Every inverter has two power ratings: continuous watts and surge watts. Most people only look at the first one. Then their inverter trips the moment the fridge compressor kicks in -- and they can't figure out why a 5kVA system can't handle a 200W fridge.
The answer is surge. And if you don't plan for it, your inverter will remind you by shutting down at the worst possible time.
Continuous Watts: The Marathon Runner
Continuous watts (also called rated or running watts) is the amount of power your inverter can deliver all day, every day, without overheating or tripping. It's the sustained output.
A 5kVA inverter with a 0.8 power factor delivers about 4,000 watts (4kW) continuously. If your total household load sits below 4kW, the inverter hums along happily. Lights, TV, fridge running steady, Wi-Fi, phone chargers -- all fine.
This is the number on the box. It's the one installers quote. And for resistive loads -- appliances that convert electricity directly into heat or light -- it's the only number you need.
Resistive loads include: kettles, toasters, incandescent and LED lights, electric heaters, hair dryers, irons. They draw the same power from the moment you switch them on until you switch them off. No surprises.
Surge Watts: The Sprinter
Surge watts (also called peak or starting watts) is the brief power spike your inverter can handle for 5-10 seconds. It exists for one reason: motors.
Any appliance with a motor or compressor draws far more power to start than to run. The motor needs a burst of energy to overcome inertia and get spinning. Once it's up to speed, the power draw drops back to the rated continuous level.
This startup spike is the surge. It lasts just a few seconds, but during those seconds, the power draw can be 3 to 5 times the appliance's rated wattage.
If the surge exceeds your inverter's peak capacity, the inverter trips -- it sees an overload condition and shuts down to protect its circuits. Your house goes dark, and you need to manually clear the fault and restart.
The Surge Table: Know Your Appliances
Here's a practical reference for the appliances most commonly found in Zimbabwe homes, showing what they draw during normal operation and what they pull during startup:
| Appliance | Continuous Watts | Surge Watts | Surge Duration | Multiplier |
|---|---|---|---|---|
| Fridge (small) | 100-150W | 300-450W | 2-3 seconds | 3x |
| Fridge (large/double) | 200-300W | 600-900W | 2-3 seconds | 3x |
| Chest freezer | 150-250W | 500-750W | 2-3 seconds | 3x |
| Borehole pump (0.75kW) | 750W | 2,250-3,750W | 3-5 seconds | 3-5x |
| Borehole pump (1.5kW) | 1,500W | 4,500-7,500W | 3-5 seconds | 3-5x |
| Air conditioner (small) | 1,000-1,200W | 3,000-3,600W | 3-5 seconds | 3x |
| Air conditioner (large) | 2,000-2,500W | 6,000-7,500W | 3-5 seconds | 3x |
| Washing machine | 500-800W | 1,000-1,600W | 2-3 seconds | 2x |
| Pool pump | 750-1,500W | 2,250-4,500W | 3-5 seconds | 3x |
| Angle grinder | 800-1,200W | 2,400-3,600W | 1-2 seconds | 3x |
| Circular saw | 1,200-1,800W | 3,600-5,400W | 1-2 seconds | 3x |
| Microwave | 1,000-1,500W | 1,500-2,000W | 1 second | 1.5x |
| Electric kettle | 1,800-2,200W | 1,800-2,200W | None | 1x |
| LED lights (10 bulbs) | 80-100W | 80-100W | None | 1x |
| TV + decoder | 80-150W | 80-150W | None | 1x |
| Laptop charger | 45-90W | 45-90W | None | 1x |
| Wi-Fi router | 10-15W | 10-15W | None | 1x |
| Phone charger | 10-25W | 10-25W | None | 1x |
Borehole pumps are the biggest surge offenders. A 1.5kW pump can spike to 7,500W for 3-5 seconds on startup. That single appliance needs more surge capacity than most homes' entire continuous load.
What Happens When Surges Overlap
The worst-case scenario isn't a single appliance surging. It's two or more appliances surging at the same time.
Your fridge compressor cycles on and off automatically -- you don't control when. If the fridge compressor kicks in at the exact moment you switch on the washing machine, both surge demands stack:
- Fridge surge: 600W
- Washing machine surge: 1,200W
- Background loads (lights, TV, router): 300W
- Total instantaneous demand: 2,100W
That's manageable for a 5kVA inverter. But replace the washing machine with a borehole pump:
- Borehole pump surge: 5,000W
- Fridge surge: 600W
- Background loads: 300W
- Total instantaneous demand: 5,900W
A 5kVA inverter with a 6kVA (4,800W real) surge rating trips immediately. An 8kVA inverter with a 16kVA surge rating handles it without flinching.
Why Surge Rating Matters More Than You Think
Here's the pattern we see over and over: someone sizes their inverter based on total continuous load, and it works perfectly -- until it doesn't.
The continuous loads add up to 2.5kW. A 3kVA inverter (2.4kW continuous) is tight but seems feasible. The installer says it'll work. And it does -- for the first few hours.
Then the fridge compressor cycles on while the TV is running and someone turns on the microwave. The surge from the fridge coincides with the microwave's brief startup spike. For two seconds, the system sees 3,800W. The inverter trips.
The homeowner resets it. Everything runs fine for another hour. The fridge cycles again while the washing machine is running. Trip. Reset. Trip. Reset.
This is the number one complaint from undersized systems. Not that the inverter can't handle the running load -- it can. The problem is surges that last 2-3 seconds but exceed the inverter's peak capacity.
How to Calculate Your Surge Requirement
Here's a practical method for figuring out what surge capacity you need:
Step 1: List Your Motor-Driven Appliances
Go through your house and identify everything with a motor or compressor:
- Fridge(s)
- Freezer(s)
- Borehole pump
- Pool pump
- Air conditioner(s)
- Washing machine
- Power tools
Step 2: Identify the Largest Single Surge
Look up the surge wattage for your biggest motor load. For most Zimbabwe homes, this is either the borehole pump or the air conditioner.
Step 3: Add Background Loads
Add up the continuous wattage of everything that's likely running when that big surge hits. Fridge (running, not surging), lights, TV, router -- these are always-on loads.
Step 4: Add a Simultaneous Surge Buffer
It's realistic to assume that at least one other motor load (typically the fridge) might surge at the same time as your largest load. Add the fridge's surge wattage to your total.
Step 5: Match to an Inverter
Your total from Step 4 is the minimum surge capacity your inverter needs.
Example: Home with a Borehole Pump
| Component | Watts |
|---|---|
| Borehole pump surge (1.5kW) | 5,000W |
| Fridge surge (simultaneous) | 600W |
| Background loads (continuous) | 400W |
| Required surge capacity | 6,000W |
A 5kVA inverter with 10kVA (8,000W) surge handles this. A 5kVA inverter with only 6kVA (4,800W) surge does not.
Always check the inverter's surge rating in its datasheet, not the marketing headline. Some manufacturers quote generous surge numbers that only last 1 second -- not the 3-5 seconds a borehole pump needs.
Why a 5kVA With 10kVA Surge Beats a 5kVA With 5kVA Surge
Two inverters can share the same continuous rating but have wildly different surge ratings. This is one of the most important spec differences to check.
- Inverter A: 5kVA continuous / 5kVA surge -- essentially no surge headroom. The moment any motor starts, total draw exceeds continuous rating and the unit trips.
- Inverter B: 5kVA continuous / 10kVA surge -- double the continuous rating available for 5-10 seconds. Motors start without drama. The fridge can cycle while the washing machine is running.
Inverter B costs marginally more but saves you from constant nuisance tripping. The surge rating is what separates a reliable daily-driver system from one that fights you every time a compressor kicks in.
Soft Starters: A Workaround for Big Motors
If you already have an inverter that can't handle your borehole pump's surge, a soft starter can help. It's a small device that ramps up the motor voltage gradually instead of hitting it with full power at once. This reduces the startup surge from 5x to roughly 1.5-2x.
A soft starter for a 1.5kW borehole pump costs $80-$150 -- much cheaper than upgrading your inverter. However, it's an additional point of failure and adds complexity. If you're buying a new system, it's better to size the inverter correctly from the start.
Quick Reference: Inverter Surge Ratings
| Inverter Size | Continuous (kW) | Good Surge Rating | Weak Surge Rating |
|---|---|---|---|
| 3kVA | ~2.4kW | 6kVA (4.8kW) | 3kVA (2.4kW) |
| 5kVA | ~4.0kW | 10kVA (8.0kW) | 5kVA (4.0kW) |
| 8kVA | ~6.4kW | 16kVA (12.8kW) | 8kVA (6.4kW) |
Look for inverters where the surge rating is at least double the continuous rating. This is the standard for quality hybrid inverters from brands like Sunsynk, Deye, and Victron.
The SolMate sizing calculator accounts for both continuous and surge requirements when recommending an inverter size. It checks your appliance list against surge multipliers to make sure the recommended inverter can handle startup loads, not just running loads.
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Key Takeaways
- Continuous watts is the power your inverter delivers all day. Surge watts is the brief spike it handles for motor startups.
- Motors and compressors (fridges, pumps, air conditioners) draw 3-5x their rated wattage for a few seconds when starting.
- Overlapping surges are the real danger -- two motor loads starting simultaneously can exceed your inverter's peak capacity even if each one is fine on its own.
- Always check both ratings when comparing inverters. A higher surge rating is worth more than a marginally higher continuous rating.
- Size for the worst case, not the average case. Your inverter needs to handle the moment when the borehole pump starts while the fridge compressor is already cycling.