When designing a solar-plus-storage system, one of the most common questions is:

What is the best ratio between PV capacity, inverter power, and lithium battery capacity?

There is no single ratio that works for every project. The ideal configuration depends on the customer's electricity consumption, backup requirements, solar resources, battery charging strategy, and budget.

However, there are practical sizing ranges that can serve as a starting point.

1. PV to Inverter Ratio: Usually 1.2–1.5

The relationship between PV capacity and inverter capacity is commonly expressed as the DC/AC ratio:

DC/AC Ratio = PV Capacity (kWp) ÷ Inverter Capacity (kW)

For example:

• 130 kWp PV + 100 kW inverter → DC/AC ratio = 1.3
• 150 kWp PV + 100 kW inverter → DC/AC ratio = 1.5

A DC/AC ratio above 1.0 is common because solar panels rarely operate at their STC-rated power for long periods. Oversizing the PV array allows the inverter to operate closer to its rated output for more hours of the day.

NREL's utility-scale PV-plus-battery reference configuration uses a DC/AC ratio of approximately 1.34, and its research notes that ratios of 1.2–1.3 are often acceptable without significant curtailment.

Practical recommendation

For many commercial and industrial projects:

PV: Inverter ≈ 1.2–1.5: 1

A typical starting point is:

1.3 kWp PV: 1 kW inverter

However, the final ratio should be checked against the inverter's maximum DC input power, MPPT voltage range, and maximum input current.

2. Inverter to Battery: Focus on Battery Power, Not Only kWh

The battery has two important specifications:

• Battery power: kW
• Battery energy capacity: kWh

These are not the same thing.

For example:

100 kW / 215 kWh BESS

means the system can theoretically discharge at 100 kW, while the battery stores approximately 215 kWh of energy.

The approximate full-load discharge duration is:

215 kWh ÷ 100 kW = 2.15 hours

Therefore, when selecting the battery, you should first determine:

How much power does the customer need, and for how many hours?

For example:

In practice, additional capacity should be considered for battery DoD, conversion losses, temperature, aging and reserve SOC.

3. A Practical Starting Ratio for PV + Battery Systems

For a commercial or industrial solar-plus-storage project, a useful starting point is:

PV : Inverter : Battery Power ≈ 1.3 : 1 : 1

For example:

130 kWp PV + 100 kW inverter + 100 kW battery PCS

The battery energy capacity is then selected according to the required backup duration.

If two hours of backup are required:

130 kWp PV + 100 kW inverter + 100 kW / 200 kWh battery

If three hours are required:

130 kWp PV + 100 kW inverter + 100 kW / 300 kWh battery

This is only a preliminary sizing method. The final design should be based on the actual load profile and operating strategy.

4. How Much Battery Capacity Does a Solar System Need?

This is where many projects are incorrectly designed.

A 500 kWp PV system does not automatically require a 500 kWh battery.

Battery capacity should primarily be determined by:

1. Daily electricity consumption
2. Night-time energy demand
3. Peak load
4. Required backup duration
5. Grid outage frequency
6. Desired self-consumption rate
7. Electricity tariff and peak/off-peak pricing

For example, if a factory has:

• Peak load: 300 kW
• Average evening load: 150 kW
• Required backup: 4 hours

The battery requirement could be approximately:

150 kW × 4 h = 600 kWh

rather than simply matching the PV capacity.

5. Different Applications Require Different Ratios

Commercial & Industrial Self-Consumption

For reducing daytime grid consumption and shifting solar energy to the evening:

PV: Inverter ≈ 1.2–1.5: 1

Battery duration is commonly designed around 2–4 hours, depending on the load profile and electricity tariff.

Backup Power

For customers mainly concerned about power outages:

The battery power rating becomes more important.

For example:

500 kW load → approximately 500 kW battery power

Then determine the energy capacity according to the required backup time.

For 2 hours:

500 kW × 2 h = 1,000 kWh

So a preliminary configuration could be:

650 kWp PV + 500 kW inverter + 500 kW / 1,000 kWh BESS

Large Utility-Scale PV + Storage

Larger projects may use higher PV DC/AC ratios and multi-hour batteries. NREL's representative utility-scale PV-plus-battery configuration uses approximately 1.34 MW DC PV, 100 MW AC inverter capacity, and 240 MWh of usable battery energy, illustrating that the optimal ratio depends heavily on the system architecture and operating objective.

6. DC-Coupled vs. AC-Coupled Systems

The ideal ratio also depends on whether the battery is connected on the DC or AC side.

In a DC-coupled system, the battery can capture some PV energy that would otherwise be limited by the inverter. This can be particularly useful when the PV array is oversized relative to the inverter.

In an AC-coupled system, the PV system and battery system generally have their own power conversion equipment. This provides greater flexibility for adding storage to an existing PV plant, but introduces additional conversion stages.

Therefore, the same PV capacity may require different inverter and battery configurations depending on the coupling architecture.

7. A Simple Rule of Thumb

For preliminary commercial and industrial projects, you can use the following as a starting point:

PV capacity: 1.2–1.5 × inverter capacity

Battery power: approximately 0.5–1.0 × inverter capacity

Battery energy: battery power × required backup hours

For example:

1 MW Commercial System

A reasonable preliminary design could be:

• PV: 1.3 MWp
• Inverter: 1 MW
• Battery PCS: 500–1,000 kW
• Battery: 1–2 MWh for approximately 2 hours at 500–1,000 kW

The final configuration should then be optimized according to the customer's actual load curve.

Conclusion

There is no universal "perfect ratio" between solar panels, inverters and lithium batteries.

A practical starting point for many C&I projects is:

PV / Inverter = 1.2–1.5

Battery Power / Inverter Power = 0.5–1.0

Battery Energy = Required Battery Power × Backup Duration

But these ratios should never replace a proper system analysis.

The best design is the one that balances solar generation, inverter utilization, battery capacity, backup requirements, system efficiency and project cost.

For a professional PV + BESS proposal, the most important data to collect from the customer are:

PV capacity → Load profile → Peak load → Daily consumption → Backup duration → Grid conditions → Electricity tariff → Required operating mode

Once these parameters are available, the PV, inverter, and lithium battery capacities can be accurately optimized.