You're on the right track thinking about hours in a month, but you need to consider peak demand and generator loading. Simply dividing the total kWh by the hours in a month gives you the average power consumption, which isn't what you need for generator sizing.Here's why and how to do it properly

Why Averaging Doesn't Work
Peak Demand

Your customer likely has periods of much higher power consumption than the average. A generator needs to handle the highest demand, not just the average. Think about when they're running air conditioning, appliances, and machinery simultaneously.

Generator Loading

Generators are most efficient (and last longer) when run at a certain percentage of their rated capacity (typically 50-75%). You don't want a generator that's barely working most of the time, or constantly overloaded.

How to Determine Generator Size from Monthly kWh Data

Unfortunately, using ONLY monthly kWh data gives you an estimate at best. The ideal method is to have a demand recording device installed at the customer's premises. This monitors peak demand over a period.Here's a breakdown of how to work with the monthly data, and the limitations

1. Calculate Monthly Average Power (kW):

As you've already done: kW_average = Total kWh in Month / Number of Hours in Month

For example: If you used 360 kWh in a month with 30 days (720 hours): kW_average = 360 kWh / 720 hours = 0.5 kW2. Estimate Peak Demand (kW) - The Tricky Part

This is where you need to make assumptions and use rules of thumb. Here are a few methods, from least accurate to slightly more accurate:

Rule of Thumb: Multiply the highest monthly average kW by a factor (Demand Factor)



Demand Factor

This factor accounts for the difference between average and peak power. The value depends on the type of load, industry, and building. A typical range for residential is 2-4, and for commercial buildings it's more likely to be 1.5-2.

Estimated Peak kW = Highest Monthly kW_average Demand Factor Example

Let's say the highest monthly average was 0.6 kW, and you use a demand factor of 3: Estimated Peak kW = 0.6 kW 3 = 1.8 kW

Reviewing Electricity Bills

Sometimes, utility bills will list the peak demand for the month, in addition to the total kWh. If you can find this, it's a much better starting point than estimating with a demand factor. Take the highest peak demand from the 12 months.

3. Account for Generator Loading

Determine the desired loading percentage of the generator. Let's assume you want the estimated peak kW to be about 75% of the generator's rated output.

Generator Size (kW) = Estimated Peak kW / Desired Loading Percentage Example

Using the peak kW of 1.8 kW, and a 75% loading: Generator Size = 1.8 kW / 0.75 = 2.4 kWRound this up to the next commercially available generator size; generators are usually sold in standard sizes, such as 2.5kW, 3kW, etc.

4. Account for Motor Starting (If Applicable)

If the customer has large motors (e.g., air conditioning compressors, pumps), you need to account for the inrush current (or starting surge) that motors draw when they start. This inrush can be significantly higher than the motor's running current. This is usually dealt with by selecting a generator with a higher kVA (kilovolt-amperes) rating than the kW rating. kVA represents the apparent power and is important for handling reactive loads like motors. A good rule of thumb is to calculate the total motor nameplate horsepower and multiply it by 2.5 to get the kVA required to handle motor starting. Convert kVA to kW by multiplying kVA by the power factor of the load, typically around 0.8 for many industrial loads.

Important Considerations and Recommendations Safety Factor

Add a safety factor (e.g., 10-20%) to the final generator size. This provides some headroom for future load growth or unexpected demand spikes.

Load Profile

The best way to size a generator is to get a load profile. This is a detailed record of power consumption over time, showing when the peaks occur and how long they last. A load profile is usually obtained from a demand recording device, or by manually recording readings from a utility meter at short intervals (e.g., every 15 minutes) for a representative period (at least a week, ideally longer).

Generator Type

Consider the type of generator:

Standby Generator

Designed to provide power only during outages. These are typically sized for peak demand.

Prime Power Generator

Designed for continuous operation. These require more careful sizing to avoid overloading.

Professional Consultation



Highly recommended! Generator sizing is complex and requires expertise. It's best to consult with a qualified electrical engineer or generator specialist. They can assess the customer's specific needs, analyze the load profile (if available), and recommend the appropriate generator size and type. They can also ensure that the generator is installed and maintained correctly.* Future Needs

Consider whether the customer's power needs are likely to increase in the future. It's often better to size the generator slightly larger than currently needed to accommodate future growth.

In summary: Your monthly kWh data is a starting point, but it's not sufficient for accurate generator sizing. You need to estimate peak demand and account for motor starting (if applicable). A load profile is the ideal way to determine generator size, and consulting with a professional is highly recommended. Flag for review