Two households in different parts of Lagos were sold the same inverter and roughly the same panels. One runs it happily for four hours a night. The other trips the inverter at eight o’clock most evenings. Nothing about the hardware differed. The difference was that the first household wrote a solar load list before ordering anything and the second guessed. Building that list takes an afternoon, costs nothing, and it is the only document that tells you whether a quotation is sensible or nonsense.

Why a solar load list comes before every equipment decision

A quotation is a chain of arithmetic. Daily energy demand sets the array, the largest simultaneous load sets the inverter, the hours of darkness before sunrise set the bank, and the heaviest starting current decides whether the unit trips or survives. Every one of those numbers comes out of the load list. When a supplier quotes panels before anyone has measured anything, the design has been done backwards, from the budget or the roof space rather than the requirement.

There is a second reason to do this on paper. A load list is the document you take with you when comparing quotations, and the one you use to check the work afterwards. It is also the fastest way to spot the two errors that cause most disappointment: an inverter chosen for the average load rather than the peak, and a bank chosen for capacity with no reserve in it. Both are arithmetic mistakes hidden behind a professional-looking invoice.

Build the list from the appliances you actually own, not the ones a brochure assumes. Note it down, because you will revise it. The first version takes an afternoon. The second, after the first long outage, usually removes half the list.

Watts and kWh answer different questions

Wattage is the speed of using power. Kilowatt hours are the amount consumed over time. Mixing them is the commonest sizing error in the Nigerian market, and the reason a family with a kettle and an iron can own a system that looks generous on paper and still collapses at dusk.

A nameplate gives you the input rating, the most the appliance draws when working hardest, not what it draws on average. A compressor fridge whose label says 200 W may average 70 W across a day. An air conditioner that peaks at 1,400 W may run at 500 W for most of an hour. Both numbers matter, and they belong in different columns.

Appliance	Indicative nameplate watts	Hours per day	Daily energy in kWh
LED bulb	12 W	6	0.07
Ceiling fan	60 W	8	0.48
Fridge freezer	200 W	24, cycling	0.85
Television	90 W	5	0.45
Laptop	65 W	6	0.39
Kettle	2,000 W	0.8 total use	1.60
Electric iron	1,000 W	0.5	0.50
Washing machine	500 W	1, on alternate days	0.25
Water pump, single phase	370 W	1	0.37

These are indicative figures to show the method, not a specification. Read the label on your own machines, and if it has faded, measure rather than trust a list on the internet.

Measuring instead of guessing

What the nameplate hides

A nameplate is a starting point rather than an answer. It gives you the maximum, says nothing about how long the appliance works, and on cheap imported equipment it is sometimes optimistic. Television sets and laptop chargers draw far less than the label suggests once the battery is charged. Old refrigerators in humid compounds draw more, because the compressor runs longer to hold the same temperature. A motor rated 0.5 horsepower delivers 370 W mechanically, and the electrical input is higher once efficiency is counted.

So treat the label as a ceiling and add your own observations. Note what runs at night, what runs only in the afternoon, and what has never worked without the generator. That last column is the most useful on the sheet.

The clamp meter and the meter reading

Two inexpensive tools turn guessing into measurement. A clamp meter that reads current is clipped around the live conductor of an appliance while it runs, and the wattage is read off as volts times amperes. Try it on a kettle, an iron, a television, a fridge compressor while it is running, and a pump while it is pumping. Five minutes each, and you have figures you can defend.

The second tool is the meter on the wall. Read it at the start and end of a normal evening, and again in the morning, then subtract. That single reading gives you the real energy of everything switched on, including the things you forgot. Many households find their measured evening use is double their estimate, almost always because of a second fridge, a security light or a sound system nobody counted.

Duty cycle: the number that decides the inverter

Duty cycle is the share of the time a load is actually working rather than standing idle. A fridge that runs for a third of the day has a duty cycle of roughly one third. Multiplying nameplate watts by hours and by duty cycle gives the figure that matters for energy, and the energy figure is what the array and the bank must supply. Dividing by the length of the solar day gives the array size.

But the inverter is not chosen from the average. It is chosen from the largest set of loads that will ever be on at once, plus the loads you will realistically add. Someone making tea while the iron is on while the fridge is starting is not unusual at nine in the evening. Add up the running watts of that combination and you have the continuous rating the inverter needs. How to size a solar inverter and battery bank works through the arithmetic in full.

Record two totals on the sheet: average daily energy in kilowatt hours, which sets the panels and the bank, and the maximum simultaneous running load, which sets the inverter. Keeping them apart is what stops a household buying a large bank with a small inverter, a common and expensive mistake.

Surge: where motors and compressors break a design

An induction motor does not start gently. At the instant the supply is applied, before the rotor turns, the motor presents what is called a locked rotor current, and it can be several times the running current. The inrush current lasts from a fraction of a second to a couple of seconds, then collapses as the motor reaches speed, but that fraction is all the inverter sees. The same applies to refrigerator compressors and to air conditioner units, where the compressor is the largest motor in the house.

Three ways to handle it. Buy an inverter with a surge rating above the locked rotor figure and leave yourself margin, because a datasheet figure is measured at a particular temperature and supply condition. Give the motor a soft start, which ramps the voltage over a second or two. Or fit a variable speed unit, the subject of our guide to running an air conditioner on solar. None is expensive next to an inverter that trips every evening at the same hour.

A worked list from a three-bedroom flat

Take a typical family home: four LED bulbs, two fans, a fridge freezer, a television, a laptop, a kettle, an iron, a washing machine and a small borehole pump. Summing the table above gives roughly 5 kWh of daily energy. The evening window from six to eleven, when the array is producing nothing useful, carries lights at about 72 W, fans at 120 W, a television at 90 W and a fridge pulling an average of 70 W while cycling, with the kettle and iron used occasionally. That is a running evening load near 350 W and a peak above 2,000 W the moment the kettle switches on.

Read those numbers properly. The 350 W figure tells you the inverter must handle at least that continuously, and the 2 kW peak plus the pump surge tells you what it must survive. A unit that runs 2,000 W continuously and starts a 370 W pump without complaint is adequate on paper, and you should still add margin for the second fridge you will eventually buy. The three-bedroom sizing walkthrough takes this same house to a panel count.

Frequently asked questions

How do I measure how much power an appliance uses?

Put a clamp meter around the live wire of the appliance while it is running, read the current in amperes, and multiply by the supply voltage, which is 230 V in Nigeria. That gives running watts. For the energy, multiply by the hours it works. If you have no meter, read the utility meter before bed and after waking instead, which gives you the whole evening rather than one appliance.

Is a solar load list the same as a bill of quantities?

No. The load list describes what the building needs. The bill of quantities describes what will be installed to meet it, with cables, protections, mounting and fitting work added. One is an input and the other an output. A good installer will produce the second from your first, and if you cannot see the first, ask for it before the second is signed.

What if the generator is not available during the sizing visit?

Say so, and plan for the absence rather than pretending. A house that runs nine hours a day on a generator needs far more panels than one that uses it for two hours in the evening, and the difference is large enough to change the design entirely. Sizing for the loads you actually use is more honest and cheaper than sizing for a habit you do not have.

Key Takeaways

• A solar load list is the input to every other sizing decision, and it takes an afternoon to build.
• Record average daily energy in kWh and maximum simultaneous load in watts as two separate totals.
• Nameplates are ceilings, not averages, and duty cycle turns them into usable numbers.
• Clamp one appliance, or read the wall meter across an evening, to replace estimates with figures.
• Motors and compressors draw several times running current at start, so surge decides the inverter.
• Keep the list and revise it after the first long outage, because real use always surprises you.

With the list finished, the next decision is the hardware, and solar inverter against generator is the comparison worth having first.

Sources: the watt, a unit of power; the kilowatt hour, a unit of energy; duty cycle; inrush current at switch-on; induction motor behaviour; supply voltages by country, including 230 V and 400 V at 50 Hz in Nigeria.