The terms you used aren’t adequate. The illumination of an area is represented in lux, and the source lumens necessary depends with how many lux you want on square meter.

As an example, in a quite-well lighted room you may have a 100W electric old-style incandescent bulb and this is spreading its 1400 lumens over almost 100 square meters of floor, walls and ceiling; that makes 1400lm/100sqm = 14 lux.

Because lux=lm/sqm.

At 14 lux, you see in your room, it’s a lot of light. Outside in the dark, maybe even a hundreth of that is enough. Consider now a 12V/2W car lamp (~15 lumens, typically the filament of these is less hot) in the same room. Its not too much light now, but you clearly see everything around. And we have now only 0.15 lux.

Let’s see what we can do with a torchlight… now.

A 300 lumens 3W Cree LED will illuminate through a torch collimator an area of…. how much? 10 meter diameter at 40–50 meters distance? Let’s say yes. And yes, it does not matter the distance, but the total area on which light is spread.

Okay, that 10m diameter lightspot (~80 sq meters) will receive the 300 lumens (minus some loss through inefficiencies) ; that means the illumination will be 300/80 = 3.75 lux

Yes, the light on the spot (3.75lux) will be slightly less than in the room (14 lux) in the first case (100W lightbulb), but far more than in the second case (0.14 lux).

Now, the point is… as i said: it does not matter the distance, but the total area on which light is spread.

You can calculate now for yourself, using the given examples and methodology.

As an even simpler way:

Take a known lumen lamp (for example a table lamp, of 600 or 1000 or 1400 lumens). Go with it outside in the designated area and see how much surface is covered by its light, when the light is enough for you. And after that, just multiply the number of lamp lumens with the estimated number of lamps to cover the entire area.

How much do LED floodlights cost?

Slimline floodlights at Razorlux cost between \$100 and \$1200 depending on their wattage. You can get a portable floodlight for between \$120 and \$1280, while a solar flood will cost approximately £11.

Please remember, when comparing prices against halogen floods, that LED lights last 20 times longer and also dramatically reduce energy cost, so you make back your investment quickly.