def consumption_diff(before, after):
    out = {}
    for key in after:
        out[key] = before.get(key, 0) - after.get(key, 0)
    return out

def consumption_to_string(dic):
    out = ''
    for key in dic:
        out += '{0:s}:\t{1:.2f}W\n'.format(key, dic[key])
    out += 'Total:\t{0:.2f}W'.format(sum(dic.values()))
    return out