A 1KW is equal to 1KVA when you have a resistive load such as an Incandescent lamps, an electric heater without fan, etc...
In this case, 100Watts = 100VA
However, for some CFL with a poor power factor of .5, this is a big deal.
In this case, 100Watts = 200VA
Unless properly documented, we generally assume a power factor of .7 for Computer, Monitors and good electric motors. However, cheap electric motor can run has low has .3. - Cheap Computer Power supply can also have a bad power factor or .5 or .6
A VA is also called "Apparent Power". This is what an electrical Transformer see. This power is not necessarily consumed but it is nevertheless needed.
All electric transformer and Inverter are rated in VA unless the load is known in advance. For example, a 120 VAC to 12VAC transformer for halogen lightning might indicate Watts or VA since this is essentially the same thing.
Otherwise, computer UPS manufacturer use an "educated/Marketing" guest. For this reason, you can find a computer UPS rated 800VA/500Watts and an others rated 800VA/540Watts. Sometime ago, I have seen an 800VA/650Watts. None of the above is necessarily better than the others. The Watts rating is all depending on what the manufacturer consider being a standard Power Factor for a computer load.
Assuming a power factor of .85 is generally adequate for a grid operated transformer. Otherwise, it might be dangerous to assume it when only a few devices (especially of the same type) are operated from a given Inverter.
For example, operating 400 Watts of CFL might require 800VA or more. A computer rooms might easily needs a power factor of .6 to operate. Large motors might have a very poor power factor when starting, needing a lot of VA per watts at that time, etc... (Here the extreme importance of the surge capacity for an inverter when you operate large motors)
When you mix and match a lot of device: Capacitive and Inductive, your resulting power factor may actually correct to a point (this is the case for the electrical grid but rarely for an Inverted). For example, a Leading Power Factor of .5 might cancel a part of a Lagging Power Factor of .4. So, the exact computation is not easy under this condition. However, in my experience, this is seldom the case (unless planned) for an Inverter under 100 KVA.
Take note that if you want to purchase an Inverter that produces a true "Sinusoidal wave form", the surge capacity is often very limited. For example, a 1KVA Inverter might be limited to a short surge of only 1.5 KVA. Considering this is very important when you have large electric motors (even some washing machine) or, in some cases, very cheap electric motors. For example, a 1/4HP inductance motors that needs to be fused with a 20AMP/120V breaker to start spinning needs looooots of VA at that time.