A tricky question!
It somewhat depends on your battery size although running anything that heats from Solar is going to 'eat' your power.

A typical solar system has between 5 and 10KwH of storage. This means that (in theory) you can run a 1Kw heater for between 5 and 10 hours and that would drain your batteries. In reality you wouldn't want to do that as you would quickly kill your batteries.

The most common forms of heating here are a wood burner, oil fired central heating and/or portable gas air heaters. Each has its plus and minus.

Hope this helps rather than adds to the confusion!

The modern (inverter) paraffin heaters are great.  Very efficient.  However, they need a power supply to operate as the have an integral fan and are electronically controlled.  The other down side is that they are expensive to run, especially in Spain where the fuel is twice the price as in France (both Leroy Merlin for comparison).  A typical 5 litre refill will last about 12 hours and a 20 litre bottle of PTX 2000 will cost you around 40 euros here.  If you travel via France regularly it's well worth buying some on the way down.
It's difficult to give specifics without knowing the size and layout of your property.  If you could describe the size of the rooms you need to heat it would be easier to advise on log stoves etc.  The butane heaters are a good all round solution.  They don't need power, are relatively cheap to buy and run, are portable (as are the paraffin) and last a good few years before they need replacing.  The main down side is lugging the heavy gas bottles around.

About 40sqm then.  One butane heater would probably be adequate except during really cold spells when a second one might be a good idea.
Alternatively, a good log burner with a butane heater as backup would do the job.  The butanes are about 100 euros to buy but a decent quality log burner is going to be in the order of 1,000 euros.
If you have access to your own logs on site then it's an economic medium/long term solution.  If not, logs were in the region of 135 - 150 euros per ton last time I looked and you'd probably go through about 1.5 tons in the average winter.
Gas bottles are around 15 euros and last about three/four days (day and evening).

Hi Paula,
I would recommend a woodburner. For us, the downside of both gas and parrafin is that they put out a lot of water vapour and we think that they smell and use up the oxygen in the room.
It's easy enough to start with a simple wood stove for little money, especially second hand. We have a pretty big stove that also heats our radiators and towel rails so the whole house is warm. They use surprisingly little wood and if you have trees to prune then is free! PM me if you would like more info, we are in Perello.

Pete,
The inverter paraffin heaters I mention don't put out water vapour and only give off a slight wiff for a minute when lighting and extinguishing.  Any heater (gas, paraffin or log) will use oxygen from the room.  It's needed for combustion so some form of ventilation is required.
Log fires with back boilers can be used for a form of central heating but the general opinion is that "cheap" log burners (with or without back boiler) are a waste of money.

can someone let me know about the basics of the generator/battery relationship??

I think the decision really is instant heat from gas or paraffin depending on how much water vapour you want & also the cost of fuel (the gas seems to be more??)

I have been hoping that I can use the generator a bit?

All figures approximate.
Butane heater:-
Cost to buy 100 Euros.
Gas bottle (exchange/refill) 15 Euros.
60 hours at 3Kw = 25c/hour
Paraffin heater:-
Cost to buy 200 Euros (plus).
20 litres paraffin 40 Euros.
48 hours at 3Kw = 80c/hour
Gas some water vapour, paraffin none.
I've no idea what the fuel costs would be of running a generator for heating but all forms of electrical heating are considered to be inefficient.
As for keeping the house warm, because of the construction of houses here (no cavity walls or insulation), any heat gathered during the day/evening will be lost overnight.