Arbitrage: Understanding Energy Cost Savings

Energy arbitrage is one of those terms that sounds more complicated than it is. At its core, the concept is simple: buy energy when it is cheap, store it, and use it when it is expensive. The same principle that drives commodity trading on financial markets applies directly to the electricity you use to run your home or business.

What has changed in the last five years is that battery storage technology has become affordable enough to make energy arbitrage genuinely practical for commercial and industrial businesses and increasingly for homeowners too. Pair a solar PV system with a properly sized battery, connect it to a time-of-use electricity tariff, and you have the basic architecture of an energy arbitrage strategy that can meaningfully reduce your electricity bills.

This article explains exactly how energy arbitrage works, what you need to implement it, how to calculate whether the numbers stack up, and where the opportunity is greatest right now.

Key definition: Energy arbitrage in the context of solar + battery storage means charging your battery during periods of low electricity tariff rates (typically off-peak hours or during peak solar generation), then discharging that stored energy during periods of high tariff rates avoiding the most expensive grid electricity entirely.

How Energy Arbitrage Works in Practice

Most grid connected electricity tariffs are not flat. They charge different rates at different times of day, reflecting the actual cost of generating and distributing electricity which is highest when demand peaks (typically early morning and early evening) and lowest when demand is low (overnight and midday).

These are called time-of-use (ToU) tariffs, and they create an immediate arbitrage opportunity for anyone who can shift their energy consumption away from peak periods.

Here is the basic sequence of how a solar + battery arbitrage system operates across a typical day:

1. Morning off-peak (midnight to 06:00): Grid electricity is cheap. If your battery is not already full from solar the previous day, the system can top it up from the grid at the lowest available tariff rate.
2. Morning peak (06:00 to 09:00): Grid tariffs spike. Your battery discharges to cover your loads, avoiding peak rate electricity entirely.
3. Midday solar window (09:00 to 15:00): Solar generation exceeds your consumption. Excess solar energy charges your battery for free rather than being exported to the grid at a low feed in rate.
4. Evening peak (17:00 to 21:00): The most expensive period on most tariffs. Your battery fully charged by solar discharges again, keeping you from using the grid during the costliest hours.
5. Night off-peak: Battery either holds remaining charge for early morning peak coverage or charges from cheap overnight grid power if needed.

The financial value of this strategy comes entirely from the tariff spread the difference between the cheapest off-peak rate and the most expensive peak rate. The wider that spread, the more value arbitrage delivers.

The Tariff Spread: Where the Money Is

The arbitrage opportunity varies significantly by location and tariff structure. Here are representative examples of the peak-to-off-peak spread in several markets:

Market	Off-peak rate (approx.)	Peak rate (approx.)	Spread
South Africa (Eskom ToU Megaflex)	R0.85/kWh	R4.20/kWh	R3.35/kWh
South Africa (Eskom ToU Homeflex)	R1.20/kWh	R3.80/kWh	R2.60/kWh
United Kingdom (Octopus Agile)	~£0.05/kWh	~£0.35/kWh	~£0.30/kWh
Australia (typical ToU)	~AU$0.15/kWh	~AU$0.55/kWh	~AU$0.40/kWh
Germany (dynamic tariff)	~€0.10/kWh	~€0.45/kWh	~€0.35/kWh
USA California (PG&E EV-B)	~$0.12/kWh	~$0.55/kWh	~$0.43/kWh

South Africa’s Megaflex tariff, applicable to large commercial and industrial users connected directly to the Eskom network, has one of the highest peak-to-off-peak spreads. This makes SA C&I businesses exceptionally well positioned for energy arbitrage strategies.

South Africa context: With the launch of SAWEM (the South African Wholesale Electricity Market), the dynamic pricing signals available to large energy users are becoming even more pronounced. Businesses with battery storage that can respond to real time price signals will have a significant cost advantage over those buying flat rate electricity.

Solar + Battery vs Battery Only: Why the Combination Wins

You can implement energy arbitrage with a battery alone charge from cheap overnight grid power, discharge during peak hours. This works, but the economics are limited by the fact that you are always paying for the energy you charge, even at off-peak rates.

Add solar generation and the economics change substantially:

• Free daytime energy: Solar generation during the midday window costs effectively zero per kWh (once the system capital is paid back). Charging your battery from solar rather than the grid eliminates the input cost entirely.
• Double displacement: Solar directly offsets daytime consumption while simultaneously charging the battery, which then offsets evening peak consumption. One solar system does two arbitrage jobs.
• Demand charge reduction: For commercial users on maximum demand tariffs, solar + battery reduces peak demand readings which can cut a separate and often very large component of the electricity bill that pure arbitrage does not address.
• Export revenue: In markets with favourable feed in tariffs, excess solar that exceeds both consumption and battery capacity can be exported for additional revenue a third revenue stream on top of arbitrage savings.

Worked Example: C&I Business in Johannesburg

Consider a medium sized manufacturing business in Gauteng with the following profile:

• Monthly consumption: 45,000 kWh
• Currently on Eskom Megaflex ToU tariff
• Peak demand: 120 kVA
• Operating hours: 06:00 to 18:00, Monday to Friday

Without solar + battery, approximately 40% of consumption falls in peak periods at R4.20/kWh, and 60% in standard/off-peak periods at R1.80/kWh average. Monthly electricity cost: approximately R113,400/month.

After installing a 200 kWp solar system and a 400 kWh BESS:

• Solar covers 70% of daytime consumption directly reducing grid draw during standard and peak periods
• Battery charges from solar midday and discharges during morning and evening peaks
• Peak grid consumption drops by approximately 65%
• Monthly electricity cost falls to approximately R52,000/month
• Monthly saving: approximately R61,400/month (R736,800/year)

At current installed costs for solar + BESS in South Africa (approximately R12,000–R15,000/ kWp all in for a system of this size), the capital cost would be in the region of R3.8–R4.5 million giving a simple payback period of 5.1 to 6.1 years on arbitrage savings alone, with a system life of 25+ years.

Important caveat: These figures are illustrative. Real savings depend on your specific tariff structure, consumption profile, load shape, solar resource at your location, battery sizing, and system design. A proper savings analysis requires a detailed load profile assessment not a back of envelope calculation.

What You Need to Implement Energy Arbitrage

A functional energy arbitrage setup requires four components working together:

1. A Time-of-Use Tariff

Without a tariff that charges different rates at different times, there is no spread to arbitrage. Check whether your current electricity supply agreement is a flat rate or a ToU structure. Most large Eskom connected businesses in South Africa are already on Megaflex or Nightsave ToU tariffs. Residential customers may need to request a ToU tariff from their municipality.

2. A Solar PV System (Recommended)

Not strictly required for arbitrage, but transforms the economics significantly. Size the solar array to cover a substantial portion of daytime consumption and provide sufficient surplus to charge the battery during the midday window.

3. A Battery Energy Storage System (BESS)

The battery is the core arbitrage tool. Key sizing considerations are:

• Capacity (kWh): Must be large enough to cover your peak period consumption after accounting for the solar contribution
• Power (kW): Discharge rate must match your peak demand. A battery with high capacity but low power output will not discharge fast enough during peak events
• Round trip efficiency: Expect 90–95% for modern LFP systems you lose 5–10% of energy in every charge/discharge cycle
• Cycle life: LFP batteries rated for 4,000–6,000 cycles at 80% depth of discharge are the current standard for commercial arbitrage applications

4. An Energy Management System (EMS)

The EMS is the intelligence layer that decides when to charge, when to discharge, and how to optimise between solar generation, battery state of charge, grid tariff periods, and your consumption profile. Without a well configured EMS, a solar + battery system will not automatically implement arbitrage it needs to be programmed with your specific tariff schedule and consumption patterns.

Most hybrid inverter systems from Sungrow, Huawei FusionSolar and Sigenergy include EMS functionality with ToU scheduling. For larger C&I systems, a dedicated EMS from suppliers like Encombi, DEIF or ComAp provides more sophisticated control including demand charge management and forecast based optimisation.

The Limits of Energy Arbitrage

Energy arbitrage is not a free lunch. There are real constraints to understand before sizing a system purely for arbitrage value:

• Cycle degradation: Every charge/discharge cycle degrades battery capacity slightly. A battery cycled daily for arbitrage will lose capacity faster than one used only for backup. Factor degradation into your long term financial model.
• Tariff changes: Electricity tariffs change sometimes significantly. An arbitrage strategy that looks excellent under current Eskom Megaflex rates may look different after a tariff restructuring. Model conservatively.
• Diminishing returns: Once you have eliminated most peak consumption, additional battery capacity delivers smaller incremental savings. There is an optimal battery size for every load profile beyond which the marginal benefit does not justify the marginal cost.
• Round trip losses: Charging from the grid at off-peak rates and discharging during peaks loses 5–10% of energy in the process. If the tariff spread is narrow, this loss can erode the economic case.

Energy Arbitrage vs Demand Charge Reduction: Know the Difference

These two strategies are often conflated but they target different components of your electricity bill:

Strategy	Targets	How it works	Best for
Energy arbitrage	Energy charges (R/kWh)	Shift consumption from peak to off-peak tariff periods	Sites on ToU energy tariffs with high peak/off-peak spread
Demand charge reduction	Demand charges (R/kVA/month)	Suppress peak demand readings using battery discharge	C&I sites billed on maximum demand with high demand charges

Many South African C&I businesses are billed for both which is why solar + BESS systems designed to address both strategies simultaneously typically deliver the strongest financial returns. A system optimised only for energy arbitrage may leave significant demand charge savings on the table.

Is Energy Arbitrage Right for You?

Energy arbitrage delivers the strongest returns when:

• Your electricity supply is on a time-of-use tariff with a wide peak-to-off-peak spread
• A significant portion of your consumption falls in peak tariff periods
• Your load profile is predictable enough for an EMS to optimise effectively
• You have adequate roof or ground space for a solar system sized to charge the battery
• Your consumption is high enough that capital costs are justified generally above 20,000 kWh/month for commercial applications

For residential users, the economics are thinner but improving as battery prices fall. South African homeowners on municipal ToU tariffs with solar + a properly configured battery are already achieving meaningful bill reductions through basic arbitrage even at smaller system sizes.

Conclusion

Energy arbitrage with solar + battery storage is not a speculative concept it is a commercially proven strategy being implemented across thousands of South African and global businesses right now. The combination of time-of-use tariffs, falling battery costs, excellent solar resources, and increasingly sophisticated energy management systems has made it one of the most financially compelling applications of distributed energy technology available today.

The fundamentals are straightforward: charge when electricity is cheap, discharge when it is expensive, and let solar generation fill the gap for free. The detail, tariff structure, system sizing, EMS configuration, and financial modelling is where specialist knowledge makes the difference between a system that delivers projected returns and one that underperforms.

In the next article in this series, we look at time-of-use tariffs in detail explaining exactly how Eskom’s Megaflex and Homeflex structures work, when the peak periods fall, and how to programme your inverter and battery to maximise savings automatically.

Running a business in South Africa and want to understand whether energy arbitrage stacks up for your specific situation? Leave a comment below or get in touch with the Green Future team.