How to Size a Generator for Your Home or Small Business

Sizing a generator correctly is one of the most important steps in choosing backup power. A generator that is too small may overload, while one that is oversized adds unnecessary cost. This guide explains how to size a generator for a home or small business in clear, practical steps.

Step 1: List the loads you want to run

Start by listing the appliances, equipment, or circuits you want powered during an outage. Focus on essentials first, such as refrigeration, heating systems, pumps, lighting, and communications.

Step 2: Identify running watts

Running watts represent the power required to keep equipment operating. These values are often found on the equipment nameplate or in manufacturer documentation.

Step 3: Account for starting watts

Some loads require extra power to start, especially motors, compressors, and pumps. This starting surge is brief but must be accounted for when sizing a generator.

Step 4: Add a safety margin

Adding a safety margin helps prevent overload and allows for future expansion. A margin of 20 percent is common for residential and small commercial applications.

Step 5: Select voltage and phase

Most homes use 120/240V single phase power. Small commercial sites may use three phase power at 208V or 480V. Generator size and ATS selection depend on this configuration.

What size ATS do I need?

Transfer switches are typically sized in amperage. In standard standby installations, the ATS must safely carry the current supplied by the generator. The correct size depends on voltage, phase, and how the generator is connected.

Do solar systems need an ATS?

Many solar and inverter-based systems do not use a traditional ATS. Instead, the inverter controls generator connection and limits current automatically. Always confirm how your system manages transfer before purchasing equipment.

Next steps

Once you understand your generator size, you can compare available equipment and confirm installation requirements.

Browse our commercial and industrial diesel generators or use the generator sizing calculator to get started.