Choosing a generator isn't just about choosing the biggest or cheapest option. It's about choosing a unit that has the right capacity for your home, business, or facility's real needs. If you choose the wrong size, you'll either end up paying too much for unnecessary power, or you'll end up with a generator that can't handle your equipment during a power outage.
This guide will show you step by step how to calculate exactly the right size generator.
Why is generator size important?
A generator that is too small will struggle to run high-load equipment (such as air conditioners, pumps, or industrial motors), overheat under load, and wear out faster. A generator that is too large will waste fuel, cost more upfront, and run inefficiently at half load. The right size protects your equipment, your budget, and your peace of mind.
Step 1: Make a list of everything you need to add
Start by making a complete list — write down all the equipment or appliances that need to work during a power outage. Divide them into two categories:
Necessary loads — Things that must work (refrigerators, lighting, security systems, medical equipment, servers, critical equipment).
Additional loads — Things that are desirable but not critical (air conditioning, unnecessary sockets, decorative lighting).
Step 2: Understand the difference between working watts and starting watts
This is where most errors in sizing occur. Every electrical appliance has two power ratings:
Working watts — The power it takes to continue working.
Surge watts — The extra power required at the moment of switching on, in a fraction of a second. Motors, compressors, and pumps can require 2-3 times the operating watts just to turn on.
If you only calculate the working watts and ignore the starting watts, your generator may not be able to handle the moment you turn on the refrigerator or air conditioner.
Step 3: Calculate total power requirement
Add together the operating wattages of all the devices you plan to run at the same time. Then identify the largest initial wattage demand of your equipment (usually a motor-driven device) and add that to your operating total. This will give you the peak demand.
Example calculation for a small business:
- Refrigerator unit: 1,200 operating watts / 3,600 starting watts
- Lighting (LED): 400 working watts
- Computers and POS systems: 600 working watts
- Security system: 200 working watts
Total running watts: 2,400 watts. Add the refrigerator's initial wattage (3,600 watts) to the running watts, and your peak demand will be about 4,800 watts — meaning you'll need a generator that comfortably exceeds 5 kW, safely, with future needs in mind.
Step 4: Add safety stock
Never select a generator with exactly 100% of your calculated load. Industry best practice recommends leaving a stock of 20-25%. This protects the generator from overloading, extends its life, and gives you room to add equipment in the future without having to upgrade.
Stage 5: Consider your growth plans
If your business is growing, or you, as a homeowner, are planning renovations, choose a generator based on where you will be in 3-5 years, not just today. It is much more economical to install the right capacity once than to replace an inadequate unit later.
Standard generator dimensions and areas of application
- 35-75 sq.m. — Small businesses, shops, residential complexes, small workshops
- 75-150 sq.m. — Hotels, clinics, medium-sized commercial buildings, construction sites
- 150-220 sq.m. — Industrial facilities, large commercial enterprises, factories
These are general guidelines — the exact size always depends on your specific equipment and initial loads.
If in doubt, seek professional advice.
Sizing a generator is more than simple arithmetic—factors such as voltage stability, power factor, altitude, and ambient temperature all affect actual performance. Miscalculation can lead to costly downtime or premature equipment damage.
At Algio Generator, our team conducts a site assessment and load calculation before recommending a generator — so you get a system that is sized right the first time — neither too small nor too large.
How to choose the right size generator: A complete guide
Don't know what size you need? Contact our team For a free consultation, we will help you calculate the right capacity for your home or business.