Re: Off Grid System with Propane Generater
First, I would humbly suggest that you get a good handle on how much (kWhrs) of electricity you will need--and if it matters, season by season (AC, water pumping for garden, etc.). Also, if you draw much power from the batteries, there is a pretty good sized penalty for conversion of electricity, charging, and inverterting back (ex: 85% charger, 80% typ. lead acid battery charge, 85% inverter -> 0.8*0.8*0.85= 54% efficiency--or you will need about 2x the kWhrs if using the batteries).
Next, a battery system that can manage your loads for 3 days or so (draining, at most, to 50% of battery capacity) is a good place to start. More batteries are expensive, fewer batteries make it more difficult to ride through solar power "blackouts" (cloudy weather, etc.) and fewer batteries is more charge/discharge cycles, and higher currents (to charge and discharge)--which makes the batteries less efficient and wear out sooner. Also, if you can only run your generator 1 day out of three--you probably would enjoy the quiet.
Next, would be to look at a battery charger for a 3kW generator--for any good sized charger, you will need to make sure that it plays well with the generator--3 kW sounds a bit small if you have a larger charger.
Lastly, conservation of electricity will be the goal... I am guessing, but just in fuel charges you are talking about ~$0.25 to $0.50 per kWhr--so anything you do to conserve power, is worthwhile.
There are things here and there that you can weigh the cost/benefits for... Example, AGM batteries cost more but are 90% efficient to charge vs 80% efficient for flooded cell--plus there is less maintenance (watering, cleaning)--but they are more prone to damaged if overcharged.
I agree with the premise of a small generator--can be more efficient to operate a small one at 80% load than a large one at 20%.
There are folks here that can recommend combo-inverter/chargers/generator controllers and such that work well and pretty much are automatic. But a sine wave inverter is not cheap (probably more on the line of $1 a watt vs the $0.10 per watt of those modified sine wave inverters you see everywhere). And, if you are running a home (motors, electronics, etc.), you will want (perhaps need) to get the true sine wave inverter to keep your appliances happy.
But, until you better describe your loads and needs--it is pretty difficult to give you cost effective answers. I would guess that a "nice" 3 kW generator/inverter/charger/battery/controller setup is going to be several times your $2,500 cost unless your "true" electrical needs are small (or you want to start with a piecemeal system). Remember that batteries are a weak spot in most systems--if you accidentally deep discharge them (or have poor maintenance), they can quickly be killed. If you are not the one always watching charge levels, starting, and maintaining the batteries/generator (wife, kids) for a manual system--it is easy to see early battery replacement down the road.
Where are you doing this? Do you have sun/wind available with little shade/wind obstructions for panels/turbines? Snow/Hot weather?
20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.