Thanks, I am not understanding, " dont worry about switching between circuits". Won't that be a problem when the grid power comes back on?
Nope. The load is always plugged into the inverter and the battery is continuously replenished by the charger while power is available. When power goes out, the battery starts being drained. When it comes back, the charger charges it back up again. 65W @ 12VDC is roughly 5.5A (rounding up slightly), so a 10A charger will allow you to run your pump and still charge the battery with 4.5A at the same time. It's true that this is a little less efficient than switching between grid power and backup power, but you'll only lose about 20% in the process (assuming charger and inverter are each 90% efficient), i.e your 65W pump is now effectively 78W, but the benefit is immediate failover without any fragile electronics (which would also suck some amount of power). And that 13W difference (the amount of a single CFL lightbulb) while the pump is running is insignificant if you're on grid power.
The alternative is a product by APC, Tripp Lite, et al., for twice or more the cost. A 100Ah battery would run your pump for 9 hours until it is 50% discharged. You can get a Trojan AGM (sealed) 100Ah battery for $200. The charger should be another $100-$150. And the inverter under $100. So no more than $450 total. For a commercial UPS solution, you're looking at a minimum of $1000, or perhaps much more for 9 hours of backup power. Most computer backup systems provide no more than a few minutes of power. Also, they're still going to be only 90-95% efficient max.
You could also eliminate the inverter inefficiency by changing your pump to a DC model that runs directly off the battery. And you could eliminate the charger and the battery itself by converting to a PV-driven pump, as I originally suggested. Since your solar thermal system can only overheat when the sun is shining, a pump that runs only when the sun is shining is ideal.
