I'm looking for some advise on how to build a hybrid battery/generator backup system for my on-grid home. The goal is to coast through small (2 hours or less) grid outages on battery power with minimal disruption but to also use a generator to handle the longer outages. I'd like the system to be quiet, fuel efficient, and as automated as is reasonably possible.
At first glance, the XW6048 seems to be just the ticket. I am sure that some of it's cost is due to also being able to support a PV array. While I am not planning for this as part of the initial project I do want to plan for it. I have 400sq of prime unshaded (though nearly flat) roof space and another 350sq of partially shaded in the morning and again in the late afternoon roof space that I'd like to press into service at a later date. Right now, the primary goal is just backup power.
I'm thinking that the first phase will be to separate my breaker panel (200amp service) into loads that I want backed up, and those I dont. I currently have many loads that would would never make sense even on a generator. These include two zones of (rarely used) baseboard heat, oven, clothes dryer, pool pump, etc. I suspect that what remains could all be fed from a 60amp sub-panel.
My current panel (30+ circuits) is monitored by 3 Brueltech 1240's. They give me 21 channels of monitoring so I have a pretty good idea of my loads. For any five minute period my total load rarely goes over 7kw and typical is far lower than that with an average under 30kwh/day. With the big loads out of the way, I would think the XW6048 would be just fine.
Assuming the XW6048 is the way to go, the remaining questions have to do with the size and technology for both the generator and the battery bank. My (simple minded) analysis here is that I would need a minimum of 8-10kwh of capacity to get two hours of runtime assuming a 2kw average load for those 2 hours and a target of 50% discharge.
My only experience with battery banks so far is with a large UPS in a data center where I hang out. It uses a two banks of 40 UB121000's and is designed to dump them all 80 in under an hour. So for my little project, it would seem that 8 would be able to handle my load and then some. Of course, Liebert doesnt expect that this system will be cycled continuously over the course of an extended outage. I'd guess they are making battery life trade offs that I dont understand. Regardless, they do expect the batteries to last 5 years in their intended role as a stand-by power source.
In my hypothetical system, two hours into an outage my batteries are down to 50% then the XW6048 tells the generator to kick in and charge the batteries to 80%. Assuming 80% efficiency for the charge, this would be about 3kw for an hour, or a bit more than half what a 6kw generator could produce and would still leave enough head room for the 2kw (average) load. Another assumption I'm making here is that the remainder of the batteries would still available to the inverter to handle any surge or short term increase in the load while charging.
I'm sure I've missed many details here, but if I am even close, it would seem that a 6kw generator and 8kwh of battery would give me somewhere around a 50% duty cycle (1 hour on 1 hour off) on the generator. While all that sounds great, I'd like to understand better the limits and factors that affect the generator's duty cycle.
So my questions to the experts are: 1) how far off are my estimates? 2) would this line of thinking work with FLA batteries? 3) Does the XW6048 "clean up" the generator output or pass it through directly? and related, 4) would I need to use an "expensive" inverter based generator for any reason other than noise control? 5) When grid power is available will I notice the load associated with keeping the batteries topped off. 6) What have I missed?
Thanks in advance for playing along.