Interest on penalty accrues for delayed payment; regulator may reduce or waive interest and require refunds. Interest on unpaid penalties is chargeable as simple interest at one and one half per cent per month from the day after the expiry of the period specified in the demand notice until payment; the Commission may reduce or waive interest for defaults due to circumstances beyond the enterprise's control, and if a judicial order reduces the penalty the interest is reduced and any excess refunded under regulation 14.