Hi @murf,
I haven’t looked much into generators before, so educated myself a bit with this article comparing inverter and non-inverter generators. That website also has a Reviews and Buyers guide, of which the first two are inverter-generators that both also support being run in parallel with another of the same type to double the power output.
Your company seems to be Scotland based, so I searched online for “best inverter generators Scotland”, which landed me at this comparison of several ‘leading’ options. I’ve also asked our engineering team internally if they’ve got any recommendations and will pass those on as/if they come through 
Hopefully some others in the community have experience with this too, and can chime in with their experiences/suggestions.
In case it’s helpful, I’ve gone through the power requirements and considerations of an OTPS system below, along with some corresponding implications for your current generator and a potential new one.
+EDITED TO INCLUDE TRANSMISSION LOSSES IN TETHER+
Operating Requirements
Supply Inefficiency Losses
Power Conversions
To get a better sense of the operating requirements, I took more of a look into the OTPS specs. While the ROV Power Supply Enclosure can output up to 1000W, that’s at a 92% efficiency (of the DC-DC conversion), so uses up 1000 / 0.92 \approx 1087 W of the available 1600W of output power from the Topside Supply Unit, which uses 1087 / 0.905 \approx 1200 W of the 1800W max input (noting the 90.5% efficiency of the AC to DC conversion).
Power Transfer (Tether)
As per our voltage drop calculator, a 125m High-Power Tether Cable using two pairs of 23AWG wires to carry 400V, with a load draw of 1087W, would have an efficiency of ~94.1%. If you’ve got the longer 250m tether then the efficiency is ~87.3% instead. Accordingly, 1000W of power in the ROV uses 1200 / 0.941 \approx 1275 W of input power at the top side, or ~1375 W for the longer tether.
Edit/Note: this calculation originally used a too high current draw, so had unnecessarily low efficiencies. The rest of the discussion uses the original result of 1300W (which can now be considered as a slightly conservative equivalent of the input power required with the shorter tether).
Power Usage Estimates
ROV
With a 15V supply (like from the OTPS), a single T200 thruster at 100% throttle will have the following approximate characteristics
| Gain | Power Usage [W] | x6 Thrusters [W] | Thrust |
|---|---|---|---|
| 100% | ~330 | ~1980 | 100% |
| 75% | ~170 | ~1020 | 65% |
| 50% | ~65 | ~390 | 34% |
So assuming you need a max of about ~35W for the 5V electronics (RPi, Pixhawk, Fathom-X, Camera, camera servo, etc - 5V 6A converter w/ imperfect efficiency), another 15W per lumen at max brightness (e.g. 60W for 4 on the standard BlueROV2), so that remaining ~900W of power for your thrusters (and other powered external devices) can get used up pretty easily if you’re driving hard. I’d recommend keeping the gain at or below 75%, or if you want that factored in automatically in ArduSub so you can have finer-grained control in your valid operating region you can follow one of the options discussed here.
For choosing a generator I’d go off the assumption that the full 1000W of ROV power is being used, which is the full 1300W of input power (conversion/transfer inefficiencies factored in).
Laptop
Laptop chargers generally don’t get above 100W, so unless you’re also powering other equipment it’s unlikely you’ll exceed that from the laptop charging output (max 250W socket), so assume you’re consistently using something like 1400W of input power all up, with relatively heavy use of the ROV.
Implications
Current Generator
Assuming your current non-inverting generator has a power factor of ~0.8 (the industry standard for non-inverting generators) it’d have a capacity of ~2800W, so generally shouldn’t be running too far above 50% load anyway (assuming I haven’t missed anything major in my estimates/calculations). It’s possible your 3.5kVA is the maximum load though, in which case if I’m understanding correctly the rated load is possibly something like 20% lower, which would have a capacity of ~2240W and would be running at ~62.5% of that. (I’m unsure about that last point, as I’m not certain if the generator VA corresponds to the max/surge power or the rated continuous/running power)
A New Generator
Generators tend to be happiest/last longer if they’re consistently used at low loads relative to their rated maximums. The ones I looked at seemed to generally indicate run time and noise levels at 25-50% usage, so that’s where I’d suggest looking for optimal performance. With that spec you’d be looking at rated running/continuous loads of 2800-4200W.