Battery Sizing Guide

1. Battery capacity versus battery power

A home storage battery has two important characteristics:

• Capacity in kilowatt hours (kWh) — how much energy it can store.
• Power in kilowatts (kW) — how quickly it can deliver that energy.

Capacity tells you “how long”, power tells you “how fast”. A 5 kWh battery can, in principle:

• Run a 1 kW load for about 5 hours, or
• Run a 2.5 kW load for about 2 hours, or
• Run a 5 kW load for about 1 hour

In practice there are limits from the battery manufacturer, and the inverter rating also matters, but this is the basic idea.

2. What “backup hours” means in SolarPVSizer

In the wizard you choose how many backup hours you would like. SolarPVSizer interprets this as:

Backup hours are based on your average daily electricity use, spread over the number of hours you select. The tool does not assume that you run your inverter at full rated power for the entire backup period.

In other words, if you select 4 hours of backup, the calculator assumes that during those 4 hours your usage will look similar to your normal average, not like every appliance in the house running at once.

This reflects how most people actually use backup systems: a mix of light and heavier loads over time, rather than a constant full load.

3. The basic sizing step

The tool starts with your monthly electricity use, converts this to a daily figure, then applies your chosen number of backup hours. A simplified version of the logic is:

1. Convert monthly kWh to daily kWh.
2. Scale this by backup hours as a fraction of 24 hours.
3. Adjust for usable depth of discharge (for example 80% usable).

The result is a theoretical minimum energy capacity that would support your typical usage for the requested backup hours.

4. Why the report may show “small” batteries for light users

For very low usage homes, the maths can produce quite small numbers. For example, a very efficient flat with low monthly consumption and only 2 hours backup may need well under 1 kWh in theory.

In reality, home storage batteries are sold in discrete sizes, and in many markets:

• Common modules start around 2.4 kWh to 5 kWh.
• Hybrid inverters below 1 kW are rare.

SolarPVSizer therefore applies internal minimums and other constraints to avoid suggesting unrealistically tiny packs. The number you see in the report is rounded and nudged to stay in a realistic range for real products.

5. How this relates to your inverter size

In your report, the inverter and battery are sized using different parts of your input:

• The inverter is based on how much power you may need at once.
• The battery is based on how much energy you typically use and the backup hours you chose.

This means that the battery will not always be large enough to run the inverter at full power for the entire backup period. That would usually require a much larger and more expensive battery bank.

The calculator instead assumes that your actual usage during outages is closer to your average, and that you will turn off non-essential loads when needed.

6. Do you ever need to worry about C-rates?

Battery manufacturers specify how quickly their products may be charged and discharged. This is often expressed as a C-rate. For example:

• A 5 kWh battery with a 1C discharge rating can deliver 5 kW continuously.
• The same battery with a 0.5C rating should only deliver 2.5 kW continuously.

SolarPVSizer does not model individual battery brands or pack configurations. Instead, it aims to keep battery suggestions in a range where typical modern lithium batteries, when correctly installed, can support the inverter sizes suggested in the report.

If you plan to run near full inverter power for long periods, discuss continuous discharge ratings and C-rates with your installer when choosing a specific battery model.

7. Interpreting the backup hours in your report

When you look at the “Backup Hours” tile and the battery kWh figure together, keep the following in mind:

• The hours are based on average expected usage, not worst case peaks.
• Short periods of higher power draw are normal and are included in that average.
• If you deliberately run many large loads at once, the real backup time will be shorter.
• If you are careful and switch off non-essential loads, backup time may be longer.

In other words, the backup hours in the report are a reasonable planning figure, not a strict guarantee for every possible scenario.

8. When you might deliberately choose a different battery size

You and your installer may decide to choose a battery larger or smaller than the report suggests if:

• You have strict backup requirements for medical equipment or business loads.
• You prefer a “lights only” backup system and are happy with a smaller, cheaper battery.
• You expect to expand the system later and want a battery that can grow over time.

The value of the SolarPVSizer estimate is that it gives you a grounded starting point that aligns with your declared usage and backup expectations.