I just got a 21kW open-frame diesel unit from Central Maine Diesel, and am in the process of installing it. It has an Isuzu engine and Mecc Alte alternator, and was assembled by GPP in New Jersey and shipped direct from them. The purchase/delivery process was easy, but I expect install to take another couple weeks at the pace I'm going. Also have to built a shed for the generator, run wire, etc. I did get the trench dug and conduit buried, which was a big step. Also have the shed foundation done. Once a few more electrical components arrive, I plan to finish things up.
As far as size, output, and fuel use, consider this. Prices go up $300-900 for about every 4-5kW increase in the 10-20kW range, so there isn't often a huge price penalty to going larger, if the need justifies it. The way I looked at it, if I was going to spend $6800 for 14kW, the jump to 21kW for $7700 wasn't big compared to the 50% increase in capacity. For our home, the minimum was 10kW. We'd need 14kW to start living comfortably, and 19kW for all possibilities.
Another thing I did was graph the fuel usage of the diesel generators in the 8-21kW range, and they all follow a linear trend that overlaps. Some of the larger units are comparable or better in fuel usage running at 25% then the smaller units are running at 75-100%. So there is often little to no penalty fuel wise. Where the big units will use more fuel than small units is if they are just idling at little to no load. Depending on your usage patterns, that may be a factor. But overall, I am expecting anywhere from 3-7 days of conservative runtime from a 55gal drum of diesel with my 21kW Isuzu unit, depending on the season and our needs. It pulls 0.45 gph at 25%, 0.74 gph at 50%, 1.05 gph at 75%, and 1.44 gph at 100%.
I would recommend spending to get the enclosure, breaker kit, sub-base fuel tank, and any other extras that are needed to make your install go more efficiently if your budget allows and/or if you're not handy with all sorts of electrical/mechanical work. In retrospect, I am probably saving 50% in material costs by doing all that myself, but it requires a lot of time/labor on top of that, and some specialized tools. As an example, GPP wanted $600 for their breaker kit (mounted near the generator to protect any downstream service wiring to the home). So far it has cost me about $300 in materials and a special crimping tool, to make the same sort of thing.
None of the work is hard, but it takes time and materials and tools, and good/expert advice and research. I've been fortunate to draw on the experience of a few folks here on TBN, which has helped immensely. I also have spent hours researching and understanding codes for both electrical and install requirements to make sure everything is done properly and safely. A local dealer quoted me $8000-10000 labor to install a generator, and in retrospect that is probably fair for materials and costs.
