Solar System Sizing Mistakes That Make Customers Waste Money

I’ve seen customers spend a lot of money on solar and still complain that the system isn’t doing what they expected.

Sometimes the problem is the inverter. Sometimes it is the battery. Sometimes it is poor installation.

But quite often, the problem started before any equipment was purchased.

It started with the system sizing.

A solar system can be built with good panels, a good inverter, a good battery and good cables and still perform badly if the system was not properly sized for the customer’s actual needs.

And this is where customers lose money.

They buy more equipment when the real problem is that the original system was never designed correctly.

Here are some of the sizing mistakes I believe customers should watch out for.

1. Sizing the system from the number of appliances alone

A customer may say:

“I have two TVs, three fans, a fridge, five bulbs, a washing machine and an AC.”

That information is useful, but it is not enough to design the system.

I need to know how those appliances are actually going to be used.

A 1HP air conditioner that runs for eight hours every day is very different from one that runs for two hours.

A refrigerator does not normally consume its rated power continuously because the compressor cycles.

Five fans running all night are different from five fans running for three hours in the evening.

The customer’s usage pattern matters just as much as the appliance list.

This is why I don’t like sizing a system simply by adding the wattage written on every appliance and calling the result the solar load.

That can produce a number, but it doesn’t necessarily produce a useful system design.

2. Choosing the inverter size without properly considering simultaneous loads

This is another common mistake.

Someone sees that the customer’s appliances add up to roughly 4kW and immediately recommends a 5kVA inverter.

It sounds reasonable.

But what happens when several appliances start at the same time?

Motors and compressors can have starting characteristics that are very different from their normal running consumption.

There is also a difference between the inverter’s apparent power rating and the actual real power it can continuously deliver.

Then there is power factor.

So I don’t just ask:

“How many watts are the appliances?”

I ask:

“What is likely to run at the same time, and what happens when those loads start?”

The inverter needs to be able to handle the customer’s real operating conditions, not just a number written on paper.

3. Sizing the battery from inverter capacity

This one causes a lot of confusion.

A 10kVA inverter does not automatically require a particular battery capacity.

The inverter tells me how much power the system can potentially deliver.

The battery determines how much stored energy is available and how long that energy can support the loads.

Those are different things.

A customer may have a large inverter but a relatively small battery.

The inverter may be perfectly capable of carrying the load, but the battery may not last very long.

For example, there is a huge difference between asking a battery to support a 2kW load for a short period and asking it to support 2kW for most of the night.

The question is not simply:

“What size inverter do I have?”

The better question is:

“How much energy do I need the battery to provide, and for how long?”

4. Forgetting usable battery capacity

This is where some quotations become misleading.

If someone tells you that you have a 10kWh battery, that does not necessarily mean you should plan to use 10kWh of AC energy from it.

There are losses in the system.

There may also be manufacturer-defined operating limits, battery management limits and other factors that affect usable energy.

So when I calculate battery requirements, I don’t treat the battery’s nameplate capacity as if every bit of it will appear at the appliances.

The distinction between battery capacity and usable energy matters.

Otherwise, the customer may be promised eight hours of backup and get something very different.

5. Adding solar panels until the inverter looks “big enough”

I’ve seen people approach PV sizing almost like this:

“It’s a 5kVA inverter, so let’s put 5kW of panels.”

Or:

“The customer wants more power, so just add more panels.”

It doesn’t work that way.

The inverter has specific PV input requirements.

You need to check things such as:

• Maximum PV input power
• MPPT operating voltage range
• Maximum PV open-circuit voltage
• Maximum PV input current
• Number of MPPTs
• Panel Voc and Vmp
• Panel Isc and Imp
• Expected temperature conditions
• Correct series and parallel configuration

You can have the correct total panel wattage and still have the wrong string configuration.

And getting the PV voltage wrong can damage equipment or cause the system to operate outside its intended range.

6. Ignoring temperature when designing PV strings

This is something I wish more people paid attention to.

Solar panels don’t have one fixed voltage under every condition.

The open-circuit voltage changes with temperature.

In colder conditions, Voc can rise.

That means a string that appears safe based only on the panel’s nominal specifications can become a problem when the temperature conditions change.

So when I’m checking a PV design, I don’t only look at the panel count.

I look at the actual panel specifications and the inverter’s allowable PV voltage, then consider the expected operating temperature.

The goal isn’t simply to make the panels produce power.

The goal is to configure them safely within the inverter’s operating limits.

7. Sizing panels without considering the customer’s daily energy requirement

This is one of the biggest mistakes.

A customer may have a 10kWh battery, but if the solar array is too small to properly recharge it after normal use, the customer will eventually become frustrated.

The battery gets used at night.

The panels recharge it during the day.

If the customer’s daily energy requirement is greater than what the PV system can realistically replace, the system can continually operate with a battery that isn’t fully recovering.

And when there are cloudy days, shading, dust, heat and other real-world conditions, the situation can become even worse.

That’s why I don’t size panels based only on the battery.

I look at the energy the customer actually needs each day and then work backwards to determine an appropriate PV array.

8. Assuming the panel’s rated wattage is what it will produce all day

A 550W panel is rated at 550W under specified test conditions.

That doesn’t mean you will see 550W from it from morning until evening.

Real-world production changes with:

• Sun intensity
• Temperature
• Shading
• Dust
• Panel orientation
• Tilt
• Cable losses
• System losses
• Weather conditions
• MPPT operating conditions

So if somebody tells a customer:

“You have 5.5kW of panels, so you’ll get 5.5kW throughout the day.”

That’s not a realistic way to explain solar production.

PV sizing has to account for real operating conditions.

9. Ignoring the customer’s future loads

I also ask customers what they intend to add later.

Maybe they currently have:

• Fridge
• TVs
• Fans
• Lights
• Laptops

But they are planning to add:

• Air conditioners
• Water pump
• Washing machine
• Electric cooker
• More refrigeration
• Borehole system

That matters.

It doesn’t mean I should automatically oversize everything massively.

But the future plan should be considered before choosing the architecture of the system.

Sometimes it makes sense to design for expansion.

Other times, the customer is better off installing a properly sized system now and upgrading later.

The important thing is to make that decision deliberately.

10. Using the same system design for every customer

This is one of the worst habits an installer can develop.

Someone installs a particular combination of inverter, battery and panels for one customer.

It works.

Then the next customer asks for solar.

The installer simply copies the same design.

Different house.

Different appliances.

Different usage.

Different roof.

Different budget.

Different battery requirements.

Different inverter.

Different environmental conditions.

Yet somehow the same design is recommended.

Solar systems are not templates.

A design that worked perfectly for one customer may be completely wrong for another.

Experience should help an installer design faster.

It should not replace the design process.

11. Sizing based on the customer’s budget instead of the customer’s requirement

This is a difficult one because customers have budgets.

Someone may say:

“I only have ₦3 million. Design something for me.”

That’s fine.

But the answer shouldn’t be to pretend that ₦3 million can buy a system capable of doing what a ₦7 million system would do.

The honest approach is to establish what the customer actually wants, determine what the properly sized system would require, and then find the best practical configuration within the budget.

Maybe some loads need to be excluded.

Maybe the battery needs to be smaller.

Maybe the customer should install in phases.

Maybe some high-energy appliances should remain on the grid or generator.

That is much better than squeezing everything into a quotation and hoping the customer won’t notice the limitations later.

12. Forgetting cable losses and voltage drop

Sizing isn’t finished when the inverter, battery and panels have been selected.

The cables also have to be appropriate.

Long cable runs, high currents and undersized conductors can create significant voltage drop and losses.

This becomes particularly important on battery connections because low-voltage systems can involve very high currents.

A system can therefore have the “right” inverter and battery on paper but still perform poorly because the installation between them was not properly designed.

The cable isn’t just a piece of wire connecting two components.

It is part of the system.

13. Leaving protection out of the original design

Another mistake is treating protection as an optional accessory to be added if there is money left.

DC isolators, appropriate overcurrent protection, surge protection, AC protection, earthing and other protective measures should be considered as part of the system design.

The exact protection arrangement depends on the system architecture and equipment specifications, but the principle is simple:

Don’t design the power system first and think about safety afterwards.

Protection is part of the design.

14. Oversizing everything “just to be safe”

Oversizing isn’t automatically good engineering.

Sometimes it is justified.

Sometimes it is simply wasteful.

I’ve seen customers with systems that have far more inverter capacity or battery capacity than their actual requirements, while another part of the system is poorly matched.

A bigger inverter does not automatically give longer backup.

A bigger battery does not automatically solve poor PV production.

More panels do not automatically solve an incorrectly configured MPPT.

The goal is not to make every component as large as possible.

The goal is to make the components work together properly.

15. Not explaining the limitations to the customer

This is probably the most expensive mistake because it creates unrealistic expectations.

If the system is designed to carry lights, fans, refrigerator, TV and a small amount of daytime load, the customer should know that.

If the air conditioner is not included, say so.

If the electric cooker cannot run from the system, explain it.

If running the water heater will significantly reduce battery backup, explain it.

If the battery is expected to last six hours under a particular load, explain the assumptions behind that figure.

A properly sized system can still disappoint a customer if nobody explained what it was designed to do.

What I look at when sizing a solar system

When I approach a solar sizing job, I don’t start by asking:

“How many panels do you want?”

I start with the customer’s actual requirement.

I want to understand:

What appliances are involved?

How much power do they consume?

How long are they used each day?

Which ones run simultaneously?

Which loads have motors or compressors?

How much backup does the customer expect?

What should run during a power outage?

What is the available roof or installation space?

What equipment is being considered?

What are the inverter’s actual specifications?

What battery technology and capacity are being used?

How much PV can the inverter safely accept?

Are there plans for future expansion?

Once those questions are answered, the system starts to make sense.

The inverter can be selected based on the expected load.

The battery can be sized based on the required energy and backup duration.

The PV array can be sized based on energy requirements and the available solar resource.

Then the cables, protection, mounting and other components can be selected to suit the design.

That’s a much better process than starting with a random number of panels and trying to make everything else fit around it.

The customer isn’t always wasting money by buying too little

This is an important point.

People often think wasting money means buying an undersized system.

Not necessarily.

An undersized system can waste money because the customer eventually has to upgrade components or continues relying heavily on the grid or generator.

But an oversized system can also waste money.

If the customer doesn’t need a huge inverter, enormous battery bank or unnecessarily large PV array, paying for them doesn’t automatically improve the system.

The best value is not the cheapest system.

It is the system that is properly matched to the customer’s actual needs.

Before you approve that solar quotation

If you’re a customer, don’t just look at the final price.

Ask the installer to explain the design.

Ask:

• What load was used for the calculation?
• What is the expected daily energy consumption?
• What inverter size was selected and why?
• What battery capacity was selected?
• What backup duration should I realistically expect?
• How much PV is being installed?
• How are the panels configured?
• Are the PV voltage and current within the inverter’s limits?
• What loads can run simultaneously?
• What loads should not run together?
• What protection is included?
• What happens if I add an air conditioner later?
• What are the limitations of the system?

A good installer should be able to explain these things without getting defensive.

You don’t need to be a solar engineer to ask questions about a system that you’re spending millions of naira on.

Final thought

I’ve come to realise that many solar problems don’t actually begin when the inverter starts beeping or the battery stops lasting.

They begin much earlier.

They begin when someone says:

“Just give me a 5kVA system.”

Or:

“Use the same design you installed for that other customer.”

Or:

“Put 10 panels there. It should be enough.”

Solar isn’t guesswork.

There is nothing wrong with using experience, standard configurations or equipment that has worked well before. But experience should guide the calculation, not replace it.

A properly sized solar system doesn’t have to be the biggest system.

It doesn’t have to be the most expensive system.

It simply has to make sense for the person who is going to use it.

Because when the sizing is wrong, the customer doesn’t just lose performance.

They pay for the mistake.