lighthouse wrote:

still would guess that 4 watts would barely cover the alarm draw.

I've been through the solar panel option for keeping a trackday car battery topped up, I tested a small panel and a larger briefcase style regulated 13W panel.
Both are now sat in my garage as for my application neither could output a meaningful voltage for charging let alone sufficient current.

The quoted wattage ratings are in perfect lab conditions and in reality you are never going to get that. Instantly you need to slash the quoted ratings by about a third to get the maximum you are likely to see (hence why I bought a 13W panel). That doesn't take into account the losses from putting under glass, indirect sunlight, clouds and useful hours of sunlight.

Unfortunately most of these panels are just a placebo effect, they light a 5V LED on the panel so you think it's doing something but in reality don't even output over 12V so are not going to charge a battery.

They do have their uses but you have to make sure you get a decent panel and realise their limitations.

I came across this video the other day and he explains the issues and what I also found far better than I can (although in a larger application but the principles still apply):

After 30 days parked up and the solar charger plugged in, she fired up first turn
Whether this was down to the charger or not, who knows
So it's a from me

I have my car parked in a garage in central London with no mains. I've recently bought one of these:

https://www.amazon.co.uk/Photonic-Universe...harger+10w

Normally my TVR's battery is practically flat in 2 weeks (alarm draw, clock), however I did have plenty of cranking power after parking it for a month with the panel attached, so pretty happy with my purchase. Even if it just delays the decharging of the battery, it's already worth the odd 40 pounds!