A fluorescent light of some type is better in my opinion, because Mother Nature doesn't let the sun shine every day....However, if you decide to use the available sunlight to propagate your plants with, I hope one of those windows is facing South.

What are you sprouting? What direction is the exposure? How many hours of direct sunlight?

Sorry but I agree with rhizo, you most definitely need supplemental light. 1 good cloudy day like in your photo and its leggy plants within hours. Windows reduce sun exposure even more when it is already low hours and low intensity this time of year.

Dave

Let's be very careful here. Sunlight is stronger than any artificial lights you'd be using. As a result, if you let direct sunlight shine on that covered seedling house, the temperature inside will rise dramatically. Probably will kill your plants. Before the seedlings germinate, you don't need any light at all.

That being the case, it comes down to how many hours of direct sunlight they'll get once the cover is removed. Since sunlight is a lot stronger than supplemental lighting, you'll need 4-5 hours of sunlight every day in order to avoid needing supplemental lighting. Seriously. Supplemental lighting, with plants right up against fluorescent bulbs, is about as bright as a cloudy day. That's OK, but as a result, if you use supplemental lighting, you'll leave it on for 18 hours a day. As noted, since you're unlikely to get 4-5 hours of sunlight every day, having supplemental lighting would be smart.

Ok thanks. I wasn't aware that they shouldn't be in sunlight while germinating. I do keep it pretty cool in there though by misting the inside of the container a couple times a day.

Also, I'm growing tomatoes, peppers, and a few herbs in the tray.

Any recommendations for a good grow light?

"Any recommendations for a good grow light?"

Just any fluorescent light (T12, T8, T5) that you can afford, and use both cool white and daylight bulbs to get all colors needed from the spectrum.

"Ok thanks. I wasn't aware that they shouldn't be in sunlight while germinating."

It's not really that they shouldn't be in sunlight. The problem often is that the sunlight isn't as direct as it needs to be.

Different plants need different amount of light. If you think in terms of lumens, you have a measure of how much light is hitting the plants. In the northern hemisphere during non-summer months, the sun is lower in the sky. It is therefore filtered by the amount of atmosphere through which it must travel. By the time it hits your plants, it might still feel warm, but only near it's apex in the sky will it be bright enough to really supply what your plants require. Most herbs would probably be just fine, but tomatoes and peppers are genetically accustomed to the very direct light of equatorial regions.

What happens is that the plants stretch to find more light. Your plants will be hindered if this occurs. It's not the end of the world, but supplemental lighting will help alleviate the problem.

Misting can be both beneficial, and detrimental. You want the surface of the medium to stay moist BEFORE germination, but once the seeds germinate, too much moisture can cause fungi to kill your seedlings. So be prepared to allow the surface to dry, while keeping the medium generally moist. Many resolve this contradiction by watering from the bottom. Just keep some water in the tray, and allow the medium to 'wick' up what it needs.

As was mentioned, a 'good grow light' is whatever supplies the seedlings what they need. Most people opt for the confluence of price, efficiency, and practicality.

I'm sorry to be so technical with parts of this reply, but there was some confusion creeping in, and hopefully this made some sense. If you need some translating, lol, please ask.

"Ok thanks. I wasn't aware that they shouldn't be in sunlight while germinating."

Rgreen48's comments are good advice after germination. But, before germination sunlight is irrelevant. It doesn't hurt, but it's not needed. What you need is warm soil.

What are you growing and how many plants? In the US lights seem to be pretty much a sine qua non. However, I have raised veggies from seed for many years without any extra lights in a glazed porch. So I will say you do not have to have lights if you are only doing a few plants and you have big windows. Grow lights are rarely used by UK domestic growers and we seem to manage to grow things. BTW the dome comes off as soon as you have germination so the heat build up shouldn't be a problem.

I have been eye balling this one, it's 26"x 22.5"(inches) that's just long enough and wide enough to get two full trays under. Agrobrite FLT28 T5 Fluorescent Grow Light System, 2 Foot, 8 Tube https://www.amazon.com/dp/B0031WU2ZM/ref=cm_sw_r_cp_apa_AXo2ybM9KF045

Misting inside a covered container a couple of times a days doesn't keep anyone cool. It just keeps the humidity high. Sunlight strongly illuminating a container like that will just make hot steam. The only way misting keeps anyone cool is if there is the opportunity for the water to evaporate. When the humidity is 100%, there won't be any evaporation.

The issue is not that seedlings pre-germination don't need to be in sunlight. They just don't care if there is any light at all. Shining light on buried seeds has absolutely no effect.

I agree with floral_uk that supplemental lighting is not absolutely essential. As I said, supplemental lighting is only as bright as a cloudy day outside, so keeping the plants outside and open to the bright sky at all angles would give them enough light. But sitting in a window allows only about half the sky to shine on them at best, and perhaps mostly a cloudy sky. The main advantage of supplemental lighting is that the "day" can be a lot longer. At higher latitudes, maybe even two times longer.

floral, out of curiosity... what directional exposure does your porch provide your seedlings? Is it unobstructed? I ask because when I look at the image of the OP, I see a corner window with a large corner support beam and no directional indication. If that set of windows is west and north, I don't think things will work out very well. However, if they were to position the tray up against an unobstructed south window, with no complete shadowing, I agree with you that just the sun would work fine. The plants may stretch a little early in the season, but they would be okay.

It faces S but I live in a tall row house so the sun doesn't come over the neighbours' houses until about 2.30 at this time of year. Certainly if I was the OP I'd move the tray to an unobstructed part of the window sill, not that spot behind the window frame.

Just by coincidence I bought a packet of basil seeds yesterday. On the pack it says 'ideal for windowsill sowing.'

Even a south facing window is never as sunny as you think it is, because it doesn't have the ambient lighting from the sky in all directions like you would outside.

I am aware of that hairmetal - its obvious. I am just saying that based on decades of experience you can start seeds on a sunny windowsill.

Hungarian Wax peppers grown indoors with no supplemental lighting.

Fava beans in the same spot in January.

That's an important point that is often neglected. The amount of illumination isn't just a matter of hours of direct sunlight, but light coming from the rest of the sky. Certainly on cloudy days, a south facing window will provide less than half the light that a plant outside could get. But even on clear days, there is substantial illumination (mainly blue light!) that comes from the rest of the sky. So a south facing window is pretty much as sunny as you think it is but still doesn't provide anywhere near the illumination that an onobscured plant outside could get.

All that being said, seedlings will do OK in a south facing window. Not ideal, but OK.

I was told by a friend that the windows in my house actually have a UV blocker coating? Even though the spot I start my seeds has two windows - one East and one large 6ft South facing, there's not enough light. I bought an adjustable 4ft LED strip and it's working great so far!
The LED has Red, Blue and White - full spectrum. And the light doesn't put off hardly any heat, so the plants can literally sit 1cm under the bulb.

Some people hate LED, but they are cheaper to run, last longer and don't get hot. That makes me feel better about running them all day with my dog at home alone.

My pepper plants are 3 weeks from seed and already have 4 true leaves. I'm happy with that.

I little bit of aluminum foil and some chopsticks to build a tent, my setup is kinda funky but functional!

I just posted another thread regarding cold frames... I might build a few this weekend, haven't decided if it's worth it yet though. Just another thought!

All glass filters some UV rays. There are UVA, and UVB rays. Most glass removes the lion's share of UVB, and at least some of the UVA. There are differences in opinion about the effects of UV light on plants, but generally, plants need the visual spectrum to thrive. Glass does filter the important spectrum of light from plants, but it varies greatly depending on the glass.

Agreed that there are differences of opinion about the value of UV to plants - clearly UV can cause molecular damage, but plants have evolved mechanisms to undo such damage. The amount of solar UV that reaches the earth is clearly far larger at altitude, but it would be hard to say that crops at low altitude are at a disadvantage because of that.