What Size Inverter Do I Need?

Most people get sized by their dealer, and most dealers size by feel. That works out fine perhaps half the time. The other half splits into two failure modes, and both cost money.

Undersize, and you trip the inverter every time the pump starts while the AC is running. You live with it for eighteen months, then buy a second inverter, and the first one was Rs. 140,000 wasted.

Oversize, and you have spent Rs. 100,000 more than necessary on capacity you will never touch, plus you have bought panels and cable sized for a system you did not need.

Sizing properly takes about twenty minutes with an electricity bill and a list of your appliances. Here is how to do it.

For help with the numbers, WhatsApp 03192070022.

Three Ways to Size, in Order of Effort

Method 1: By your electricity bill

Fastest, and accurate enough for most households.

Use your highest bill of the year, not your average. Sizing to a mild March bill means the system falls short every June, which is precisely when you need it.

Method 2: By appliance load

More work, more accurate, and necessary if your usage is unusual in any way — a workshop, a farm, a shop, or a house with heavy pumping.

Add up what actually runs at the same time. Not everything you own. The washing machine and the iron rarely run at 2am, and no household runs every appliance simultaneously.

Method 3: By roof space

Only relevant as a ceiling. If you have 400 square feet of usable roof, you cannot install a 12kW array regardless of what your bill says. Roughly 60 to 70 square feet per kilowatt of panels is a working figure, so a 6kW system with 7kW of panels needs around 420 to 490 square feet.

When roof space is the binding constraint, size the array to the roof and accept that you will offset part of your bill rather than all of it.

The Appliance Load Table

The Column Everyone Ignores

Look at the right-hand column again. That is where sizing goes wrong.

Anything with a motor draws far more current at the instant it starts than it does while running. A 1HP water pump runs at 750W and spikes to around 2,500W for a second or two. A non-inverter air conditioner can pull three times its running load at startup.

Your inverter has to survive that spike or it trips. And the spike is not just about the individual appliance — it stacks on top of everything already running.

Worked example. You are running two 1.5-ton inverter ACs at 2,800W, a fridge at 300W, eight fans at 480W and lighting at 200W. That is 3,780W, comfortable on a 6kW inverter. Now the water pump starts and adds a 2,500W spike. You are momentarily at 6,280W, over the limit, and the inverter cuts out.

This is why two houses with identical bills can need different inverters. The one with a pump and an old non-inverter AC needs more headroom than the one running only modern inverter units.

What to do about it. Check the surge rating of any inverter you are considering, and check how long it sustains it. Almost every brochure lists a peak surge figure. Very few state the duration, and two seconds versus five seconds decides whether your compressor starts. Some units surge to double their continuous rating, which effectively solves the problem. Ask the question before you buy.

Inverter AC Versus Non-Inverter AC

This single distinction changes your sizing more than almost anything else, and it is worth spelling out because the naming is confusing.

An “inverter AC” is an air conditioner with a variable-speed compressor. It has nothing to do with your solar inverter. It ramps up gradually instead of slamming on, so it has almost no starting surge and it draws less power once the room reaches temperature.

A non-inverter AC has a fixed-speed compressor that switches hard on and off. Every cycle produces a surge.

Two 1.5-ton inverter ACs sit comfortably on a 6kW system. Two 1.5-ton non-inverter ACs on the same system will trip it when both compressors happen to start together. If your ACs are non-inverter and you are not replacing them, size up one bracket.

Adding Headroom

Whatever number the methods above give you, add roughly 20%.

Not because the calculation is wrong, but because households grow. A new AC arrives. A son moves back with a family. An EV charger goes in. A tenant takes the upper portion. Every one of those adds load, and adding capacity later costs far more than buying it now.

Twenty percent is the right amount. Fifty percent is money sitting idle. If your load audit says 5kW, buy 6kW. If it says 8kW, buy 10kW if the budget stretches, or 8kW with a model that can run in parallel with a second unit later.

That parallel capability is worth asking about. Many hybrids run up to six or nine units together, which means a 6kW inverter today can become 12kW next year without discarding anything.

Sizing the Panels

Panel capacity should exceed inverter capacity. This surprises people who assume the two should match.

An array rated 6kW delivers 6kW for perhaps two hours on a perfect June day and considerably less the rest of the time. In December haze in Punjab, output can drop 50% to 60%. An array sized exactly to the inverter produces a trickle for three months a year.

Most modern hybrids allow 130% oversizing, and some allow 200%. Using that headroom keeps you generating usable power through the months that actually determine whether the system frustrates you. The inverter clips the excess on bright summer afternoons and you barely notice.

Working figures for a 6kW inverter: 12 to 14 panels of 550W to 620W, giving 6.6kW to 8.7kW of DC capacity, needing around 420 to 490 square feet of roof.

Check two limits before finalising the string design. Maximum PV input power, and maximum open circuit voltage. Open circuit voltage rises on cold clear mornings, and a string sized for a hot afternoon can exceed the inverter’s ceiling at 7am in January in Islamabad or Quetta. That damages the input stage.

Sizing the Battery

Battery sizing is a separate calculation from inverter sizing, and conflating them is common.

The inverter determines how much you can run at once. The battery determines how long you can run it.

Work out your backup requirement in kilowatt-hours. Take the load you want to keep alive during an outage, multiply by the hours you want it to last, and add roughly 20% for inefficiency and depth-of-discharge limits.

Example. You want fans, lights, fridge and a router running for five hours overnight. That is roughly 1,000W, so 5kWh of delivered energy, or around 6kWh of usable battery capacity once losses are accounted for.

Note that you do not need to back up everything. This is where dual-output inverters earn their place: they split your circuits into heavy and essential, drop the air conditioners during an outage, and keep the essential circuits running for hours instead of forty minutes. Choosing that feature can halve the battery bank you need.

Battery rates move almost weekly in this market and the pack is often the largest single item in a system budget, so anything printed here would be stale within days. WhatsApp us now for the latest price of the battery at 03192070022.

Worked Examples

5 marla house, one AC, bill around Rs. 18,000

Load: one 1.5-ton inverter AC at 1,400W, fridge 300W, six fans 360W, lighting 150W, TV 100W. Total around 2,310W, with a 0.5HP water pump adding a 1,200W spike.

Answer: 5kW. Comfortable headroom, handles the pump start, room for a second AC later. Our Crown Yorker 5KW price guide covers the budget option in this bracket.

10 marla house, two ACs, bill around Rs. 35,000

Load: two 1.5-ton inverter ACs at 2,800W, fridge and freezer 600W, ten fans 600W, lighting 250W, washing machine intermittent, 1HP pump with a 2,500W spike.

Answer: 6kW. The most common answer for Pakistani homes, which is why this class has the widest model choice. Full comparison in our 6kW solar inverter price in Pakistan guide.

1 kanal house, four ACs, bill around Rs. 65,000

Load: four ACs at 5,600W, refrigeration 800W, fans and lighting 1,000W, 2HP pump with a 5,000W spike.

Answer: 10kW. Eight kilowatts would work on paper but leaves nothing for the pump start on a hot evening.

Small shop or showroom, bill around Rs. 45,000

Load: continuous cooling and heavy lighting, running most of the day rather than in peaks.

Answer: 8kW. Commercial load profiles are flatter than domestic ones, so headroom matters less than sustained capacity and thermal performance.

Farm or tube well, no reliable grid

Load: dominated by pump starting current rather than continuous draw.

Answer: size to the surge, not the running load. A 2HP pump needs an inverter that can deliver 5,000W momentarily. A 5.5kW unit with a surge rating of double its continuous output handles this comfortably where a nominally larger unit with a weak surge rating will not.

Commercial or light industrial, three-phase connection

Answer: 12kW and above, and the single-versus-three-phase decision matters more than capacity at this scale. Our 12kW hybrid inverter price in Pakistan guide covers that decision in detail.

Five Sizing Mistakes

Sizing to your average bill instead of your peak. June is what the system has to survive.

Ignoring surge current. The most common cause of an inverter that trips despite being “big enough” on paper.

Adding up every appliance you own. Nothing runs everything at once. Size to realistic simultaneous load.

Matching panels to inverter capacity. Oversize the array or accept poor winter output.

Buying no headroom to save Rs. 20,000. Then spending Rs. 200,000 on a second inverter eighteen months later.

Quick Reference

Once you have your number, our guide to choosing the best inverter for a home in Pakistan covers which specific model to buy at each capacity, and current stock with specifications is on our products page.