A three-bedroom flat in Ikoyi runs a fridge, a television, fans, a kettle, a washing machine and a borehole pump. A similar flat in Kano adds an air conditioner and a second fridge. The two need very different systems, and the gap between them is money. Working out how to size solar system for a 3-bedroom flat is not a matter of taste or a rule of thumb about the number of rooms. It is four calculations done in the right order, and the order is what most people get wrong.

How to size solar system for a 3-bedroom flat, step by step

The chain runs load, then energy, then inverter, then array, then bank. Each step consumes the output of the one before. If the load list is wrong, every number after it is wrong in the same direction, and the most common direction is optimistic. A solar system is a chain, and a weak link shows up as a bank that is empty by nine in the evening. Work through it once on paper and you will know within an afternoon whether the quotation you have been given is sensible.

Throughout this worked example the numbers are illustrative. They describe a plausible family home in Nigeria, not your home, and the point is the method rather than the answer. Replace every figure with one you have measured, and read how to build a solar load list before you start.

Step one: the load list

Go room by room. Record what is plugged in, what its label says, roughly how long it runs each day, and whether it runs in the evening or only during the day. The evening split matters more than people expect, because the array is doing nothing useful after sunset and everything used then comes out of the bank at full cost. A system designed on the daily total alone will disappoint you every single evening.

Load	Watts	Hours a day	kWh a day
Four LED bulbs	12 W each	6	0.29
Two ceiling fans	60 W each	8	0.96
Fridge freezer	200 W, cycling	24	0.85
Television	90 W	5	0.45
Laptop	65 W	6	0.39
Router and decoder	15 W	16	0.24
Kettle	2,000 W	0.8	1.60
Electric iron	1,000 W	0.5	0.50
Washing machine	500 W	1, alternate days	0.25
Microwave	800 W	0.3	0.24
Borehole pump	370 W	1	0.37
Total			6.1

Read that total as a planning figure, not a measurement. The kettle line is the one to check, because it depends on how many family members are home and how much tea is drunk. A flat where nobody is home during the day is a different building from one with a stay-at-home parent running a small business from the spare room.

Step two: the evening peak and the duty cycle

Daily energy decides the panels and the bank. Peak simultaneous load decides the inverter. They are different questions, and conflating them is the standard mistake. From six in the evening until eleven, this flat runs lights at 48 W, two fans at 120 W, a television at 90 W, a fridge averaging 70 W while it cycles, and about 15 W of router and decoder. That is roughly 345 W running continuously. Add a kettle at 2,000 W and the peak climbs towards 2.4 kW, and the borehole pump adds a starting current several times its 370 W running figure for a second or two.

So the inverter must handle 2.4 kW continuously, and must start a 370 W pump while the rest of the house is running. On a 230 V supply, 2.4 kW is a little over 10 A. A 3 kW inverter would just about do it and would leave no room for surge, so the sensible choice for this home is a 5 kVA unit. That headroom is not waste. It is what lets you add a second fridge, an air conditioner or a freezer later without replacing the inverter, and our guide to running an air conditioner on solar explains why one extra load changes the whole design.

Why 48 V suits this size of system

At the same power, a higher bank voltage means lower current and therefore thinner cable, smaller breakers and less voltage drop on long runs in a compound where the batteries sit away from the house. A 48 V bank is the usual choice once a system passes about 3 kW, and the trade-offs are set out in 24 V against 48 V solar systems. For this example the 5 kVA inverter is a 48 V machine, so the bank must be 48 V.

Step three: array and battery bank

The array has to produce 6.1 kWh a day, plus losses for heat, dust, cable and the mismatch between what the panels can make and what the batteries will accept. The production figure you need is the peak sun hours for your own coordinates, which you can look up on the Global Solar Atlas rather than taking from a sales brochure. This example assumes four and a half peak sun hours, which is a reasonable planning figure for much of southern Nigeria in the drier months, and lower in the harmattan or the rains.

Six point one kilowatt hours divided by four and a half hours is 1.36 kW of array before losses. Adding about fifteen per cent for dust, heat and mismatch gives roughly 1.5 kW, so six 250 W panels. That array is designed to refill the bank, not to run the house from midday onwards.

How many hours of darkness the bank must cover

From sunset until sunrise is the period the bank is doing all the work, and for this system it is the period that defines whether the design works. Assume 45 per cent of the daily energy is used after sunset, which is 2.7 kWh at the sockets. Inverters are not perfect, so the bank must supply about 3 kWh. If you allow the bank to be discharged to 80 per cent, the nominal capacity required is a little over 3.75 kWh, so a 5 kWh bank is the sensible next step up rather than the 4 kWh that sits uncomfortably close to the limit. Battery bank sizing takes this calculation apart in more detail.

At 48 V, 5 kWh is about 104 Ah, so four 100 Ah 12 V batteries wired in series is the natural build. The choice between lithium and lead–acid is a separate argument about cycle life and weight rather than about capacity.

Sanity check against the loads people forget

A design that survives the table but not the household is the standard disappointment. Walk the flat again and look for what was not in the list: the second fridge that appeared after the first broke, the freezer bought for harmattan, the CCTV cameras and the router for them, the extra lighting on a converted storey, the immersion heater, the sewing machine, the hair dryer, the pressure pump. Each one is small. Together they routinely add a kilowatt hour a day.

Component	Figure	How it was reached
Daily energy	6.1 kWh	Sum of the load list
Evening peak load	2.4 kW	Running loads plus the kettle
Inverter	5 kVA at 48 V	Peak load with headroom for surge and later additions
Array	1.5 kW, six 250 W panels	Daily energy divided by peak sun hours, plus losses
Battery bank	5 kWh, four 100 Ah in series	Night energy, inverter losses, discharge limit and reserve

Two honest caveats. The first is that a home which expects to be fully independent through a long stretch of cloud cannot do it on a bank of this size at any sensible cost, and the answer is a generator as a backstop rather than a bigger bank. The second is that an air conditioner, if it is going to run through the night, will roughly double the array and change the inverter, which is why it belongs in the load list from the start rather than being added later. A system bought for today’s loads and then stretched is the most expensive way to buy solar.

Frequently asked questions

How many solar panels does a 3-bedroom flat need?

For a home using around 6 kWh a day, in a location with about four and a half peak sun hours, six 250 W panels is a reasonable starting point before losses. Add fifteen per cent for dust, heat and mismatch. A home that adds an air conditioner or a freezer needs noticeably more, which is why we would rather you measured first than take a number from any article, including this one.

Do I need a generator as well?

If your load includes anything you cannot afford to lose during an outage, keep a generator. A solar system handles the base load well and cuts fuel use dramatically, but a week of cloud is a week of cloud. A small generator covering the odd essential load is often cheaper than a bank large enough to cover a whole bad week, and much cheaper than a bank large enough to cover a whole bad month.

Can I add air conditioning later without replacing everything?

Sometimes. If you sized the inverter and the array with margin, a small unit can be added. If you did not, the inverter usually has to change and the array usually has to grow. This is the strongest argument for putting the air conditioner on the load list now, even if you do not plan to buy one for two years.

Key Takeaways

• To size solar system correctly, work in order: load, energy, inverter, array, bank.
• Daily energy in kWh sets the panels and the bank. Peak simultaneous load sets the inverter.
• A kettle and a pump dominate the peak, so the inverter needs real headroom above the average.
• Look up peak sun hours for your own coordinates rather than accepting a quoted figure.
• Leave a reserve in the bank, because a bank that reaches empty every night has no margin left.
• Count the loads you forget, because the second fridge is the most common one.

Once the numbers are settled, the next thing worth reading is how to size a solar inverter and battery bank, which covers the two components the whole chain converges on.

Sources: the kilowatt hour, a unit of energy; duty cycle; supply voltages by country, including 230 V single phase in Nigeria; how a photovoltaic system is put together; deep-cycle batteries and depth of discharge; Global Solar Atlas, for peak sun hours at your coordinates.