The whole idea of a hybrid car is the ability to store excess engine power at some times and deliver it when needed, so an engine can run efficiently all the time, generating excess power when cruising at slow to moderate speeds, and delivering that power for short bursts of acceleration.

Regenerative power recovery during braking also helps.

In this way, you can cut the conventional power requirement of the engine from say 100 hp to only 50 hp, since you never need that top-end power from the engine.

The weak point is that the batteries that do this work are VERY heavy. Here's an example of a standard off-the-shelf heavy duty battery. Sample 12 volt battery. (I just pulled this one at random off the web). It is a 12 volt battery with 125 ampere-hour capacity. It weighs 99lb (45 kg).

Now, what can we do with this battery? In simple energy terms it represents 12x125 watt-hours i.e. 1500 watt-hours or, with 100% efficiency about 2 horsepower-hours. If we wanted to use this battery alone as the buffer for a hybrid electric car, and demand 50 hp top-up from it, it would last for 2/50 hours (a touch over 2 minutes).

So if we went driving in this car and were in a hurry, looking for zippy performance, it's good for about 4 traffic lights, and then it's used up and our car is back to a mere 50 hp and performs like a dog.

By modern standards with gasoline powered cars, this is unacceptable, and you'd want about ten times this battery capacity to give reliable power availability.

There are some further problems that going for high currents and deep discharge severely limits battery life. You know what a bind it is, and how expensive, when your standard auto battery dies after just a year or two. Imagine having to replace TEN of them every 3 months!

And remember, these 10 batteries weigh 450 Kg (990 lb), and the racks that mount them and make them secure in a crash (don't want half a tonne of acid-filled bombs thrown around, do we) will add quite a bit to vehicle mass. So for a car of equivalent performance to gasoline, we need to shed over half a tonne from the existing car mass, or build-in more power to compensate.

The batteries also take up space - more than is saved by the smaller engine. The saving in engine weight is cancelled by the extra weight of wheel-hub motors and heavy duty generators.

Remember that car manufacturers have been working intensely for 100 years to reduce unnecesary vehicle mass and increase available space. There isn't this sort of mass margin and space available within a conventional vehicle envelope.

So to make a hybrid car "as good as" an existing gasoline car is simply impossible. It's a bit like building a spaceship. Every Kg you add to the useable bit at the top, adds mass to the support structure and the engines to drive it, and makes the whole thing bigger and more expensive.

Hybrid cars will need to be slower or bigger, or lighter and less capable than their conventional alternatives. You cannot beat a gasoline engine. Various bits of technology might get you to shave up to 10% of the fuel consumption with a very small hybrid component, but it will only be efficient in big cars that are expensive and inefficient anyway. Much more savings would come from going to small conventional cars with e.g. 70-90 mpg diesel engines.

At the end of the day, gasoline is a wonderfully efficient way to run vehicle transport. Hydrogen technology is unnecesarily expensive and dangerous. When we run out of conventional gasoline, what the world will do is to manufacture synthetic gasoline and use it in the existing infrastructure, rather than swith to a more expensive and dangerous alternative that needs wholly new infrastructure.