How to Size a Solar Inverter and Battery for a Nigerian Home

The four numbers that determine whether a solar system works — daily watt-hours, battery bank, panel array and inverter size — with the derates that separate a system that runs all night from one that dies at 9pm.

Updated 2026-09-10

Most solar installations that disappoint in Nigeria were not badly built. They were badly sized. The panels are fine, the wiring is fine, the installer was competent — and the battery bank is too small for what the household actually runs, so the system works in the first week and gives up by 9pm once the routine settles.

Sizing is arithmetic. Here is all of it.

Step 1: Daily watt-hours

List every appliance with its wattage, how many you have, and how many hours a day it actually runs. Then:

`` daily watt-hours = watts × hours × quantity ``

A realistic family home without air conditioning:

Two things to get right here.

Refrigerators cycle. They do not draw their rated power continuously. An inverter-type fridge runs roughly 8 hours out of 24; a conventional one can run 12 or more. Using the nameplate wattage with 24 hours will triple your estimate and massively oversize the system.

Air conditioning changes everything. A single 1.5HP inverter AC drawing 1,200W for six hours adds 7,200 Wh — more than the entire list above. If you want AC on solar, you are designing a different and considerably larger system.

Step 2: The battery bank

This is where undersizing happens, because two derates get left out.

`` battery watt-hours = daily Wh × autonomy days ÷ (depth of discharge × inverter efficiency) ``

Depth of discharge is how much of the battery you are allowed to use. For tubular or flooded lead-acid it is 50% — draining below that destroys the plates and halves the life. For AGM or gel, 60%. For lithium (LiFePO4), 80%.

Inverter efficiency is what you lose converting DC to AC. A good hybrid inverter runs 88–93%. Use 0.9.

For our 4,552 Wh home with one day of autonomy on lead-acid:

`` 4,552 × 1 ÷ (0.5 × 0.9) = 10,116 Wh ≈ 10.1 kWh ``

On lithium at 80% depth of discharge the same load needs:

`` 4,552 × 1 ÷ (0.8 × 0.9) = 6,322 Wh ≈ 6.3 kWh ``

That is the whole lithium argument in two lines: you buy 37% less battery capacity for the same usable energy, and it lasts three to four times longer.

Autonomy days is how long the system must run with no sun. One is normal. Two if a dark night is genuinely unacceptable. Doubling autonomy doubles the battery and does not change the panel requirement at all.

Step 3: The panel array

`` panel watts = daily Wh ÷ (peak sun hours × 0.75 derate) ``

The 0.75 accounts for heat, dust, wiring losses and the fact that panels are rated at laboratory conditions. Nigeria's climate is sunny but hot, and heat reduces panel output.

Peak sun hours are roughly 4.0–4.5 for most of southern Nigeria and 5.0–5.5 in the north.

For our home in Lagos at 4.5 hours:

`` 4,552 ÷ (4.5 × 0.75) = 1,349 W ``

That is three 450W panels or three 550W panels. Anyone quoting you two panels for this load is guessing.

Step 4: The inverter

Size it on total connected load plus surge headroom, then round up to the next standard size.

`` inverter = total connected watts × 1.25, rounded up ``

Our home has 8 + 210 + 80 + 20 + 150 + 60 = 528 W connected. With surge headroom that is 660 W, so a 1,000W inverter. A 650W unit would technically fit the arithmetic but leaves nothing for a second television, a washing machine or the day you add a fan.

The surge factor matters because motors draw several times their running current for a fraction of a second at startup. A fridge compressor or a pump will trip an inverter sized exactly to running load.

The four numbers, summarised

Lead-acid or lithium

Lead-acid is cheaper upfront, widely available, and you can only use half of it. Life is 3–5 years with daily cycling.

Lithium costs roughly twice as much, you can use 80% of it, it weighs a fraction as much, it charges faster, and it lasts 8–12 years.

Over a ten-year horizon lithium is usually the cheaper option, and if you are cycling the battery daily it is the only sensible one. Lead-acid makes sense for occasional backup where the battery sits charged most of the time.

Reading a quote

Ask for these four numbers explicitly. If a quote lists components without showing the daily watt-hour figure it was sized against, the installer did not size it — they picked a package.

Then check:

• Does the battery capacity account for depth of discharge, or did they quote raw capacity?
• Is the panel array sized with a derate, or at nameplate?
• Is the inverter sized on connected load or on daily consumption? These are different numbers.
• What happens at 9pm in the rainy season?

Get at least three quotes for the same bill of materials. In Nigeria, quotes for identical specifications routinely differ by more than 50%.