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How To: Choose A Solar Panel
Help! How do I choose a solar panel? ... is one of the most popular questions we are asked.
As with most things, there are different points to consider when choosing a panel:
Q6 - How do you size a system? (and calculate the actual power needs)
What we'll do here is go through the different points in broad terms (and then cover the actual power calculation in Q6).
The wattage of the solar panel you need is perhaps the most important thing to get right.
Why? Well, if you underestimate the amount of power you need you could be very disappointed with the results. (Like expecting a tiny car to pull a huge caravan).
Overestimate and you might end up spending more than you need to. (It might be fun but most people don't buy an expensive Ferrari for a quick trip to the shops).
There are three main stages to a solar powered system.
- Power generation (the solar panel)
- Power storage (the battery)
- Power use (your items you want to run off solar power)
To work out the wattage correctly, the panel needs to be sized according to how much power you are going to use. (Later on you're going to need a bit of info on each electrical item you need to be solar powered).
The battery, no matter how large or small, is a storage container for solar power gathered in daylight for use immediately or for later on. Highly important. For example without a battery, a solar powered torch would be rather pointless in the dark!
Just remember that to maintain the power storage at a constant level the solar panel needs to put in to the battery the same amount of power as is going out of the battery now (or will go out of the battery later on).
However, at this point you don't yet need to consider the size of battery you are charging (unless you are sizing for an uninterruptible power supply (UPS) system). You just need to know that you will need one.
Some quick ideas of power ratings needed:
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. (There tend to more gadgets needing power in a motorhome than in a caravan).
For laptop charging you need at least 25W to provide a useful trickle charge.
See Q6 - How do you size a system for more details.
One of the reasons we like to supply a range of different manufacturers panels is to provide lots of different sizes. (By that we mean width, length, thickness AND power rating).
You can also add panels together to get the overall size you need. So if a 120W panel is too large in area, but you have room for two 60W panels side by side, then this would work too.
Remember that for the equivalent wattage a crystalline panel will be smaller than an amorphous panel. (If you haven't come across these terms before, you may want to scan through our Glossary).
For large orders we can have even have panels made to your specifications.
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 on items such as:
- the quality of the junction box and frame
- the packaging and instructions
- cables and other accessories that might be included
(It also depends on how good a deal we can get from the manufacturers!)
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 above the surface they are fixed to 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.
Generally speaking if weight and portability is your main concern (perhaps you're a touring cyclist) 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 solar 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 (because it is part of your boat deck), or need it to fit on a curved area, you might consider either a rollable Powerfilm panel or a Solara semi-flexible kit.
You may also want a rollable panel because you want to temporarily fix it in place with bungee cords (for example) on a boat or over a tent. Again the rollable or flexable panels are very handy. You can browse our flexible panels by their power rating or in solar kits for particular usage.
In sizing a system, the aim is to balance the power going into 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) for each hour under standard test conditions (1000W/m sq and 25oC).
A quick sunshine hours guide for the UK:
A summer's day will give you the equivalent of 4 hours sunshine in the UK. A 10W panel will give 40W in that day.
On a winter's day, you'll get the equivalent of 1 solid hour of sunshine and so a 10W panel will give 10W in that day.
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:
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.
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.
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.
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.
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.
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