Alternative Energy

A typical system looks like the diagram above.

There are several possible architectures. Some hybrid inverters have PV MPPTs built in; others use separate MPPT charge controllers. Some systems have separate grid and generator inputs. For example, Victron's Quattro architecture specifically supports two AC sources such as grid and generator.

This is the most important step.

Don't start by asking: "What size inverter should I buy?"

Rather start with: "What does the house actually need to run?"

Create a simple load schedule.

• 1 Refrigerator @ 200W for 10h = 2kW

• 15 Lights @ 10W for 5h = 0.50kW

• 1 WiFi AP @ 20W for 10h = 0.20kW

• 1 PC @ 150W for 5h = 1.50W

• 1 Washing Machine @ 700W for 1h = 0.7kW

• 1 Microwave @ 1.2kW for 0.5h - 0.60kW

• 1 Kettle @ 2kW for 0.5h = 1kW

• Then calculate:

Daily energy

Eday=∑(W×hours)

So, for example:

• 7.5 kWh/day is required as calculated above

But energy consumption isn't enough.

You also need maximum simultaneous power.

Imagine these are operating simultaneously:

• Refrigerator = 200 W

• Lights = 150 W

• TV = 150 W

• Computer = 300 W

• Kettle = 2,000 W

• Microwave = 1,200 W

Total:

Ppeak=4,000WP or 4kW

Therefore you don't want a 3 kW inverter.

A sensible starting point might be a 5 kW inverter, subject to surge requirements, continuous ratings, temperature derating and the actual load profile.

You should similarly distinguishes between energy consumption and peak power requirement when designing residential backup systems.

This is extremely important for hybrid systems.

Don't necessarily put the entire house behind the inverter.

Create:

Essential / backup DB

For example:

• Lights

• Refrigerator

• Wi-Fi

• TV

• Alarm

• Security

• Selected plugs

• Garage door

• Selected office equipment

And:

Non-essential DB

For example:

• Electric geyser

• Oven

• Pool pump

• Large air-conditioners

• Electric heaters

• Welder

• EV charger

This dramatically reduces the inverter and battery requirements.

For example:

Whole house Peak: = 12 kW

Essential loads Peak: = 4 kW

A 5 kW hybrid inverter may therefore be perfectly suitable for the backup system, rather than needing a 12 kW inverter.

Look at four things.

A. Continuous output

If your calculated maximum essential load is: 4.0 kW

I'd normally look at: 5 kW minimum

rather than selecting a 4 kW unit with no margin.

B. Surge capacity

Motors and compressors can produce substantial startup currents.

Examples:

• Refrigerator

• Freezer

• Pump

• Air conditioner

• Pool pump

Therefore check:

Continuous power

and

Peak/surge power

in the inverter datasheet.

C. Battery voltage

For residential systems, 48/51.2 V battery systems are common.

A 5 kW load at approximately 50 V means:

I = 5000W / 50Vdc =100A

After allowing for inverter losses, the battery current can be even higher.

This is one reason high-power residential systems should generally use a properly engineered higher-voltage battery architecture rather than trying to obtain large power from a small low-voltage battery bank.

D. AC inputs

If you're integrating a generator, this becomes critical.

You want an inverter/charger that can properly manage:

Grid + battery + PV + generator.

Some systems have separate AC inputs. Others use a shared AC input with generator detection/control.

Now calculate the battery.

Suppose:

Essential loads: = 1.5 kW average

Required backup: = 8 hours

Then:

• 1.5kW×8h=12kWh

But 12 kWh shouldn't necessarily be your nameplate battery size.

You need to account for:

• Maximum depth of discharge

• Inverter losses

• Battery temperature

• Battery ageing

• Reserve SOC

• Battery manufacturer's allowable continuous current

For example, if you want to use approximately 80% of the nominal capacity:

Battery = 12/0.8 = 15kWhBattery

Then you might choose something around: 15–16 kWh nominal rather than exactly 12 kWh.

Likewise you need battery sizing around required autonomy during a mains failure and remember that larger batteries provide longer backup autonomy.

This is a common design mistake.

A battery can have enough kWh but still not be able to deliver enough kW.

For example:

If Battery = 10 kWh it doesn't automatically mean: Battery can supply 10 kW.

Suppose the battery's maximum continuous discharge is: 100 A

At approximately 51.2 V then

51.2Vdc × 100A = 5.12kW

So approximately 5 kW is available before considering losses and manufacturer limits.

For a 5 kW inverter, that's potentially appropriate.

But if you install a 10 kW inverter with one 5 kWh battery

you could have a serious battery-current limitation.

For lithium batteries, check:

• Battery voltage

• Maximum charge current

• Maximum discharge current

• BMS communication

• CAN/RS485 compatibility

• Approved inverter list

• Number of batteries allowed in parallel

• Cable requirements

• Battery fuse/breaker requirements

Don't simply assume:

"48 V battery = compatible with 48 V inverter."

The BMS communications and current limits can be equally important.

For example, Sunsynk's current guidance specifically requires checking battery compatibility and battery configuration, and its setup documentation specifies capacity and charge/discharge parameters.

Now we determine the PV size.

Suppose your house consumes: 20 kWh/day

and your design location effectively provides: 5 peak-sun-hours/day (harvesting Time)

The theoretical PV requirement is:

20 / 5 = 4kWp

But that's not what I'd necessarily install.

You need to account for:

• Temperature

• Dust

• Cable losses

• MPPT losses

• Inverter losses

• Panel degradation

• Non-optimal orientation

• Winter production

• Battery charging

• Cloudy weather

Therefore you might design around:

5–6 kWp

rather than exactly 4 kWp.

Actual PV yield should be calculated using local solar resource, orientation, inclination and shading, not a generic multiplier.

Now choose the panel.

For example: 550 W panel

If you need approximately: 5.5 kWp

then: 5500/550=10

So: 10 × 550 W = 5.5 kWp

But don't stop there.

You must design the strings.

Suppose the panel datasheet says approximately:

• Voc = 50 V

• Vmp = 42 V

• Imp = 13 A

If you put 5 panels in series:

Voc = 5 × 50 = 250VVoc

Vmp = 5 x 42 = 210V

Five panels might therefore form: 5S

Two such strings in parallel: 5S 2P

Total: 10 panels

PV power: 5.5 kWp

But this is where professional design becomes important.

Check three things:

1. Maximum PV voltage

You must calculate cold-weather Voc, not simply use the panel's nominal Voc.

Cold temperatures can increase Voc. Standards explicitly warns that PV voltage rises at lower temperatures and that the MPPT's maximum PV voltage must not be exceeded.

2. MPPT operating range

Your string Vmp needs to sit comfortably inside the inverter's MPPT operating range.

3. Maximum MPPT current

Two strings in parallel: 13A + 13A = 26A

The inverter MPPT must be able to accept that current.

For an inverter with multiple MPPTs, don't automatically put everything on one MPPT.

This is particularly useful where:

• Roof orientations differ

• One roof is east-facing

• Another is west-facing

• Shading differs

• String voltages differ

Avoid mixing substantially different orientations on the same MPPT unless the manufacturer's design rules permit it.

(See Figure 2)

Now we add the generator.

This is where hybrid systems become particularly interesting.

The generator needs to supply:

A. House load and potentially:

B. Battery charging at the same time.

For example:

House load: 3 kW

Battery charging: 3 kW

Required: 6 kW

Then a generator smaller than this could be heavily loaded.

Current guidance, for its systems, advises a generator with approximately twice the inverter's rated output to allow battery charging while supplying loads, although the exact generator requirement is model-dependent.

Don't apply that as a universal rule to every inverter. Always use the particular inverter manufacturer's generator specifications.

Suppose: 5 kW hybrid inverter

Possible generator: 8–10 kVA

But then we need to check:

• Generator continuous kVA

• Generator continuous kW

• Power factor

• AVR

• Frequency stability

• Voltage stability

• Minimum loading

• Maximum charging current

• Generator input current limit

• Neutral/earth arrangement

• RCD/earth-leakage arrangement

• Automatic start/stop capability

Generator guidance, for example, provides model-specific minimum generator sizes and warns that generator sizing must also account for charging and peak loads.

A sophisticated hybrid system can work like this:

Grid feeds Inverter

Generator feeds Inverter

Battery is a Load

House is a Load

Normal operation: PV → loads

Excess PV: PV → battery

If Grid is available:

Grid → supplement loads / charge battery according to settings

When Grid fails:

Battery → loads

Battery SOC becomes low Then Generator starts

Generator:

Generator → loads + battery charging

When Battery reaches target SOC:

Generator stops

If Grid returns:

Generator stops / grid resumes

This automatic generator strategy is a core feature of properly designed hybrid systems.

We describe exactly this approach: battery/inverter supply the loads first, then the generator can automatically start at a critical battery level and stop after recovery

A typical control strategy might be:

GRID FAILURE

▼

Battery supplies loads

▼

SOC declining

▼

SOC reaches 25%

▼

START GENERATOR

▼

Generator stabilises

▼

Transfer/enable generator input

▼

Generator supplies loads + charges battery

▼

Battery reaches 70%

▼

Stop generator

▼

Battery resumes

The actual SOC thresholds should be selected based on the battery, generator, load profile and manufacturer's recommended settings.

This is often overlooked.

The generator isn't simply another "230 V supply."

The inverter must be able to accept the generator's:

• Voltage

• Frequency

• waveform

• neutral arrangement

• earthing arrangement

• maximum current

The generator may also need:

• AVR

• electronic governor

• dry-contact start

• 2-wire start

• remote start module

Some hybrid systems can directly control generator start/stop; others require an external controller.

Your design will typically need appropriate protection for:

Grid → inverter

• Main isolator

• Circuit breaker

• Surge protection where required

• RCD/earth-leakage arrangements as applicable

Generator → inverter

• Generator breaker

• Isolation

• Appropriate protection

• Correct neutral/earth arrangement

Inverter → backup DB

• Main breaker

• RCD/RCBO arrangement as required

• Surge protection where required

PV

• DC isolator(s)

• String protection where required

• DC SPD where required

• Correctly rated PV cable/connectors

Battery

• DC-rated fuse/breaker

• Battery isolator

• Correct cable size

• Appropriate busbars/distribution

• BMS protection

Important: AC breakers and DC breakers are not interchangeable simply because their ampere rating is the same.

Cable sizing is not: "5 kW system = X mm² cable."

You cable size by calculate it from:

• Current

• Cable length

• Installation method

• Ambient temperature

• Grouping

• Voltage drop

• Short-circuit rating

• Cable insulation

• Manufacturer requirements

For DC: I = P/V

For a 5 kW inverter at 51.2 V: I = 5000 / 51.2 ≈ 97.7A

After considering inverter losses, you may be above 100 A.

That is why the battery-to-inverter cable can be substantially larger than the PV string cable.

Suppose:

5 kW inverter

Battery: 51.2 V

Approximate DC current: I = 5000 / 51.2 = 97.7A

Allowing for inverter efficiency: I ≈ 103A

Now calculate the cable based on:

• 103 A design current

• cable length

• allowable voltage drop

• installation method

• temperature

• protection device

Then select the cable and protection accordingly.

Never select the battery cable solely from the inverter's nominal power.

For a professional system, monitoring isn't an afterthought.

Monitor:

• Grid power

• PV power

• Battery SOC

• Battery voltage

• Battery current

• House consumption

• Generator power

• Generator runtime

• Inverter temperature

• Alarms

• Faults

This lets you diagnose problems remotely and understand actual energy behavior.

Residential systems, for example, use GX monitoring to monitor battery SOC, consumption, PV, generator and grid parameters.

This is the sequence I'd recommend you use for your residential hybrid system:

1. SITE SURVEY

↓

2. LOAD LIST

↓

3. DAILY ENERGY kWh

↓

4. PEAK LOAD kW

↓

5. ESSENTIAL vs NON-ESSENTIAL LOADS

↓

6. BACKUP AUTONOMY

↓

7. BATTERY kWh

↓

8. BATTERY kW / CURRENT

↓

9. INVERTER kW

↓

10. PV ENERGY REQUIREMENT

↓

11. PV ARRAY kWp

↓

12. PV STRING DESIGN

↓

13. MPPT VOLTAGE/CURRENT CHECK

↓

14. GENERATOR kVA

↓

15. GENERATOR CONTROL

↓

16. AC PROTECTION

↓

17. DC/PV PROTECTION

↓

18. BATTERY PROTECTION

↓

19. CABLE SIZING

↓

20. EARTHING/BONDING

↓

21. MONITORING

↓

22. SETTINGS

↓

23. COMMISSIONING

↓

24. TEST GRID FAILURE

↓

25. TEST GENERATOR START

↓

26. TEST GENERATOR STOP

↓

27. HANDOVER

Given Specifications from Customer

• Grid connected

• 5–8 hour backup

• Solar during the day

• Battery at night

• Generator for extended outages

• Essential loads only during outages

Load assessment

Average daily consumption: 18 kWh Essential backup energy: 10kWh

Essential peak load: 4.0 kW

Desired battery reserve: 20%

PV target: ~5 to 6 kWp

Inverter: ~5 kW class

Battery: ~15 kWh class

Generator: ~8–10 kVA class*

*Illustrative only; final generator size must be calculated against the selected inverter's charging capability and generator requirements.

When you're choosing actual equipment, create this table:

Hybrid inverter: kW, surge, battery voltage, MPPTs, AC inputs

Battery: kWh, usable kWh, kW, max current, BMS

PV panels: W, Voc, Vmp, Isc, Imp, temperature coefficient

MPPT: Max PV voltage, current, power, MPPT range

Generator: kVA, kW, AVR, start/stop, frequency

PV isolator: DC voltage/current rating

PV SPD: DC PV voltage rating/type

Battery fuse: DC voltage + interrupt rating + current

Battery isolator: DC Voltage/current

AC breaker: Voltage/current/breaking capacity

RCD/RCBO: Correct type and rating for application

Cable: Current, length, installation method, voltage drop

Monitoring: Meter/CT/shunt/GX/BMS communication

Earthing: Equipment + array + battery + generator requirements

If you're learning this as an installer / designer, remember these:

Rule 1

Load first. Equipment second.

Rule 2

kWh and kW are different.

Battery capacity is energy.

Inverter capacity is power.

Rule 3

Battery kWh isn't enough.

Always check battery maximum charge/discharge current.

Rule 4

PV voltage must be checked at temperature extremes.

Don't just add panel Voc numbers.

Rule 5

Generator size must consider charging + loads.

Not simply the inverter rating.

Rule 6

Protect every energy source.

Grid, generator, PV and battery each require appropriate protection.

Rule 7

Use the manufacturer's wiring diagrams.

Especially for:

• Generator neutral

• Earth bonding

• CT placement

• BMS communication

• Parallel batteries

• Multiple inverters

• AC coupling

Rule 8

Commission by testing failure scenarios.

Don't just switch the system on.

Test:

• Grid available

• Grid failure

• Battery low

• Generator starts

• Generator supplies load

• Battery charges

• Generator stops

• Grid returns

• PV production

• Excess PV charging

• High-load operation