Per-month source of the kwh array: kwhSource[i] labels kwh[i]. Only accepted alongside kwh: exactly 12 entries, or []. Values: "manual" (user-entered), "estimate" (client-computed), or "unknown" (no value supplied). Omitted or null, the server fills it in: months whose kwh value is unchanged keep their stored label, others become "unknown" if 0 and "manual" otherwise. The server never infers "estimate", so send kwhSource whenever any month is estimated.