How to prevent power overload when loads exceed a generator's capacity
You can do what you require with the addition of one relay with a changeover contact and a little rewiring.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. (a) Existing circuit. (b) Syrup priority relay modification.

How it works:


RLY1 is an AC relay. Coil voltage should match the generator voltage. Contact rating should have a minimum voltage rating to match the generator and current rating of at least that of the load.
The SYRUP and WATER thermostats are normally closed and will open when temperature is exceeded.
When the SYRUP thermostat closes the relay will energise and power the syrup heater. The water circuit will be disconnected.
When the SYRUP reaches temperature its thermostat will open, the relay will drop back to its shown position and the water circuit can work as normal.








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Figure 2. Using a thermostat with a changeover contact makes the modification simple.

If you could replace the thermostat with one with a changeover contact then the modification becomes very simple.









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Figure 3. (a) Existing circuit. (b) Syrup priority current sensing relay module.

This circuit does not require modification of the syrup heater. It is similar to the socket monitor in Jeroen3's answer. It requires the use of a current sensing relay with an AC coil rating to match the current drawn by the syrup heater. 

The problem with this circuit is that if the water heater is on then there is a transient high-current load when the syrup heater first switches on. Both heaters will be on for a fraction of a second until RLY1 moves. This may trip out your generator.









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Figure 4. A current limiting lamp to prevent overload on switchover.

This circuit uses a lamp to limit current while the relay energises. When it does relay contact 1b bypasses the lamp and applies full power to the heater. Meanwhile the water heater is already disconnected so current is limited.

This solution relies on finding a relay that will energise on the low current but survive with the high current running through the coil.



Fuses and circuit breakers have been omitted for clarity.