Nothing really. Even the chips used are generally the same in both 500 VA and 600/650 VA. 500 VA is fine, even I use 500 VA. Just that if the price difference is not huge you can consider 600 VA since 600 VA would provide more backup time and also handle more load though not very much since the difference is only 100 VA. Just consider the max bang for buck factor when deciding, that's all.

What is the load you are using on this 500VA backup? is it a projector?

if it is a projector which draws approx 300Watts of power then the 500VA UPS will fail immediately. I am 100% sure the 500 VA will fail !!!!

If you are investing in a projector for more than 50k please invest another 3-4 k for a mininum 800VA UPS or 1 KVA (best) that will give you at least 5-10 mins of backup. Also please keep in mind that as the batteries age then the backup time also reduces. Hence if your 800 va UPS gives 5 mins backup when new will eventually give you 1-2 mins backup after 1 yr.

The sizing of the UPS should be done using a calculation as below,

1. Take the Amp and Volt reading of the projector and multiply the Volt x Amp to get the total VA.

2. Easier approach, if the projector is rates in max Watts or just Watts, then convert to VA by multiplying by 1.43.

3. Add a factor of safety FOS of at least 50% (30% for additional load and 20% for battery life degradation + misc factors not known)

Example Optoma HD65 (my new projector) is rated 265max Watts. Since this is at test conditions, assume it to be approx 300W (OMG thats big!!! Considering that an average DVD player draws 10-12 W only)

- 300w X 1.43 = 430 VA

- 430VA * 1.5 FOS= 645 VA

I have tested this using a 500VA APS and a 800 VA microtek UPS systems using the HD65. The 500 VA APS failed immediately. 800VA UPS gives me 5 mins backup. Best option is to use a Auto Shut Down feature on most projectors that will shut down the PJ if there is no source signal. I set it to 1 min.

Always , Play safe!!