How to Size an Inverter for Your Home | Mercury Direct

With the current cost of fuel and grid electricity, an inverter is no longer a luxury in Nigeria. It is a necessity. The most common mistake homeowners make is trying to buy one machine that powers everything in the house at once. That is the expensive way to do it.

The smartest, most economical approach is selective loading: only powering what you absolutely need, and actively switching off what you are not using. Here is how to size your system using the real rules of energy management in a Nigerian home.

The 300W Sweet Spot: Maximise Runtime, Minimise Cost

For the average Nigerian household, the best setup does not need a massive, expensive machine. The secret to long backup lies in managing your continuous draw. If you keep your continuous consumption around 300 watts, you can comfortably get about 10 hours of overnight backup using an affordable bank of two 200Ah deep cycle batteries.

Here is what a clean 300W load profile looks like:

The golden rule is simple: if you leave a room, turn off the fan. If nobody is watching, switch off the TV from the wall. Turning off what you are not actively using is how a modest, well-managed system comfortably outlasts a bigger, careless one. For this profile, the Mercury 2kVA Complete Pure Sine Wave Inverter System with 2 × 200Ah batteries or the smaller Mercury 1.5kVA Solar Hybrid is your match. See a full breakdown of what this class of inverter can power for more detail.

The Battery Maths Behind 10 Hours

The 10-hour figure is not a guess. It comes from the standard runtime formula we use across Mercury Direct:

• The formula: Backup Hours = (V × Ah × DoD × Efficiency) ÷ Load Watts
• Worked example: Two 12V 200Ah batteries in series make a 24V 200Ah bank. At 24V × 200Ah × 0.80 depth of discharge × 0.85 efficiency, divided by a 300W load, you get about 10.9 hours.

That is your roughly 10 to 11 hours of overnight backup. Two things to keep in mind. First, once you add a fridge or push the load into the 500W to 600W range, the same bank drops from roughly 10 to 11 hours to about 5.5 to 6.5 hours, depending on cycling, surge, and how disciplined the household is, so load discipline is everything. Second, while a GEL bank can be discharged deeply, doing it hard every single night shortens its service life. Charging from solar during the day takes the strain off the batteries and stretches their lifespan. You can run your own numbers on the battery runtime calculator, and read how to extend battery life.

Why This Beats a Generator at the Pump

Here is the arithmetic that makes the inverter win. As of mid-June 2026, market reports put petrol in Lagos around ₦1,200 per litre, with independent marketers selling higher, and diesel close to ₦1,650 per litre. Fuel prices now move quickly, so the exact pump price may differ by location and week, but the direction is clear: every extra hour on generator is becoming more expensive.

The inverter still needs to be charged from the grid, solar, or another energy source, so it is not free power. But it removes the daily petrol burn that makes generator backup so expensive. In Lagos, many Band A customers currently pay around ₦209.50 per kWh, depending on DisCo, tariff class, and feeder. Even at that level, grid charging is still far cheaper than running a petrol generator for hours every day, and solar can cover most of it.

Moving Up: Adding an Inverter Air Conditioner

If you want premium comfort and overnight air conditioning, the maths changes entirely. You cannot run an AC properly on a basic two-battery setup, and you should never attempt it on a small two-battery system such as a 2kVA or 2.4kVA inverter setup. To enjoy AC backup you must meet three conditions:

• Inverter-type AC: the kind with a variable-speed compressor that drops its power draw once the room is cool. A typical 1HP inverter-type AC is the realistic starting point.
• Pure sine wave: you need a pure sine wave inverter to avoid stressing the compressor. See pure sine wave versus modified sine wave.
• Battery and solar: runtime depends heavily on battery bank size and solar support, not just the inverter rating, so plan for a 4 by 200Ah GEL bank as a minimum, or move to lithium.

If the goal is regular overnight AC backup, lithium is strongly recommended. It charges faster, handles deeper cycling better, and is far more practical than repeatedly draining lead-acid batteries every night. Our guide on which battery is best for an inverter explains the trade-offs. For this level of comfort, look at the Mercury 3.5kVA Solar Hybrid, sized with the right battery bank and panels. The 3.5kVA load guide shows exactly which appliance combinations it handles.

The Heavy Lifters: Pumps, Washing Machines, and Freezers

Do you want near-total independence from the grid, including the ability to pump water, run a washing machine, or keep heavy chest freezers running through the day? These appliances have large inductive motors with heavy startup surges. Trying to run them on small battery banks will quickly stress your batteries or trip the inverter into overload protection. See whether an inverter can carry a fridge for the surge logic.

To support these loads, your home needs a robust 48V architecture: a high-capacity Mercury 5.5kVA Solar Hybrid (48V) Complete System paired with a large GEL or lithium bank and solar. A 48V platform is the threshold where serious motor loads become practical to run on stored energy. For full off-grid living with several heavy motors, a larger 48V system may be needed, so it is best to message us for a custom sizing. The 5.5kVA load guide has the full picture.

Summary: Which Mercury System Is Yours?

Still weighing your options? The full inverter buying guide walks through every decision, from kVA sizing to battery chemistry. If you are adding panels, start with monocrystalline versus polycrystalline solar panels.