"In the insurance context, the right to contribution arises when several insurers are obligated to indemnify or defend the same loss or claim, and one insurer has paid more than its share of the loss or defended the action without any participation by the others. Where multiple insurance carriers insure the same insured and cover the same risk, each insurer has independent standing to assert a cause of action against its coinsurers for equitable contribution when it has undertaken the defense or indemnification of the common insured. Equitable contribution permits reimbursement to the insurer that paid on the loss for the excess it paid over its proportionate share of the obligation, on the theory that the debt it paid was equally and concurrently owed by the other insurers and should be shared by them pro rata in proportion to their respective coverage of the risk. The purpose of this rule of equity is to accomplish substantial justice by equalizing the common burden shared by coinsurers, and to prevent one insurer from profiting at the expense of others." (Fireman's Fund Ins. Co. v. Maryland Casualty Co. (1998) 65 Cal.App.4th 1279, 1293 77 Cal. Rptr. 2d 296.) The right to equitable contribution "is predicated on the commonsense principle that where multiple insurers or indemnitors share equal contractual liability for the primary indemnification of a loss or the discharge of an obligation, the selection of which indemnitor is to bear the loss should not be left to the often arbitrary choice of the loss claimant, and no indemnitor should have any incentive to avoid paying a just claim in the hope the claimant will obtain full payment from another coindemnitor." (Fireman's Fund Ins. Co. v. Maryland Casualty Co., supra, 65 Cal.App.4th at p. 1295.) "In choosing the appropriate method of allocating defense costs among multiple liability insurance carriers, each insuring the same insured, a trial court must determine which method of allocation will most equitably distribute the obligation among the insurers 'pro rata in proportion to their respective coverage of the risk,' as 'a matter of distributive justice and equity.' As such, the trial court's determination of which method of allocation will produce the most equitable results is necessarily a matter of its equitable judicial discretion. " (Centennial Ins. Co. v. United States Fire Ins. Co. (2001) 88 Cal.App.4th 105, 111 105 Cal. Rptr. 2d 559.) "In keeping with the fundamental principle that a trial court has discretion to select a method of allocating costs among insurers with the aim of producing the most equitable results based on the facts and circumstances of the particular case, the courts have adopted a number of different ways of apportioning the burden among multiple insurers. These various methods have included, among others, the following: (1) apportionment based upon the relative duration of each primary policy as compared with the overall period of coverage during which the 'occurrences' 'occurred' (the 'time on the risk' method) ; (2) apportionment based upon the relative policy limits of each primary policy (the 'policy limits' method) ; (3) apportionment based upon both the relative durations and the relative policy limits of each primary policy, through multiplying the policies' respective durations by the amount of their respective limits so that insurers issuing primary policies with higher limits would bear a greater share of the liability per year than those issuing primary policies with lower limits (the 'combined policy limit time on the risk' method) ; (4) apportionment based upon the amount of premiums paid to each carrier (the 'premiums paid' method) ; (5) apportionment among each carrier in equal shares up to the policy limits of the policy with the lowest limits, then among each carrier other than the one issuing the policy with the lowest limits in equal shares up to the policy limits of the policy with the next-to-lowest limits, and so on in the same fashion until the entire loss has been apportioned in full (the 'maximum loss' method) ; (6) apportionment among each carrier in equal shares (the 'equal shares' method) ." (Centennial Ins. Co. v. United States Fire Ins. Co., supra, 88 Cal.App.4th at pp. 112-113.) "The costs of defense must be apportioned on the basis of equitable considerations not found in the insurers' own contracts, since the insurance companies who must share the burden do not have any agreements among themselves. The courts have expressly declined to formulate any definitive rules for allocating defense costs among carriers, because of the 'varying equitable considerations which may arise, and which affect the insured and the ... carriers, and which depend upon the particular policies of insurance, the nature of the claim made, and the relation of the insured to the insurers.' " (CNA Casualty of California v. Seaboard Surety Co. (1986) 176 Cal.App.3d 598, 619.) Questions as to whether the nonparticipating insurer breached the duty of good faith and fair dealing toward its insured or otherwise acted tortiously are not at issue in an equitable contribution action. (Id. at p. 621.) Equitable contribution does not depend on fault; it is based on an equitable apportionment of contractual undertakings. (Hartford Accident & Indemnity Co. v. Superior Court (1994) 29 Cal.App.4th 435, 441.)