thedoogs I'm assuming you mean 0.6-1.5kWh total across the period, rather than peaks of 0.6-1.5kW.
There's a couple of likely causes:
1) the firmware on your inverter/batteries just doesn't react fast enough to load changes, so you're getting more than desirable grid draw and/or battery drain as the load fluctuates.
2) cheap/poorly positioned CT clamp. Check it's clamped around the meter tails properly and also at the inverter end. Interference from either the neutral or your consumer unit is the most likely issue.
Also, if you have any other clamps (say for an EV charger, keep them as far apart as you reasonably can.
3) although CT clamps should be within 5% generally, they're not very good at fluctuating low loads, so this is why you notice it more at night.
It's definitely worth you looking in to this, as currently you're potentially "wasting" £100/year.
