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Subject Need a system that does both Grid Intertie and backup power
Author Sascha Deri
> I am very interested in the medium grid system. I would also like to
> have protection from blackouts etc. Is it possible to change the
> inverter in the medium system to a Trace SW 4024 so that at some future
> time I could add batteries? If not, would it be cheaper to put in a
> medium system and then add a manual switch that could change the panels
> from the sun tie inverter to a very small inverter, battery system that
> could power the water pump and maybe the freezer? The average system
> seems to be too expensive and has more batteries then we require.
> Thanks for your help. Jacquie

Hello Jacquie,

Lots of folks are hoping to have the best of both worlds with these utility intertie systems by using the Trace SW4024 inverter instead of the SunTie. The main problem is that the SW4024 requires batteries to operate all the time even if you are using it in a utility intertie system - you can not just add the batteries later. You can certainly build a utility intertie systemn around the SW4024 and use a small battery bank but you lose some of the efficiency of the SunTie and the system ends up being quite a bit more expensive.

If you want a very small system for protection against black outs there is a simple option: use a Trace DR inverter (small & inexpensive and will easily run water pumps, fridges and other non-electronic loads) with a small battery bank. The DR inverter is used all the time in non-solar back-up power applications and since it is also a battery charger it will just charge the batteries when the grid is available and will automatically turn on when it senses that the grid is gone. When the DR is acting as a battery charger it is using utility power like any other load but the solar electric system with the SunTie inverter is providing a portion of your power anyway so the solar electric system is indirectly charging your batteries. The only tricky part to this system is that you will have to put the loads that you want to operate during a black out on a sub circuit breaker panel that is separated by a manual transfer switch that you flip when the lights go out. Of course you can also have an automatic transfer switch to do this but they are quite expensive. Also please let us know what size your well pump is so that we can size the back-up power inverter.
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