How much artificial light?

Hi folks, first post

I'm currently starting to build my first outdoor growing space, a polytunnel. I have just put in an access path and started to dig some footings, most frame materials are sourced and a sheet of plastic is in the post somewhere. I have buried a mains water pipe and a duct to put some power in if needed. I live on the Isle of Lewis at about 58 degrees north but due to the magic of the sea currents it doesn't really get cold, just a bit wet and windy with some sea salt on that wind.
Having been inspired by the Winter Harvest Handbook I am aiming to grow and sell small scale veg year round eventually.
I was thinking some LEDs would extend the growing day during shoulder seasons but I was wondering if there is some sort of guide as to how much light might be needed?
A small amount of LEDs won't use much power obviously and I am happy to basically use an extension lead in the duct, plugged in indoors via RCD as and when needed. A lot more power would sensibly need professional input now.
What's available seems to either be a bit of extra light for a houseplant to small scale intensive small space greenery that I'm not into.
Is it a suck it and see or any suggestions of what could sensibly be part of the build straight off? Waterproof leds the preference.

Thank you

Jason

Hi John, welcome!  I'm hoping one of our far northern (or southern) growers chimes in here (like Skandi).  How cloudy are you from mid Oct to mid Feb?  My hunch is that you're moderately cloudy and that your plants could use some extra light.

I'm far to the south of you (45 degrees).  My only data point for you is that I have a greenhouse that is passive solar (no ambient overcast light from the north due to an insulated wall).  I planted cabbage and cauliflower last fall and they overwintered - growing quite slowly.  About a month and a half ago they started growing faster and we harvested last week.  I'm 70% overcast during the winter months and I didn't give them extra light.

Hey I'm 57North so similar light levels, from my memories of house sitting on Lewis.. it's cloudy! Since in December/January you're looking at 5 hours daylight tops you will need full lights to "grow" anything. and you will need to have then on for a minimum of 5 hours a day. unless you have basically free electric I don't think that is going to work out cost wise for sales. You can keep things alive in your greenhouse all winter no problem without lights but you won't get any growth, just think of your lawn, we cut ours in early October and just cut it again last week. 6 months with less than 4cm growth despite the temperatures this winter being generally around 6C.

Commercial LED's for plant growing are horribly expensive and use huge amounts of power. you can easily start seedlings or do micro-greens under cheaper set ups either using led's or just using florescent tubes. but as soon as the plants get much over 10cm tall these systems do not provide enough light to push it past the top of the plants.

Thank you for the replies.
So not much use adding a bit of lighting for the plant's benefit unless I go big? So I'll go home

When using LED grow lights with poor outside lighting, I have been successful in tenting a space blanket over the bed. The reflective material helps to enhance the effect of the lights.