1.5HP Air Conditioner on Solar: What Size System Do You Need?

The Startup Surge Is the Whole Sizing Problem

This is the single most common sizing mistake we see, so it is worth being blunt about it.

A 1.5HP inverter air conditioner pulls roughly 1,100W while it is running comfortably. For a few
seconds every time the compressor starts, it pulls somewhere around 2,500W. That short spike is
called inrush current, and it is what kills undersized systems.

Now do the arithmetic that matters. A 1.5kW inverter cannot start a 1.5HP air conditioner. If you
size the inverter to the running wattage, the compressor tries to start, the inverter sees a demand
far beyond what it can deliver, and it either trips on overload or stalls and retries. That cycle
repeats every time the thermostat calls for cooling.

What you hear in the room is the AC starting and stopping every few minutes, and what you see on
the inverter is an overload fault. The instinct is to blame the panels. The panels are fine. The
inverter is too small to start the load.

Sizing rule of thumb: size the inverter for the surge, not the running draw, and leave
headroom above that. A non-inverter unit draws more at steady state and surges much harder, so it
needs a larger inverter than an inverter-type AC of the same horsepower.

Running the AC on Solar During the Day Is a Different System

Everything above assumes the AC runs at night on stored battery power, because that is when the
solar is not producing and the grid is often off. It is the harder case, and it is what the table
above is sized for.

Running the AC during the day is easier in one way and harder in another.

Easier, because the panels are producing at the same time the AC is drawing, so
you need far less battery storage. If your AC runs from 11am to 4pm on a clear day, the array can
feed it more or less directly.

Harder, for three reasons:

• Panel output peaks at midday and falls all afternoon. A 1.5HP AC running flat
  out in the afternoon, when panels are already declining, is the worst possible overlap. A system
  that copes at noon can fail at 3pm.
• Heat reduces panel output exactly when you need it most. Nigerian afternoon
  temperatures push panel output down by a meaningful margin at the same time your cooling demand
  peaks.
• You still need the inverter sized for surge, and now you also need enough
  panels to carry both the AC and everything else running at that hour.

If your goal is daytime cooling, tell the engineer at the consultation, because the array is
sized differently and so is the inverter. Saying “I want the AC on during the day” at the quote
stage costs nothing; adding solar later costs more.

What the Air Conditioner Actually Costs You Per Month

Take the 8.8kWh per night figure from above and run it monthly.

• 8.8kWh per night, roughly 30 nights a month is about 264kWh a
  month.
• At a grid tariff of roughly N100 per kWh, that is around N26,000 a month in
  grid electricity.
• Running the same 264kWh from a generator at about N400 per kWh of fuel is closer to
  N105,000, which is where most people feel the difference.

Those figures are illustrative, not a quote: your tariff and your generator fuel cost are yours,
and both move. But the shape of the calculation is the right one, because it tells you whether
air conditioning is the thing you size the system around. For most Nigerian households it is, and
once you know that, the sizing conversation stops being a guess.