What it means
The defining feature is the shape of the cost curve. Building the network, whether pipes, cables or track, costs an enormous amount up front, while connecting one more household costs very little, so average cost per customer keeps falling as the customer base grows.
Duplicating that network is wasteful. If two firms each lay their own water mains down the same street, both carry the full fixed cost while serving only half the households, so both end up with higher costs than a single provider would have had.
That is the economic argument for accepting one supplier and controlling its prices instead. Regulation usually takes one of three forms: a cap on the price the operator may charge, a cap on the return it may earn on its asset base, or public ownership of the network itself.
Each approach tries to leave the operator enough profit to keep investing while stopping it from charging whatever a captive customer would be forced to pay. A common modern variant splits the business in two.
The network stays a regulated monopoly while the services running over it are opened to competition, which is how many countries handle electricity, telecoms and rail. Consumers choose their retailer even though only one set of wires reaches the house.
Natural monopolies are not permanent. Technology can erode them, as mobile networks did to fixed-line telephony and as rooftop solar and batteries are slowly doing to parts of the electricity grid.
Managers in these industries watch closely for the point at which the fixed-cost advantage stops being decisive.
In practice
Real-world examples.
Example
A city water authority is the only supplier of mains water to 400,000 homes because no rival could realistically dig a second set of mains. The regulator therefore sets a five-year price control that allows a 4% real return on the authority's asset base. Customers get no choice of supplier, but they do get a price ceiling.
Example
A national rail infrastructure company owns every metre of track and signalling in the country. Train operators compete for passengers and pay access charges to use the track, which keeps the monopoly layer regulated while the service layer stays contestable.
Example
A telecoms provider is the only firm willing to lay fibre to a village of 900 homes, because the $2,700,000 build cost could never be recovered twice over. The government awards a subsidised contract on condition that rivals may rent capacity on the new line at published wholesale rates.
Formula
Calculation
Average cost per customer = (total fixed cost + variable cost per customer x number of customers) / number of customers.
Take a regional water network with fixed costs of $500,000,000 a year covering pipes, plants and maintenance, plus $200 a year of variable cost for each household served. If a single operator serves all 1,000,000 households, total cost is $500,000,000 + ($200 x 1,000,000) = $700,000,000, so average cost per household is $700,000,000 / 1,000,000 = $700.
Now let two operators build competing networks, each serving 500,000 households. Each still carries the $500,000,000 of fixed cost, so each has a total cost of $500,000,000 + ($200 x 500,000) = $600,000,000 and an average cost of $600,000,000 / 500,000 = $1,200 per household. Competition has pushed the cost of supply up by $500 per household, which is the signature of a natural monopoly.Case study
Seen in the real world.
Northmere Grid Company is an invented business used here purely as an illustrative example. Northmere owned the only electricity distribution network across a rural county, serving 260,000 connections from an asset base of $1,800,000,000, at a network cost of about $520 per connection each year.
When a challenger proposed building a parallel network in the county's three largest towns, Northmere's analysts modelled the result. The challenger would take the 90,000 densest connections, leaving Northmere's largely fixed network costs of $135,200,000 spread across 170,000 customers instead of 260,000. Average network cost for the remaining, mostly rural, customers would climb from $520 to roughly $795.
The regulator accepted the analysis and refused the second licence, tightening Northmere's price control and requiring it to publish open access terms instead. This fictional case shows the standard regulatory bargain: one network, prices set by the regulator, and access for anyone who wants to sell services over it.
Watch out
Common mistakes.
- Calling any dominant firm a natural monopoly. The label applies only where a single supplier genuinely has lower costs than several would, which is a claim about cost structure rather than about market share.
- Assuming a regulated monopoly is guaranteed its profit. Price controls set allowed revenue, not actual profit, and an operator that overspends on its network absorbs the difference itself.
- Treating a whole utility industry as monopolistic. In most utilities only the network is a natural monopoly, while metering, retail supply and maintenance can all be competitive.
Questions
People also ask.
Why not break a natural monopoly into competing firms?
Splitting the network would duplicate the fixed costs and raise the average cost of supply, which is the opposite of what competition is supposed to achieve.
How do regulators decide what price to allow?
Most use a building-block method: an allowed return on the asset base, plus depreciation, plus efficient operating costs, divided by expected volumes.
Can a natural monopoly disappear over time?
Yes, new technology can shrink the fixed-cost advantage, as mobile and satellite services have done to parts of the fixed-line network.
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