Technical guide, , by Green Automatica engineering team · 3 min read
How to size a solar system for a factory or commercial building
A practical, step-by-step method for estimating solar PV and battery size from your own consumption data, with a worked example.
Most underperforming commercial solar systems were sized from a single electricity bill and a sales target. A better system starts with how and when your site actually uses power. This guide shows the method we use, so you can check any proposal you receive.
Step 1: collect twelve months of bills
Bills show your monthly consumption (kWh), maximum demand (kVA or kW) and any power factor charges. Twelve months reveal seasonal changes, such as harvest peaks or cooling loads in hot months.
Step 2: measure your daytime load
Solar produces power in daylight. What matters is how much of your consumption happens between roughly 08:00 and 17:00. A power logger on the main incomer for at least one representative week shows your load profile hour by hour, including weekends.
If most consumption is at night, a solar system without batteries will save far less than the annual total suggests.
Step 3: reduce waste first
Every kilowatt-hour you stop wasting is one you do not need solar to produce. Correcting power factor, fixing compressed air leaks and adding drives to pumps and fans are usually cheaper per kWh than additional panels. See our energy audit service.
Step 4: estimate the array size
In many parts of Kenya, each kilowatt-peak (kWp) of well-installed, unshaded panels produces roughly 4 to 4.5 kWh per day averaged over the year. Your site's orientation, tilt, shading, dust and temperature all change this, so confirm with site data.
| Worked example | Value |
|---|---|
| Weekday daytime consumption (measured) | 400 kWh per day |
| Assumed yield per kWp | 4.2 kWh per day |
| Array size to cover daytime load | 400 ÷ 4.2 ≈ 95 kWp |
If you cannot export surplus power, size the array to your lowest regular daytime load (for example weekends), or add batteries or export limitation. Otherwise you pay for generation you cannot use.
Step 5: size batteries only for a clear purpose
Batteries are justified when you need backup during outages, want to shift solar into the evening, or want to replace generator hours. Size them for the critical loads and backup time you actually need:
Battery capacity (kWh) ≈ critical load (kW) × backup hours ÷ usable depth of discharge ÷ inverter efficiency
Example: 20 kW of critical load for 3 hours, with 90% usable depth of discharge and 95% inverter efficiency, needs about 20 × 3 ÷ 0.9 ÷ 0.95 ≈ 70 kWh of battery.
Step 6: check the site and the connection
- Is the roof structure strong enough, and how many years of life does it have left?
- Is there shading from tanks, parapets or neighbouring buildings?
- Where will inverters sit, and how far is it to the main switchboard?
- How will solar work with your generator and changeover?
What a good proposal should include
- Your load profile and the assumptions used
- Expected annual generation and how much of it you will use on site
- A single-line diagram showing protection and connection
- Equipment datasheets and warranties
- Savings and payback based on your real tariff
The figures in this guide are illustrative estimates, not a design. A qualified engineer should confirm sizing from measured data. Request a site assessment and we will size a system from your own load profile.