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For businesses assessing commercial solar, the payback period is the point at which cumulative savings recover the capital invested in the system. It is a useful first measure of return, although it does not describe the full financial performance of an asset expected to operate for twenty years or more.
A credible return model begins with the installed cost and the electricity expenditure the system can realistically avoid. It then tests those cash flows against tariff escalation, solar production, self-consumption, financing, maintenance, equipment degradation and future component replacement. The answer is specific to the operation and to the commercial purpose of the project.
A facility with consistent daytime demand may recover its investment primarily through reduced grid purchases. A site with high peak-period charges may create additional value through load shifting and demand management. Another business may assign significant financial value to continuity when a grid interruption would halt production, damage stock or interrupt customer service.
How a commercial solar payback period is calculated
Simple payback divides the installed capital cost by the annual savings generated at the initial operating assumptions. A R12 million system producing R2.4 million in annual electricity savings would have a simple payback period of five years.
The calculation becomes reliable only when the savings estimate reflects the way the electricity account is structured. Solar generation can reduce grid energy purchased while the facility is operating. Battery storage may move energy into expensive tariff periods, reduce demand charges where the tariff allows and increase the amount of solar used on site. Fixed service, administration and network charges may remain payable and should not be counted as avoided expenditure.
The baseline also needs to be consistent. Comparing one unusually high electricity bill with an annualised solar estimate can overstate the return. A feasibility study should use sufficient historical data to capture seasonal production, operating changes, shutdown periods and variations in demand.
Simple payback is the starting point
Simple payback gives management an accessible measure of how quickly capital may be recovered. It gives the same weight to a rand saved in year one and a rand saved several years later, and it excludes the cash flows that continue after the payback date.
Net present value brings projected future cash flows into today’s money using an agreed discount rate. Internal rate of return shows the discount rate at which the project reaches a net present value of zero, allowing the investment to be compared with other uses of capital. A lifecycle model can also incorporate financing costs, maintenance, insurance, performance degradation, inverter or battery replacement and the expected residual value of the system.
The three measures answer different questions. Payback indicates when the initial capital is recovered. Net present value estimates the value created over the assessment period. Internal rate of return provides a percentage return that can be tested against the organisation’s investment hurdle rate.
The assumptions that move the return
Self-consumption is one of the strongest variables in a commercial solar model. Energy used as it is produced directly replaces an electricity purchase. Surplus generation has a different value and may be curtailed, stored or exported under the rules and rates available at the site. An optimistic export assumption can shorten the modelled payback without improving the actual operating result.
System yield also needs to move from a headline estimate to an engineering forecast. Orientation, shading, temperature, soiling, equipment efficiency, downtime and gradual panel degradation all affect annual production. The financial model should apply the same performance assumptions used in the technical design.
Capital structure changes the cash-flow profile. An outright purchase places the investment on the business’s balance sheet and allows it to retain the full operating saving. Financed ownership adds interest and repayment obligations. A power purchase agreement or lease can reduce the initial capital requirement, while the contract price, escalation terms, duration and performance obligations determine the resulting benefit.
Battery storage needs its own return case
A battery can improve the commercial performance of a solar project when it performs clearly defined functions. These may include peak shaving, tariff arbitrage, load shifting, increased use of surplus solar, power-quality support and continuity during interruptions. Each function should be quantified separately to avoid assigning the same saving to more than one part of the model.
The combined solar-and-storage payback may be longer than the payback on solar alone because the battery increases the initial investment. Its contribution can still support the broader business case where peak-period costs are high or an outage carries a measurable operational cost. Battery cycling limits, usable capacity, efficiency, degradation and likely replacement timing should be included in the assessment.
Electricity tariff increases change future cash flows
Following its revised determination, Eskom’s average tariff increase for standard direct customers is 8.76% for 2026/27, while NERSA approved an average increase of 8.83% for 2027/28. The increase that reaches an individual business still depends on whether it is supplied directly by Eskom or through a municipality, as well as its tariff category and demand profile.
A simplified sensitivity test shows the effect. If a business purchases R1 million of electricity a month and the entire amount increased first by 8.76% and then by 8.83%, the monthly cost would rise to approximately R1.184 million after the second increase at the same consumption level. Actual accounts will differ because fixed, energy and demand components do not necessarily move at the same rate, and municipal tariffs vary by distributor.
Solar reduces the volume of grid energy purchased and therefore part of the expenditure exposed to future energy-price increases. The ROI model should show the approved tariff path separately from longer-term escalation assumptions so that management can see how much of the projected return comes from current evidence and how much comes from the forecast.
The project objective determines the return model
Rentech’s Shoprite Centre project in Paulpietersburg illustrates how the commercial objective shapes the system. Rentech installed a 556.8 kWp grid-tied solar system with 375 kW of inverter capacity. The design used the available roof area to co-generate electricity with the grid and limit the centre’s exposure to rising electricity costs over time.
The financial assessment for this type of project begins with the electricity the centre can consume while the system is generating, the tariff value of those avoided purchases and the production expected from the installed capacity. Storage income and resilience savings should not be added when the installed architecture does not provide those functions.
A manufacturing site, cold-storage facility or logistics operation may present a different investment case even at a similar system size. Its operating schedule, demand charges, critical loads and cost of interruption can change both the technical configuration and the sources of return.
The cost of delay belongs in the same model
A delay moves the entire savings stream further into the future. If a fully assessed project is expected to save R2.4 million a year at its initial assumptions, a six-month delay in operation would defer approximately R1.2 million in projected savings. The eventual cost may differ as tariffs, equipment prices, exchange rates and financing terms change.
The comparison should use realistic project dates. Feasibility work, approvals, procurement, construction and commissioning are part of the delivery programme. A decision model can compare the cash flows from proceeding on that schedule with those from investing later, using the same technical scope and transparent assumptions.
A credible ROI model shows where the return comes from
The investment case should identify the baseline electricity cost, the tariff components the system can reduce, expected annual production, self-consumption, curtailment or export, battery dispatch where applicable, operating costs, degradation, replacement allowances, financing and the evaluation period. Base, downside and upside cases then show how the result changes when the principal assumptions move.
Rentech’s EPC process brings load analysis, tariff modelling, system design, procurement, construction, commissioning and ongoing support into one technical and financial assessment. The resulting payback period can then be considered alongside net present value, internal rate of return and the operating outcomes the energy system is designed to deliver.