Do You Need a Battery for a Solar Power System?

Solar panels and batteries perform different jobs.

Solar modules generate electricity when sufficient sunlight is available. A battery stores part of that energy so it can be used later.

For that reason, the question “Do I need a battery?” cannot be answered simply by looking at the solar array size. First, you need to define what the storage system is expected to do.

1. Use daytime solar energy in the evening

During the day, a solar system may produce more electricity than the home is using at that moment.

In the evening, solar production falls while people return home and switch on lighting, cooking appliances, air conditioning and other loads.

Battery storage can keep part of the daytime solar energy and make it available later.

The U.S. Department of Energy describes energy storage as a way to capture electricity and use it at a different time from when it was generated.

2. Backup power during outages

One of the clearest reasons to install storage is to keep selected loads running when the external grid fails.

There is an important detail, however.

Solar panels alone generally do not keep a home powered during a grid outage. Standard grid-connected systems are designed to shut down with the grid for safety.

To continue operating, the property needs a system capable of islanded operation: a suitable inverter, correct switching architecture and energy storage.

Not every battery and not every inverter automatically provides whole-home backup.

3. Define critical loads

Before selecting battery capacity, make a list of the devices that must keep operating during an outage.

For example:

— refrigerator; — internet equipment; — lighting; — heating controls; — pumps; — gates; — work computers; — medical equipment; — air conditioning.

Then determine their simultaneous power demand and the required backup duration.

That is much more useful than simply saying, “I want a 10 kWh battery.”

4. Understand the difference between kW and kWh

This is one of the most important concepts.

kWh describes stored energy.

kW describes power — how much load the system can supply at a given moment.

A 10 kWh battery stores a certain amount of energy, but that does not mean it can supply an unlimited load.

Real performance depends on battery output power, inverter power, depth of discharge, efficiency, temperature, backup settings and state of charge.

Energy capacity and power therefore need to be evaluated together.

5. Do you need a battery if the grid is reliable?

Not necessarily.

If outages are rare, much of the electricity is consumed during daylight hours, and the main goal is financial efficiency, a solar system without storage can be entirely reasonable.

The solar installation can reduce the amount of electricity purchased from the grid without adding the cost of a large battery system.

6. When storage becomes especially valuable

Battery storage is particularly useful when several of the following apply:

— grid outages are frequent; — there are important critical loads; — much of the property's demand occurs in the evening; — the customer wants higher solar self-consumption; — energy independence is important; — grid quality is a concern.

7. Larger is not automatically better

A battery that is too small may not meet the objective.

But an oversized battery can also be inefficient financially: it costs more and a significant portion of its capacity may rarely be used.

Storage size should therefore follow the use case.

A good design answers four questions:

1. What must remain powered? 2. What is the peak power of those loads? 3. How many hours should they operate? 4. How should the system recharge after an outage?

Only after that should the battery, inverter and system architecture be selected.

Sources

1. https://www.energy.gov/cmei/systems/homeowners-guide-solar
2. https://www.energy.gov/cmei/systems/solar-and-resilience-basics
3. https://www.energy.gov/cmei/systems/solar-integration-solar-energy-and-storage-basics