Electric bike conversion kit

the max motor wattage is almost certainly 250W.
(battery is 36V × 10.5Ah = 378Wh.)

@jjamgomes Your kit appears to be a 36V EU-legal mid-drive conversion kit, and those are almost always 250W nominal.

The clues:
• 36V
• 378Wh battery
• Dutch seller / Dutch invoice
• Product image looks like a common Bafang-style mid-drive kit
• EU / NL road-legal e-bike kits are typically sold as 250W continuous

So the answer is almost certainly:

Max legal rated power: 250W nominal

And in practice:

Likely peak output: ~500W to 650W

That’s the short burst power you may see under:
• hard acceleration
• hill climbing
• heavy cargo load (like a Babboe)

Battery-side reality check

Your battery is:
• 36V × 10.5Ah = 378Wh

If the controller pulls:
• 15A, then theoretical electrical peak =
36V × 15A = 540W
• 18A, then theoretical electrical peak =
36V × 18A = 648W

That lines up with what these kits usually do.

Best estimate for your specific kit

For your exact setup, I’d estimate:
• Nominal motor power: 250W
• Likely controller limit / peak electrical power: ~540–650W
• Usable real-world peak at the wheel: a bit lower after drivetrain losses