You will see the performance chart for your unit will tell you what sort of delta-T (temperature rise) you will get at various GPM and incoming water temps. The newer units are much improved in terms of efficiency of the earlier ones. I couldn't imagine doing one powered by electricity though. I don't have natural gas available in my area and the electric one I looked at would consume I think 120amps when running. Between that and my furnace it would be a problem for only having 200amp service.