Unlike the mains utility, which for residential applications and smaller industrial applications has unlimited scope to support the starting of motors and other electrically disruptive equipment, a diesel generator must be properly sized to ensure it meets the required demands of the application.
Firstly its important to understand the rating of a generator that you might purchase. Generators are usually sold in kVA or sometimes kW. These are not same and the relationship between the two is expressed as the power factor, which is typically 0.8, so kVA x pf = kW. So if you have a 100kVA generator, at a 0.8 power factor, it will produce 80kW of electrical power.
In simple terms the kW portion of the generator is the real work done. This powers your lights, heaters and moves the air from your fans. The difference between the kVA and kW value is "wasted" power, thats called kVAr. It flows around the circuit, but does nothing useful.
Broadly these can be split into two categories - resistive and inductive. Inductive loads cause "real" and "wasted" part whereas resistive loads cause only the "real" part.
If you only have resistive loads, then the sizing is easy. You can safely add the ratings of all the items you have and work out how many kW you need. The only thing you really need to do is check the load acceptance of the generator. All generators have a load acceptance which limits the single step load to prevent the engine slowing down more than 10%. Typically this single step it might be 60% (you would fine this on the engine or generator specification sheet), so if you want to apply your full load in one attempt then you need a generator where 60% of its rating represents the load you have just calculated. If you can turn everything on gradually, you won't need it to be as big, which saves you money.
If you have inductive loads it gets more complicated. You need to be able to calculate how much "real" power ( that's kW) you need and how much "wasted" (that's the kVAr) power you will have.
When you have the kW and kVA values, you can use the kW as above. Then with the kVA you need to do a similar calculation to the alternator. With the alternator taking this kVAr (the difference between the kVA and kW), what happens is the voltage drops. This voltage drop will cause light flicker and potentially some equipment to turn off. 30% is considered the maximum, but for many controls its too much and 15% is a much more reasonable figure again you need to look at the generator or alternator specification sheet.
Two or Four Pole alternators
Its an important distinction - do you know the difference between two and 4 pole machines. Here is a short explainer video showing the difference. The faster an engine runs, the more fuel it will consume, so knowing how long you expect to run for is critical.
Speed drop on Alternator Voltage
One final consideration - as the engine speed falls, the alternator voltage also drops a little further, so you need to take this into account as well - back to the alternator specification sheet!
While for small home applications where the outcome is not critical, you can perhaps size your own, it is highly recommended to get a professional to size it for you in most situations. A local electrician is often an ideal starting point.