import csv, json
by = {}
for r in csv.DictReader(open("readings.csv")): by.setdefault(r["meter"], []).append((r["read_at"], int(r["register_kwh"])))
out = {}
for m, rs in by.items():
    rs.sort(); use = {}
    for (t0, a), (t1, b) in zip(rs, rs[1:]): use[t1[:10]] = (b - a) % 100000
    out[m] = {"total_kwh": sum(use.values()), "peak_day": max(use, key=use.get)}
json.dump(out, open("usage.json", "w"))