Rule of thumb for panel sizing

[prodata]

OK, this is a question about sizing a solar PSU designed to supply 12v only and to provide guaranteed (@ eg a 98% level) power YEAR-ROUND for a location at 55N (UK, so winter light levels can be low for some days at a time and it's the winter conditions that will be important for sizing). This is for an instrumental PSU and so the power required is actually tiny - let's say 0.2A draw at 12v DC.

What I'm after is an informed guess at a factor to multiply the current draw required so as to size the panel (which will be fixed and south-facing and at whatever angle gives the optimum winter power). I'm thinking that a starting point would be to size a 12v lead-acid battery in Ah at about the same as the panel wattage (no formulaic reason for doing so, it just feels about right).

I'm also thinking that a factor of about 50 might be about right (ie 100W panel, 100Ah battery), but would appreciate comments.

[Sunking]

This has been discussed thousands of time and why there is a Stickie that answers all your questions.

[Mike90250]

Without direct sun, most PV panels just don't produce much power.

200mA @ 12V is 2.4w x 24 hrs = 60 watts daily. (always round up)

5 days no sun = 300 watts consumed.

An "average" 50ah AGM battery (not gel) holds 600wh of power, half of which is useable. So far, so good, low loads, low loss AGM battery. After 7 days, you will be really hurting the battery, but the sun comes out for at least 3 hours, 1 x a week.
Sooooo in that three hours you have to harvest enough power to recharge and overcome any losses.
Panels produce 80% of their nameplate power. You have to replace 420wh in 3 h which would need about 170w of panels.

[ZoNiE]

That is probably one of the best summaries I have come across.

Sucks to be in Seattle, I suppose.

This has been discussed thousands of time and why there is a Stickie that answers all your questions.

[prodata]

Many thanks for the replies - need to digest and then maybe comment further.

[PNjunction]

Sucks to be in Seattle, I suppose.

It can be done in areas with low or intermittent solar insolation. All it means is that you'll be paying more for relatively larger solar arrays, and higher capability batteries than flooded, like certain low internal resistance agm's which can handle being hammered by a large solar array when there IS sun. Lifepo4 is also an alternative for advanced setups which don't care about deficit-charge or psoc styles of operations - but mistakes here will be costly for sure.