To keep batteries healthy, FULLY charging them on a regular basis is crucial. A small amount of generator run time can help accomplish this.
Modern MPPT charge controllers combined with a large array can do an excellent job of bulk, float and equalize charging. If cloudy weather or high electricity use don't allow this, the most efficient way to use a generator is to use it for the bulk charge phase. Doing this in the morning for an hour or so will allow the solar system to supply a finishing charge for the rest of the day. In this way each part of the system does what it is suited for.
General Notes and Sizing
You get what you pay for. Homeowner grade generators do not provide long life. Low speed models (1800rpm) are preferable for extended run times but not essential for occasional charging. In order of increasing quality and size some options are:
1. Honda gas
2. Air-cooled propane (e.g. Onan)
3. Liquid-cooled diesel or propane (e.g. Kohler)
One important feature is the ability to get ALL the output into one leg of 115V AC to feed the battery charger. Some models need to be rewired to do this. It is also possible to get a transfomer to take 230V from two legs of the generator and combine it into one leg of 115V.
Sizing involves taking the desired charge rate and derating the generator output for altitude and charger inefficiency. A C/5 Rate will work for bulk charging. The charger will reduce this as the battery charges to avoid damaging the battery.
Deduct 3% for each 1000' above sea level up to 6000' and 6% for each 1000' beyond that.*
Charger efficiency can be estimated as 75%. Using the example from the previous page and supposing we are at 4000' above sea level:
12,720WH is the total battery capacity
12,720WH / 5H = 2,544W (C/5 rate)
2,544W / 88% = 2,891W
2,891W / 75% = 3,855W
So we would need a 4kW generator for this example.
Next page: Battery Care