A friend here in Morelia has that same solar heater. She can't say enough good about it. She adores it. Last time I was at her house, she insisted that I go up on the roof to admire it. She says she could make tea with the hot water that comes directly from the spigot on the tank. I felt the hot water (she made me put my hand under the faucet that comes from the tank!), and she's right: she could. She says that there are days the water gets so hot that the tank boils over a bit.
She lives alone and says that she has used exactly half a small tanque estacionario of gas in a year, for cooking. Her house has two bathrooms, kitchen, and laundry room that all use hot water. The tank is plumbed on the roof and connected to the hot water pipes of her 100-year-old house; she did not have to replace any plumbing to install it. She has never run out of hot water, even in Morelia's rainy season.