on grid, but want off grid capability?

[mudhole]

I installed a 5kw, grid tie using a sunnyboy inverter, it has been up and working for about 2 years now.

My question is: worse case scenario, we lose the grid, what hardware do I need to run well pump intermittently(240) and freezer? could I reconfigure my array to use an outback charge controller, and then maybe a midnite inverter, pure sine wave? from farm equipment i could scavenger 4 large deep cell 12v. I could just bypass my current inverter.

does this sound plausible?

[Mike90250]

1) you will need batteries, 48V, about 100ah per KW used ( this is to provide ripple filtering for the inverter, another calculation involves Load Size and duration )

2) you will need a Hybrid inverter, or just create a totally separate system using your existing HV DC PV array and a HV DC charge controller

[mudhole]

thanks mike90250, so 1. when you say 100ah per Kw, did you mean per Kw-h? or whatever my peak Kw is?, 2. my thought was to just have a completely different inverter, just for off grid.

[mudhole]

so i estimate my well pump at 1 kwh per day and my chest freezer 1kwh per day, how do i size out the batteries and the inverter?, i calcuated out a start up amperage of around 20 amps if both units started at the same time. If my array is 5kw and i get average 5.5 sun hours a day....

[SunEagle]

so i estimate my well pump at 1 kwh per day and my chest freezer 1kwh per day, how do i size out the batteries and the inverter?, i calcuated out a start up amperage of around 20 amps if both units started at the same time. If my array is 5kw and i get average 5.5 sun hours a day....

Wait. So both of your loads only use 1kWh per day. but how many watts do they draw when they are running? The size of the batteries and inverter are based on what the load draws when it is running. If it runs 24 hours a day then you can say it draws (1kWh / 24hours = 41.66 watts) but I think is will draw a lot more then that when it is running. To big of a load may cause the inverter to shut down due to low battery voltage.

So as Mike states you will need about 100ah per each kW of load.

[mudhole]

they both use around 365 kwh a year per, so per day 1 kwh per, the start up amps are 10 amps per, the running amps are 5 amps each, one at 220v x 5 amps , one at 110v at 5 amps., so if both a are running, roughly 1.5 kw load.

so i need 150 ah battey? and at very minimum 2kw inverter?

I just had another idea, is there just a way to trick my grid tie sunnyboy to not shut down due to grid being down? and then hook up UPS between whatever I want to run?

[mudhole]

of course I would disconnect the main line coming into the house from the grid....i am just wanting to be prepared if something goes bump in the night for a Longggg time

[Ampster]

............

I just had another idea, is there just a way to trick my grid tie sunnyboy to not shut down due to grid being down? and then hook up UPS between whatever I want to run?

It is not a trick per se, it is called AC coupling and it is what most hybrid inverters are capable of doing. That ability was implicit in the earlier suggetions for hybrid inverters. Depending on how old your Sunny Boy the interaction can be seamless or clunky. The latest specification of UL1741SA allows the hybrid grid forming inverter to modulate the GT grid dependent inverter. Older GT inverters can only be on or off but that can still provide some functionality to charge batteries and serve loads.

[SunEagle]

they both use around 365 kwh a year per, so per day 1 kwh per, the start up amps are 10 amps per, the running amps are 5 amps each, one at 220v x 5 amps , one at 110v at 5 amps., so if both a are running, roughly 1.5 kw load.

so i need 150 ah battey? and at very minimum 2kw inverter?

I just had another idea, is there just a way to trick my grid tie sunnyboy to not shut down due to grid being down? and then hook up UPS between whatever I want to run?

Answer to your last question. NO. I do not know of any device that tricks the inverter to think there is a grid. But there are inverters that can charge a battery system or provide a limited wattage output.. For that matter the one I am thinking about is made by SMA and has the secure power option.

[mudhole]

so if i was able to charge a UPS, hooked up to my SMA, I would be able to run the freezer etc, well pump at night(would have to figure out how to get 220v), but either way, I would need 100ah battery per KW, so in my case 150 ah battery.

[mudhole]

this just seems so clumsy, DC to AC to DC to AC

[SunEagle]

this just seems so clumsy, DC to AC to DC to AC

They do make water pumps that run directly from a solar panel and actually have a float switch to turn it off if the tank gets full.

[Mike90250]

thanks mike90250, so 1. when you say 100ah per Kw, did you mean per Kw-h? or whatever my peak Kw is?, 2. my thought was to just have a completely different inverter, just for off grid.

For every KW of power the inverter produces, there needs to be about 100ah of battery ( @ 48V. or 200ah battery @ 24V) to "buffer" the 120Hz surges the inverter draws. too small of battery and the inverter starts distorting it's output and generating excess internal heat. If you have 2kw of loads, you need 200ah of battery to keep the inverter safe. ( all in the fine print of the manual)
This is totally different than the amount of battery required to sustain a load for a set amount of hours.

[littleharbor2]

You can go with a separate hybrid inverter or another option is using a Sunny Island inverter in conjunction with your Sunny Boy GTI. This combination may require only certain models of Sunny Boy inverters. so maybe or maybe not an option for you.

[mudhole]

so, the hybrid inverter will still run when the grid goes down? if so, how does that work without the danger of linesmen getting electrocuted?