The most common method of voltage control for a hydro or wind based generating system is with a voltage sensing diversion controller. When the battery and loads are satisfied, any surplus energy must be diverted to a diversion load      . Heating elements make the most reliable loads but other types of loads such as lighting can also be used. Some inverters and other charge controllers have auxiliary relays that can be used to turn other types of loads like pumps on and off as needed. We carry water heating elements in 12, 24, and 48 volt. If using a conventional electric water heater, the existing thermostat will not hold up for 48 volt or higher DC operation. Modification to the water heater may be necessary. Air heating units are the more reliable device. We make air heating units that are for continuous use at 100% duty cycle in a 50 C ambient without a fan. They are superior to everything we have seen on the market. We took this approach due to the number of failed units and near fires we have seen from other brands. That said, the air diversion load must be mounted as specified on a fire proof wall or other suitable support with suitable fire proofing as needed. The unit must be kept free of any combustible materials or falling debris. In short, the same as you would treat a plug in floor heater. For battery based systems, the National Electric Code requires a back-up diversion. There are several ways to satisfy this requirement:

1. Grid-tie battery based inverter…..the inverter sells power to the grid as the primary diversion. If the inverter fails or the grid goes down in a power outage, a back-up diversion controller will dump to a heating element or ? preferably a DC device not powered from or interacting with the inverter.
2. Off-grid inverter….. Primary diversion done by voltage sensing relay or a diversion controller and a diversion load. Secondary diversion is best done with another diversion controller and a DC air heating load.
3. Off-grid, using an MPPT type controller in hydro format will also need a back- up controller which can be powered from an aux relay in the inverter but I prefer the more reliable or foolproof method of a diversion controller with the air heating load.