The xantrex XW system - when I looked at it last, would use the grid to charge batteries when the pricing was advantageous. I'm sure it could be configured either way.
at work, we have UPSes between the generator and equipment so the equipment doesn't see the 10s downtime between power fail and genset pick up the load (we keep all the generators hot 24/7 so they're ready to go without warmup). this keeps battery requirements to a minimum (lower TCO on the UPSes).
CHP - combined heat and power. you can use the waste heat from your generators to heat your house when you need heat. this is usually done by tapping in to the cooling system to indoor radiators. any heat you need gets taken from the generator's waste heat, and the generator's radiator thermostat gets left alone so that when you don't need heat it maintains proper cooling.
you only get 35-40% fuel efficiency out of the generator when it's running, you can get another 25-30% fuel efficiency if you can use all the "waste" heat from the radiator.
also remember you can use the waste heat from your computers for space heating as well. not sure how you're managing that now.
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