Why Inverter Sizing Matters

The inverter is the brain and muscle of your solar system. It converts DC power from your panels and batteries into the AC power your house runs on. Get the size wrong and you'll either waste money on capacity you never use, or deal with constant overload shutdowns that leave you in the dark at the worst moments.

Let's break down what the numbers actually mean and how to choose the right size for a typical Zimbabwe home.

kVA Is Not kW

Every inverter has a kVA rating stamped on it -- 3kVA, 5kVA, 8kVA. Most people read "5kVA" and think "5,000 watts." That's not how it works.

kVA (kilovolt-amperes) is apparent power. kW (kilowatts) is real power -- the actual watts your appliances consume. The relationship between them is governed by a number called the power factor, which for most residential inverters sits around 0.8.

The formula is straightforward:

Real power (kW) = kVA x Power Factor

So a 5kVA inverter with a 0.8 power factor delivers about 4kW of continuous real power. Not 5kW.

What Happens When You Undersize

If your running loads exceed the inverter's continuous capacity, it trips. The screen flashes an overload error and the unit shuts down to protect itself. Everything goes off -- lights, fridge, Wi-Fi, everything -- until you manually reset it after reducing the load.

This is the most common complaint from people who bought a 3kVA inverter thinking it would "handle everything." It handles everything right up until someone switches on the kettle while the fridge compressor is running.

What Happens When You Oversize

Oversizing isn't dangerous, but it wastes money. A 10kVA inverter powering a home that never draws more than 3kW works fine -- it just cost you an extra $800-$1,200 that could have gone toward more battery capacity or extra panels.

Inverters also have a small standby power draw. Larger inverters consume slightly more power just sitting there, which nibbles at your battery reserves overnight.

The Real Problem: Surge Loads

Here's where inverter sizing gets tricky. Many appliances with motors or compressors draw 3 to 5 times their rated wattage for a few seconds when they start up. This is called the surge or inrush current.

Your fridge is rated at 150W. But when the compressor kicks in, it pulls 450-600W for 2-3 seconds. A borehole pump rated at 1,500W can surge to 5,000-7,500W on startup.

Common Appliance Wattages (Continuous and Surge)

Why a 3kVA Inverter Can't Start a 1.5kW Borehole Pump

This is the question that trips up most first-time solar buyers. The maths looks simple: 1.5kW is less than 3kVA (2.4kW real), so it should work, right?

It doesn't. Here's why:

1. The pump's continuous draw is 1.5kW, but the surge on startup hits 5,000-7,500W for a few seconds.
2. A 3kVA inverter typically has a surge rating of around 6kVA (6,000VA). Sounds enough?
3. But surge capacity is measured in VA, not watts. Apply the power factor: 6kVA x 0.8 = 4.8kW real surge capacity.
4. The pump needs up to 7.5kW on startup. The inverter trips instantly.

Even if the pump's surge falls at the lower end (5kW), you're right on the edge -- and if the fridge compressor happens to kick in at the same moment, it's game over.

The Sweet Spot for Zimbabwe Homes

After sizing hundreds of systems for Zimbabwe households, clear patterns emerge:

5kVA -- The Workhorse

A 5kVA inverter (with 10kVA surge capacity) handles most 3-4 bedroom suburban homes comfortably. It can run:

• Fridge and freezer simultaneously
• Lights throughout the house
• TV, decoder, Wi-Fi
• Laptop and phone charging
• Microwave (one at a time, not with the kettle)
• Washing machine

It will not comfortably handle a borehole pump, a large air conditioner, or multiple heavy loads at once.

8kVA -- The Heavy Lifter

An 8kVA inverter (with 16kVA surge capacity) is the right choice if you have:

• A borehole pump (the main driver for stepping up from 5kVA)
• Multiple air conditioners
• A larger home with 4+ bedrooms
• A workshop with power tools
• An electric geyser you want to run during solar hours

3kVA -- Essential Backup Only

A 3kVA inverter is fine for a small flat or cottage where you're only covering lights, fridge, TV, and charging. It's the entry point for "keep the lights on during load shedding" setups. But understand its limits -- it will not run a kettle, microwave, or any motor-driven appliance comfortably.

Stackable Inverters: Build Now, Expand Later

Some inverter brands (Sunsynk, Deye, Victron) support parallel stacking -- you install one 5kVA unit now and add a second one later to get 10kVA. This is a smart strategy if:

• Your budget only stretches to 5kVA today, but you plan to add a borehole pump or air conditioning next year
• You want redundancy -- if one unit fails, the other keeps critical loads running
• You're building in phases and your load will grow over time

Make sure the specific model you buy supports parallel operation. Not all do, and mixing different models or firmware versions can cause problems. Ask your installer before committing.

How to Size Your Inverter: The Quick Method

1. List your appliances and their wattages (check the rating plate on each one)
2. Add up the continuous wattages of everything that might run simultaneously
3. Identify motor loads and multiply their wattage by the surge factor (see the table above)
4. Find the largest single surge in your list -- this is your peak demand moment
5. Choose an inverter whose continuous kW rating exceeds your total running load AND whose surge rating exceeds your peak surge

Example: Typical 3-Bedroom Home in Harare

A 3kVA inverter (2.4kW continuous, ~4.8kW surge) handles this on paper. But it's tight -- add a washing machine or a second fridge and you're over. A 5kVA inverter gives you comfortable headroom and room for your household to grow.

Key Takeaways

• kVA is not kW. Multiply by 0.8 to get real usable power.
• Surge loads are the hidden trap. Always check both continuous and surge ratings.
• 5kVA is the sweet spot for most Zimbabwe homes; step up to 8kVA if you have a borehole pump or heavy loads.
• Don't oversize just to be safe. That money is better spent on batteries or panels.
• Consider stackable inverters if you want to start small and expand later.