I guess that the question is quite self-explanatory. I am hoping to invest in a solar panel soon, but I am a little worried about it getting inefficient once government subsidies run out and the generated energy is not obligatorily paid-off by governments anymore. Any other suggestions to cope with the peaks? Heating water in a tank for instance?
I think this is best answered as two questions.
How efficient is it to use a battery as storage when excess electricity is generated by solar panels?
What are the advantages and disadvantages of using an electric car's battery for this purpose?
I'll take a stab at both, but there are probably others around who can answer each one better.
Question 1: whether it's worth using a battery
Good modern batteries have a round-trip efficiency of 85-90%. Average grid losses in the UK are in the region of 7%, although it is not clear that this figure is applicable for distributed generation - possibly it should be lower. From a pure efficient-use-of-electricity perspective, then, it will nearly always be better to export energy to the grid to be used than to store it locally.
However, you have asked about a future where feedin tarrifs and the like no longer apply, so presumably you are interested in what makes economic rather than purely energetic sense. In this case yes, it makes sense to store excess energy locally and use it later. Whether it is best to do this with a battery or to use the energy to heat water depends a lot on your circumstances, and in particular on your need for heat. A well-insulated water tank probably has a higher round-trip efficiency, at least over moderate timescales, and large capacities will certainly be more affordable than with batteries, but you lose flexibility in what the energy can be used for.
Question 2: Whether an electric car is a good way to provide this battery
This is a "solution" beloved of many, simply because if there is a large takeup of electric cars there will be high-capacity batteries plugged into many homes for a large proportion of the time. Advantages:
Assuming that you need a new car, and that an electric one will suit your needs, then the marginal cost of an electric car over a ICE car is probably less than the cost of buying the equivalent batteries and associated electronics separately.
No installation is required.
The car is not always connected. With solar power in particular, as others have pointed out in comments to the question, for somebody with a daily commute to work the car will not normally be connected when solar power output is greatest.
Unless you know in advance that you will not need to drive your car for x hours, you can only allow the top n% of the battery capacity to be used for solar power smoothing. This puts a damper on the first advantage above, in that the marginal cost of an electric over an ICE car is probably greater than the cost of the relatively small battery capacity that you will actually be using with the solar system.
Linked to the last point, unless you can plan your journeys far ahead, in order to make the system useful for solar smoothing you must accept a reduction in your car's range - it may not be fully charged when you want to use it.
In a practical sense, I don't know whether current electric cars support smart use of the batteries in this way in software.
My conclusion from this is that it's probably better to install dedicated batteries for the solar system than to rely on using a car. However, if you already have an electric car for other reasons, and you don't need it to be at 100% charge all the time, then it may be worth investigating use it with your solar system.
 Source: http://www.raytheon.com/newsroom/technology_today/2011_i1/energy_storage.html, section on lithium ion batteries. They might not be the type used in a house, but they are the type used by electric cars, so they seem relevant.
 There are a number of scenarios in which storage may help with efficiency, e.g. to deal with choke points in the grid layout, or where storage is more efficient than running a thermal power plant at a low capacity factor, but these are all best managed by the utilities and are not really applicable to a home generator.