Detecting that the PV is generating excess is pretty easy actually.
You'd need a diode in line with the panels feed to the invertor (which you probably have anyway) - you then just hook up the batteries on the side of the wires from the panels - this way when there is sufficient current from the PV the batteries will get charged, when the PV stops generating the batteries will discharge into the invertor and at a certain point (which would need fairly basic battery protector circuitry - caravans have this) the battery will stop discharging and the setup you have now will just let the utility power take over.
No risk of the utility power being used to charge the batteries, and they'll leech off any surplus power the PV's generating at any given point in time, until they're fully charged. They will also smooth off the supply from the panels to a large extent meaning that you'll get pretty much your peak power output for a lot of the day as the batteries discharge when the panels aren't getting much light - of course only if there's something requiring the power.
A diode is just a one-way valve for electricity - it will only allow current to flow in one direction, so all that can happen in this circumstance is power going to the invertor.
Batteries are a definite issue - you don't want to use Lead Acid or Gelly Cells indoor as they emit hydrogen when charging which can lead to some interesting effects.
NiMH batteries are *relatively* inexpensive, and available fairly easily and can take a decent charge - they're what most commercial UPS's use these days. Depending on what you want them to do (say constant drain of 200W for 10 hours - 2kWh or so. A 2.2kWh NiMH commercial battery pack can be had that weighs 35kg and is 65x17.5x14cm - that's not that big really. What's that? 2 foot x 8" x 6"? The 3.5kWh pack from the same supplier is the same dimensions, but a yard tall (95cm by their spec)
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You'd need a diode in line with the panels feed to the invertor (which you probably have anyway) - you then just hook up the batteries on the side of the wires from the panels - this way when there is sufficient current from the PV the batteries will get charged, when the PV stops generating the batteries will discharge into the invertor and at a certain point (which would need fairly basic battery protector circuitry - caravans have this) the battery will stop discharging and the setup you have now will just let the utility power take over.
No risk of the utility power being used to charge the batteries, and they'll leech off any surplus power the PV's generating at any given point in time, until they're fully charged. They will also smooth off the supply from the panels to a large extent meaning that you'll get pretty much your peak power output for a lot of the day as the batteries discharge when the panels aren't getting much light - of course only if there's something requiring the power.
A diode is just a one-way valve for electricity - it will only allow current to flow in one direction, so all that can happen in this circumstance is power going to the invertor.
Batteries are a definite issue - you don't want to use Lead Acid or Gelly Cells indoor as they emit hydrogen when charging which can lead to some interesting effects.
NiMH batteries are *relatively* inexpensive, and available fairly easily and can take a decent charge - they're what most commercial UPS's use these days. Depending on what you want them to do (say constant drain of 200W for 10 hours - 2kWh or so. A 2.2kWh NiMH commercial battery pack can be had that weighs 35kg and is 65x17.5x14cm - that's not that big really. What's that? 2 foot x 8" x 6"? The 3.5kWh pack from the same supplier is the same dimensions, but a yard tall (95cm by their spec)
Can't say as to the cost of them I'm afraid.