions. In the UK allow around 4 hours equivalent sunshine in summer and 1 in winter. Ie In Winter a 10W panel will give 10W over a whole day, whereas in summer it will give 40W. These are fairly conservative figures – some companies use up to 6 hours in summer. You can do the same calculations with the Amps (people are often more familiar with Amps).
TYPES OF PANEL: There are two main types of solar panel: amorphous and crystalline. In general, amorphous perform better than crystalline under low light conditions and don´t suffer as much power loss in hot temperatures. However, in good conditions, the efficiency of amorphous panels is lower, and they are physically larger than crystalline panels of the same wattage. Rollable, folding and flexible panels are generally amorphous, whereas crystalline panels tend to be aluminium framed and glass fronted.
MOUNTING: Crystalline framed panels have aluminium frames with mounting holes in, so you can screw them onto something. They need to be raised up by around 10mm to allow the air to circulate underneath. Flexible panels generally have holes in the corners to use for mounting. Some can be glued down, sewed on, velcroed on etc. The output from a solar panel will be greatest when it faces into direct sunlight. In the UK that means it should be mounted facing south at an angle of around 45 degrees. Perhaps more important is to make sure that there is no shading over the panel.
BYPASS DIODES: If part of a solar panel is in shadow then output from the whole panel goes down, unless there are bypass diodes between the cells to isolate them.
BLOCKING DIODES: These prevent power from going back into the panel from the battery at night. The larger framed panels tend not to have blocking diodes fitted, whereas the smaller portable panels generally do have them fitted. If you use a charge controller you don’t need a blocking diode, because the charge controller performs that function (amongst other things).
CABLING: Some panels have built-in cables coming out of the box on the back (the junction box). In this case you need to be aware of the connectors on the end – can you connect to them? Where there are no cables, you need to wire the cable into the junction box, usually just a case of striping it back a little and screwing it in.
VOLTAGES: Solar panels for 12V battery charging are usually rated at 15-17V. Those for 24V battery charging are rated at around 34V.