The ohm is a familiar unit for measuring electrical resistance It’s also a new solar heating monitor from solar water heating manufacturer Sunnovations (ohm.sunnovations.com). The Ohm is a production meter for residential use that eliminates the need and costs of an inline flow meter. The Ohm uses a resistance temperature device to measure the tank temperature and calculate the gain for each day. The Ohm requires Internet access at your home and uses a third-party website to access monitoring data. The company has published on its website the results of independent testing of the unit’s accuracy.