You shouldn't be using incandescent lamps at home - let alone in a tent.
Unless you mean they are "40 watt equivalent" - in which case they're actually going to be 7 to 11 watt fluorescent.
A single 20 watt fluorescent is fine for a family-size tent - after all, it's as much as most people have in a 12x12 living room. I usually only have a 9 watt spiral running myself.
That said, even 20 watts is a bit of a luxury when on batteries - almost 2 amps.
Even one or more 4 or 8 watt 12 volt linear fluorescent inspection lamps would do the job - cheap as chips - though curiously not outdoor rated - but at least all the high voltage bits are inside.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.