Mick,

SMA has something:

http://www.sma.de/en/products/off-grid-inverters/smart-load.html

Dick Ratico
Solarwind Electric
Bradford, VT

--- You wrote:
Step three is most intimidating to me: We need a way to control 240 volt AC
heating elements in a fine pulse width modulated PWM fashion, to protect the
battery from destructive overcharge in event of a utility blackout. I say
240 volt AC because the amperage goes off the chart if trying to divert big
energy from a 48 volt battery. Mr. Ohm would tell us that it's better to use
a higher voltage/lower amperage, and on/off control is less sparky with
alternating current.
--- end of quote ---

