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In general, one innovative aspect of the subject matter described in this specification can be embodied in a load control system configured to perform load control operations comprising: measuring a voltage on an electrical grid at a plurality of times; determining, for each time of the plurality of

In general, one innovative aspect of the subject matter described in this specification can be embodied in a load control system configured to perform load control operations comprising: measuring a voltage on an electrical grid at a plurality of times; determining, for each time of the plurality of times, a difference between a specified normal voltage and the respective measured voltage at that time; summing the differences between the specified normal voltage and the measured voltages to determine a curtailment level; determining whether the curtailment level exceeds a first threshold curtailment level; and in response to determining that the curtailment level exceeds the first threshold curtailment level, adjusting an amount of power drawn by a load.

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1. A system comprising: a power source coupled to an electrical grid and configured to provide power to the electrical grid, wherein the power source is configured to reduce a voltage on the electrical grid as a consequence of a current drawn by the electrical grid exceeding a threshold current;a fi

1. A system comprising: a power source coupled to an electrical grid and configured to provide power to the electrical grid, wherein the power source is configured to reduce a voltage on the electrical grid as a consequence of a current drawn by the electrical grid exceeding a threshold current;a first load control system coupled between the electrical grid and a first load, wherein the first load control system is configured to perform load control operations comprising: measuring the voltage on the electrical grid at a plurality of times, and for each measuring of the voltage: determining a difference between a specified normal voltage and the respective measured voltage at that time to obtain a difference value;determining a curtailment level that is a summation of each of the difference values determined at each of the plurality of times, wherein the summation is reset when the difference values are each below a threshold for a specified duration of time;determining whether the curtailment level exceeds a first threshold curtailment level; andin response to determining that the curtailment level exceeds the first threshold curtailment level, adjusting an amount of power drawn by the load. 2. The system of claim 1, comprising: a second load control system coupled between the electrical grid and a second load; anda system controller configured to communicate with the first and second load systems over a data communications network, wherein the system controller is configured to provide the first threshold curtailment level to the first load control system and to provide a second threshold curtailment level to the second load control system. 3. The system of claim 2, wherein the system controller is configured to send, to a user device, instructions for presenting a user interface for selecting curtailment thresholds and to receive, from the user device, first and second selections for the first and second threshold curtailment levels. 4. The system of claim 3, wherein presenting the user interface comprises presenting a plurality of names for a plurality of loads, and wherein receiving first and second selections for the first and second threshold curtailment levels comprises receiving an ordered list of the names, and wherein the system controller is configured to determine the first and second threshold curtailment levels using the ordered list of names. 5. The system of claim 1, wherein the power source comprises an inverter coupled to a direct current (DC) bus coupled to a plurality of DC power sources, and wherein determining that the curtailment level is the summation of each of the difference values determined at each of the plurality of times comprises multiplying the summation of each of the different values determined at each of the plurality of times by a weighting factor. 6. The system of claim 5, wherein the electrical grid is coupled to a utility grid, and wherein at least one of the DC power sources is a photovoltaic power system, and wherein the load control system is configured to begin summing each of the difference values determined at each of the plurality of times as a consequence of determining that the electrical grid is not receiving power from the utility grid. 7. The system of claim 1, wherein the load control system comprises: a communications device configured to communicate over a data communications network; anda power controller configured to adjust the amount of power drawn by the load. 8. The system of claim 7, wherein the load control system comprises a microcontroller configured to receive, using the communications device, the first threshold curtailment level from a system controller. 9. The system of claim 1, wherein the load control system comprises a circuit breaker, and wherein adjusting the amount of power drawn by the load comprises tripping the circuit breaker. 10. The system of claim 1, wherein the weighting factor is variable over the plurality of times. 11. A load control system configured to be coupled between an electrical grid and a load, wherein load control system is configured to perform load control operations comprising: measuring the voltage on the electrical grid at a plurality of times, and for each measuring of the voltage: determining a difference between a specified normal voltage and the respective measured voltage at that time to obtain a difference value;determining a curtailment level that is a summation of each of the difference values determined at each of the plurality of times, wherein the summation is reset when the difference values are each below a threshold for a specified duration of time; andas a consequence of determining that the curtailment level exceeds a threshold curtailment level, adjusting an amount of power drawn by the load. 12. The load control system of claim 11, wherein determining that the curtailment level is a summation of each of the difference values determined at each of the plurality of times comprises beginning summing each of the difference values determined at each of the plurality of times as a consequence of determining that the electrical grid is not receiving power from a utility grid. 13. The load control system of claim 11, comprising: a communications device configured to communicate over a data communications network;a power controller configured to adjust the amount of power drawn by the load; anda microcontroller configured to receive, using the communications device, the first threshold curtailment level from a system controller. 14. The load control system of claim 11, comprising a circuit breaker, wherein adjusting the amount of power drawn by the load comprises tripping the circuit breaker. 15. A method performed by a load control system coupled between an electrical grid and a load, the method comprising: measuring the voltage on the electrical grid at a plurality of times, and for each measuring of the voltage: determining a difference between a specified normal voltage and the respective measured voltage at that time to obtain a difference value;determining a curtailment level that is a summation of each of the difference values determined at each of the plurality of times, wherein the summation is reset when the difference values are each below a threshold for a specified duration of time; andas a consequence of determining that the curtailment level exceeds a threshold curtailment level, adjusting an amount of power drawn by the load. 16. The method of claim 15, wherein determining that the curtailment level is a summation of each of the difference values determined at each of the plurality of times comprises beginning summing each of the difference values determined at each of the plurality of times as a consequence of determining that the electrical grid is not receiving power from a utility grid. 17. The method of claim 15, comprising receiving, using a communications device configured to communicate over a data communications network, the first threshold curtailment level from a system controller. 18. The method of claim 15, wherein adjusting the amount of power drawn by the load comprises tripping a circuit breaker.