A direct-drive LED flashlight powered by Alkaline D cells can have a run-time measured in weeks, not hours. It's not bright, but on a dark night it's better than a dead regulated light.

For many applications a 1 watt LED in a 2+ D cell light is more than adequate. If you have an old Maglite® Flashlight which takes between 2 & 6 D cell batteries you can do a lot worse than upgrading with a 1 watt Terralux Ministar LED. For $12.95 you turn a crummy light into a pretty darn good LED flashlight with a lot of throw. The 2 D maglite in my truck has one. Not as bright as the 3 watt in my 3 D Mag, but the runtime should be longer.

Edit: According to the review it doesn't need to be a Maglight. Lots of old D cell lights out there. Dunno, I've only upgraded my Maglites.

besides...buying a pack of D's makes me feel like I'm preparing for hurricane season..

On the electrical thing....if you're going to run your house on a generator, you really should have a transfer switch installed. It breaks the connection to the utility and connects the generator to the home's panel. It also has its own individual circuit breakers for safety. Most places require a transfer switch as part of the electrical code if you are going to run home circuits off a generator.

There are automatic transfer switches, but for the average home owner a less expensive manual one will work just fine. It makes connecting a generator much safer and easier for everyone. You run one 240v extension cord from the generator the transfer switch, push a button, and your house is safely powered.

Either directly connect your devices to the generator using extension cords (or surge supressors) or use a transfer switch. Never, ever backfeed a generator into house wiring without one.

fluorescent tube lanterns,and the big ole Mag-lites use D cells

D size alkaline cells cost about twice as much as AA cells, but contain about 6 or 7 times the energy.
The cost per watt hour from D cells is therefore about one third of that from AA cells.

No alkaline cell is very efficient at high discharge currents, but for a given current D will be more efficient than AA.

As an example, a 0.7A flashlight bulb will only run for about an hour and a half on AA cells, but will run for 12 hours or more on D cells.

The extra weight and bulk of D cells is of little consequence for home use.

I use alkaline D cells in flourescent and LED lanterns, in Maglights, and in fans.

I also have home made lights that use 6 D cells in a plastic holder that is connected via a few yards of cord to a miniature screw base bulbholder with a screw base LED torch bulb.

The very light weight of the bulb and holder means that it is easily hung from any improvised support and lights a whole room.
If the batteries leak, then only a cheap battery holder needs replacement rather than a costly lantern.
The light is turned on/off by a switch near the battery holder, much more convienient than a lantern that might be out of easy reach.
The run time is over 100 hours from 6 alkaline D cells.

The length of time you can run a flashlight or other electronics on a given battery depends heavily on the load. For example, with flashlights, today's LED flashlights often have multiple levels and even the lower levels provide adequate ambient light when you don't need a lot of task lighting. My preference is always to use minimal lighting as much as possible, so using the lower levels on a flashlight is normal for me.

At a reasonable 250mA current draw, that single D cell will last 16 hours. But the AA will last 9 hours, so the D cell provides less than twice the runtime even though it contains 6 or 7 times the total energy as the AA cell. Now which one looks more efficient?

Louisiana Father-Son Team Rescues 120 From Flooding

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Duracell appears to rate their Procell D (PC1300) at around 60 hour at 250 mAh at 70F, though its hard to figure out their non-specs.

Duracell Data sheets

The real advantage to a D comes at much lower discharge rates, 10 mAh or less. A 3D direct-drive LED will be pretty dim at 5 mAh, but it will be a long, long time between "dim" and "dead".