solar cell area to gain 12V 40amps?
I do not know what "blue cell which looks like cracked glass" you are talking about, but I have seen cells on sale rated up to 173 W/m2 nominal power - these where best quality, selected cells. The ratings are however usually given as open circuit voltage times short circuit current, and even with the best cells, the most power you are ever going to get even with the cell facing directly the sun, is 70% of that, which is about 120 W/m2. But that nominal power you are only going to get at full sunlight with the cell facing exactly in the direction of the sun. Yesterday I was making tests with some small cell around 14:00, and when the cell was in horizontal position, I was able to get about 67% of the maximum power it had facing the sun. However when the cell was in shade, the output was only about 6% of the maximum power.

If you are interested in getting 12 V &amp; 40 A only at clear sky, if you buy the best quality cell of rated nominal power 173 W/m2 (real maximum power 120 W/m2), the rough calculation would be:

$$12 \mbox{ V} \times 40 \mbox{ A} = 480 \mbox{ W}$$

$$\frac{480 \mbox{ W}}{120 \mbox{ W}/\mbox{ m}^2 \times 67\%} \approx 6 \mbox{ m}^2$$

That might fit on a van if it is a moderately large one, but if you want it to provide that power also when there are clouds, or any shade, you will need a panel at least 10 times as large. If you don't buy the highest quality selected cells, but go for most common panels, they have only 114 W/m2 nominal rated power, the required area will be 50% more. 

If you would want to supply that 480 W round the clock, you will need batteries, account for the loss of power while charging (10% - 50% depending on what batteries and charge controllers you use), and take into account that sun only shines a small part of the day. 

I read that from a panel in horizontal position, in London, UK, in December, you get on average 0.434 Wh of energy per each nominal W of panel power per day. In July the value is about 10 times as much Wh per day. 

That means to provide 480 W continuously even in December, you would need a panel of nominal power 26544 W, which would be at least 154 m2 in size. And that's not taking into account loss on charging the battery.