Do I need a solar panel?
For those campers who want to ‘get away from it all’ (including an electric hook-up point), adding a solar panel to your vehicle will potentially allow you to use many of life’s electrical necessities whilst away from a mains electrical supply. Equally as important, a solar panel will ensure that your leisure battery is fully charged and ready to supply power to your GPS tracker unit, alarm and power-hungry motor-mover.
Choosing the right panel
Choosing the right panel can be a challenge. 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, but overestimate and you might end up spending more than you need to.
There are three main parts of any solar powered system.
- Power generation (the solar panel)
- Power storage (the leisure 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.
The leisure battery (however large or small) is a storage container for the solar power gathered in daylight for use immediately, or for use later on. To maintain the power storage at a constant level the solar panel needs to put into the battery the same amount of power as is being taken out of the battery now (or will go out of the battery later on). At this point, you don’t need to consider the size of the leisure battery you are charging (you just need to know that you will need one).
What size panel do I require?
As a rule, with the ever-increasing reliance on battery-powered gadgets that require almost constant charging, you’ll need a panel with a minimum output of 60W (to put that into perspective, for laptop charging you need at least 25W to provide a useful trickle charge).
The best way to work out what size panel you require is to ‘size’ your system.
How do I work out my power usage to ‘size’ my system?
In sizing a system, the aim is to balance the power produced by the solar panel (and stored by the battery), with the power being drawn from the battery over a period of days or weeks (depending on how it is being used).
A 10 Watt panel will give 10 Watts for each hour under the standard test conditions (1000W/m2 at 25oC. This is approximately the W/m2 of the Sun over the Earth’s surface). But the amount of sunlight varies according to the geographical location and the time of year (i.e. the season). A typical UK summer’s day is the equivalent of 4 hours’ sunshine in the UK. A 10W panel will, therefore, give 40W in that day.
On a typical winter’s day, however, you’ll get the equivalent of just 1 hour’s sunshine. The same panel will therefore only provide 10W in that day. These are conservative figures to cater for the worst-case scenario.
Some (simple?!) steps for sizing a 12V system:
- Firstly, list all of the 12V electrical appliances you use in your leisure vehicle and find their power rating (Wattage). This information can be found on the equipment’s information plate or in the product handbook. If you can only find the current rating in Amps, simply multiply this by 12 to convert it to Watts. Lighting power rating is defined by the wattage of the lamp or bulb.
- Next, calculate your daily total hourly requirement by first estimating how many hours you would use each appliance for over a typical week, then divide by 7 for a daily rate.
- Next, multiply each appliance (or light/lamp’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. To do this simply divide the daily total Watt-hours you require (calculated at step 3 above) by the hours of usable light you expect in an average day. This will give you your minimum panel wattage. As a guide in the UK, allow 1 hour of light in winter, rising to 4 hours’ light by mid-summer.
- Then calculate your battery size. To do this multiply your daily Watt-hour requirement (see step 3) by 7 to create a weekly requirement, and then divide this by 12 to convert back to Amp Hours, which batteries are rated in. Multiply this result by a factor of two to give the correct battery size.
- Finally, calculate the correct current rating of 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.
We did say it could be a bit of a challenge!
An Example System
In our example, in one week we wish to run a TV for 4 hours and a light (lamp) for 5 hours. Using the steps listed above….
- The TV has a power rating of 65W and the light (lamp) 8W
- Daily hours requirement – TV = 4 hours / 7 days = 0.6 hours per day. Daily hours requirement – Light = 5 hours / 7 days = 0.7 hours per day
- Daily total Watt Hours (Wh) – TV = 65W x 0.6 hours = 39Wh per day. Daily total Watt Hours (Wh) – Light = 8W x 0.7 hours = 6 Wh per day. Therefore, the total daily Wh required is 39 + 6 = 45Wh
- Panel Size = Daily total Wh / number of hours of sunlight. Calculating for the worst-case scenario (i.e. 1 hour of sunlight falling on the panel) = 45Wh / 1h = 45W
- Battery Size (in Amp hours where Amp hours (Ah) = Wh x Factor of 2 / Volts (12V) = 45 Wh x 7 days x Factor 2 / 12V = 630Wh / 12V = 53Ah
- Minimum Charge Controller rating (Amps) = Rated power of chosen panel (step 4 above) / 16.5V (output of panel) = 45W / 16.5V = 3A
Do I need a solar regulator / controller?
Larger solar panels can easily over-charge your leisure battery unless a controller or regulator is added to restrict the current to safe levels. As a general rule-of-thumb, divide the Ah rating of your leisure battery by 10. If the result is MORE than the rating of your panel then you wouldn’t normally require a regulator or controller. In reality, with leisure batteries of 80Ah or more, ANY solar panel fitted (and certainly the 45W panel in our example above) will require a regulator or controller.
Where can I get help in choosing a solar panel that meets my needs?
Fortunately, we at KCE are here to take the pain out of sizing your system and selecting the right solar panel and controller for your needs.
Here’s a link to download the Camping and Caravaning Club’s excellent datasheet on Solar Power (pdf do.cument download).