DIY Grid-Tied Solar Panels – How To Calculate Solar System Size

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What Is A Grid-Tied Solar System?

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DIY grid tie solar system design methodology

There are two basic options when considering a solar panel installation for your home.

The electrical output of the system can be connected to the regular electricity supply, in which case it’s called an on-grid or grid-tie solar system, or it can be stand-alone, which is known as on off-grid system.

How does a grid tie solar system work?

A solar panel system connected to the electricity grid is a grid-connected or grid-tied solar system.

It is installed with a mutual agreement between the electricity supplier and the owner of the solar panel system that the output voltage level from solar panel power to the electricity grid power is at the same voltage and frequency.

The utility companies supply homeowners with electricity within strict limits to ensure that the voltage doesn’t spike too high or fall too low under heavy loads.

The frequency of solar system inverters has to be monitored so that the output is perfectly in-phase with the grid supply. Mis-match in frequency phase or value in cycles-per-second (Hz) can cause problems with induced current due to harmonics and interference with other electrical equipment.

Electricity can go either way in a grid-tied solar panel system i.e. from solar panels to the electricity grid or from the grid into the house.

When the solar system creates more than a home needs, the surplus is fed into the national grid and bought by the utility company.

Let’s call the power from the solar panel system the output power and from the electricity grid the input power.

Grid-tied inverter solar panel system principles

A grid-connected solar panel system is similar to large power plants connected to the grid, it’s just that the voltage and current on the bigger national production scale is very high.

The principle of operation of grid-tied solar panel system is that the direct current (DC) generated by the solar panels is converted to alternating current (AC) by a solar inverter which supplies it to the house or the local load through a switchboard and sends the excess electricity to the utility grid.

This was pioneered by Hoffman Electronics in 1959. In 1973, first grid-connected domestic solar panel system was built by University of Delaware and it’s called as Solar One.

From that time to present, grid-connected solar power is revolutionizing home-based electricity generation – millions are benefiting by using and selling the power of the sun.

These grid-connected solar panels systems are being installed everywhere and promoted in the fight against climate change.

Electricity consumption on our planet is increasing almost exponentially with time and electric vehicles are of course a reality.

Generating electricity at home is a very viable option and installing a grid-connected solar panel system is very beneficial for the present and future.

How Many Solar Panels Do I Need To Power My House?

There is no straight forward answer to this question. It depends on many factors:

Household energy consumption

How much electricity is consumed per day, per month or per year at your house?

Household energy consumption - solar grid-tie installation step 1

Divide kWh by number of days to find household daily energy consumption – Courtesy Peco

It can easily be worked out from your utility bills and is the first place to start when designing any solar panel installation. With seasonal changes energy consumption changes and is not consistent throughout the year.

A solar system can be designed to cater for minimum, maximum or average consumption patterns for your home.

The first stage of the solar system design is to find these values and is the essential first step for solar panel sizing and determining the number of panels required .

Irradiation or insolation level at your geographic location

solar insolation map of the USA for grid-tie solar system design

Solar panel output affected by insolation – WRI

The amount of solar radiation available at your location varies each day, month and year.

Irradiation is the term used to express how strong the sunshine radiation is at any instant in time and is measured in watts (W) per square meter.

Insolation is the amount of radiation over an area over time and is measured in kilowatt-hours (kWh) per square meter.

Both are important from solar system design perspectives. Irradiance can be measured easily by a solar power meter but usually insolation values are taken from maps provided for the purpose’

Generally, in the summer months irradiation is highest and lowest in winter. This is because the position of sun in the sky changes continuously.

A solar system can be designed for minimum, maximum or for average solar irradiation, the latter being the obvious choice for all-round performance.

Irradiance and therefore insolation levels at your location has a direct impact on the number of solar panels your home will need.

Roof space for solar panels

The amount of roof space or ground space which is available to install solar panels also defines size of your solar system and number of solar panels in it. The type and efficiency of solar panels used becomes an important consideration.

The solar array should face south as much as possible (for the northern hemisphere) and north (for southern hemisphere).

Smaller roofs may not have enough area to provide all household needs and feed excess electricity into the grid. In this case the solar panel arrays would be split and a proportion of them installed in the yard in an unshaded location.

Does solar panel orientation matter?

Orientation refers to the lateral placement of a solar panel array. In the Northern hemisphere the sun rises in the East and tracks across the sky before setting in the West.

It rises higher in the sky during the middle of the day but this movement is not as pronounced as the travel from East to West.

Increase solar panel output by automatic solar tracker

Dual-axis tracking maximises solar output

The best possible orientation is towards the South in order to maximise solar panel power output. If your roof faces East or West then the system would generate about 20% less than due South. South-East or South-West are good options too.

The very worst case is facing North. If you really can’t mount any panels on a roof then garden-mounting should be considered.

Mounting at ground level also raises the possibility of auto-tracking, an arrangement that tracks the sun and points the panels perpendicular to it’s rays for maximum power generation.

This kind of installation can produce up to 60% more electricity depending if it’s single or dual-axis control (more of this later.)

Best angle for solar panels

Roof angle also has impact on solar energy production. Flat mounted panels can lose something like 15% of their maximum value at the correct pitch.

If the pitch of your roof is anywhere between 25 to 65 degrees that will be fine for most purposes.

If you do have the possibility of making seasonal changes then tilting the solar panel array to your location latitude plus 15 degrees in summer and minus 15 degrees in winter will maximise the power output. (https://sam.nrel.gov/)

Tilt angle in degrees

Percentage increase in production - Southern USA

Percentage increase in production - Northern USA

Flat

Assume 100 comparison only

10

108 - 8% increase

109 - 9% increase

20

113 - 13% increase

115 - 15% increase

30

115 - 14% increase

118 - 18% increase

40

114 - 14% increase

117 - 17% increase

50

110 - 10% increase

116 - 16% increase

From the table above we can see that the increase in solar system power output due to tilt angle is more pronounced in the North than in the South.

If an array is installed with no tilt at 90 degrees (perpendicular) the power outputs falls by between 25 to 30 percent. This is the worst case scenario and should be avoided.

Types of solar panels for houses – what type of solar panels are most efficient for DIY installation?

For the same amount of electricity generated, the type of solar panels chosen will determine how many need to be installed.

Mono-crystalline solar panels are the most efficient and require less space for the same amount of electricity produced compared to poly-crystalline solar panels.

Thin-film solar panels will acquire even more space than poly-crystalline for the same amount of electricity generation.

TYPE

Monocrystalline

Polycrystalline

EFFICIENCY

Up to 22%

About 15%

LIFETIME DURATION

25 to 30 years

20 to 25 years

COST

Expensive

Cheaper

The three types of panels look quite different and they all have their place. The vast majority of homeowners choosing a DIY solar installation would use monocrystalline panels to optimise space and maximise efficiency.

The physical dimensions of domestic-use solar panels varies but tend to be big, because they are more cost-effective to install.

Panel sizes approaching 6 ft long and 4 ft wide are not uncommon, each weighing about 40 lbs. A structural roof survey is necessary if many panels are to be installed.

How are solar panels rated?

Solar panels for domestic use tend to have large outputs – between 200 and 400 watts per panel. However, this rating can be misleading.

Manufacturers rate their panel outputs according to laboratory conditions called STC (Standard Test Conditions) which basically states a panel’s wattage output at irradiance of 1000 watts/m2 and a temperature of 25 degrees C.

These conditions of STC are not realistic. Irradiance can vary a lot, depending on location and orientation, while a solar panel’s output reduces as it heats up, which it does when it faces the sun full-on.

A more realistic test standard is provided by NOCT standard (Normal Operating Cell Temperature) which gives a power rating at an irradiance of 800W/m² and temperature of 20 degrees C and open to a breeze of 1 meter per second.

It also specifies a tilt angle of 45 degrees – overall it’s a much more realistic way of assessing solar panel output power.

NOCT Solar Panel Testing Standard

  • Irradiance of 800W/m2
  • Temperature of 20 degrees C
  • Tilt of 45 degrees
  • Open to 1 m/s breeze

As a general rule-of-thumb a quality solar panel will generate approximately 75% of the wattage stated under the standard STC test conditions.

It means that when you buy a 100 watt panel, it’s really a 75 watt panel for all practical purposes.

The table below shows compares STC and NOCT power rating in watts for 5 popular solar panels:

Solar Panel Model

STC Rating (Watts)

NOCT Rating (Watts)

% Difference

LG NeON® 2 Black – LG345N1K-L5

345

259 

-25% 

SolariaPowerXT® Pure Black™

400

295

-28%

Trina Residential MBB

335

254

-26%

N330E HIT® Black Series AC module

330

306

-17%

REC Alpha Series

380

289

-24%

Average electricity consumption per day

Your energy consuming behavior

There is a saying that goes: “energy conserved is energy generated”. How you consume energy in your home makes a big difference in the size and cost of the solar installation you need.

Less electricity consumption means a smaller solar system in general. It obviously defines how many solar panels your house will require.

This is the reason in some countries EPC or Energy Performance Certificates are issued to houses and buildings which rate them according to the energy consuming behavior of that building.

However, your personal energy consumption and your home’s energy efficiency is not the whole picture.

AC units and solar panels

Large AC units draw heavy current at start

Many homes have large air conditioning units or other appliances that use hefty motors to run compressors or pumps.

An unfortunate characteristic of compressors used in AC units is that the motors stop and start regularly as the temperature is maintained.

An a.c. motor can use 3 to 4 times its normal running current when starting, particularly from cold. This spike in amps has to be accommodated by the solar system.

One way around this is to install an electronic soft-starter, which limits the current draw while the motor is coming up to full speed.

Solar panels return on investment – it’s payback time!

Whether from financing or savings, you want to know how cost-effective your investment in your solar system is.

A payback period of 7 to 10 years is considered normal for a solar installation and it can be more or less, depending on your exact circumstances. Your budget will also define the number of solar panels which can be installed to power your house.

The average cost of a US domestic home solar installation works out at around $3/watt (2020) and the average size is 5kW – total cost is approximately $15,000 for a solar panel array of 20 panels.

Equipment Needed For Solar Energy – DIY Solar System Equipment List

Essential components of grid tied solar system

  • Solar panels

Solar photovoltaic panels convert the sun’s light to electricity in direct current (dc) form. A number of panels are connected together to supply a specific voltage and design current to the load.

When the panels are connected in series, the voltage adds up and current remains constant.

When panels are connected in parallel, the current adds up and voltage remains constant. Series and parallel connection of solar panels can be combined in a solar array to get specific voltage and current design values.

Domestic solar - panels in series and parallel connections

Solar panel array voltage and current can be adjust by combining series and parallel connections

  • Junction boxes

The junction box collects the direct current from panels individually or together from the array of solar panels to the equipment controlling the electrical supply.

There’s a bit more to them than just a weather-proof enclosure for making connections. They also contain blocking diodes which allow current to flow in one direction only, so that power can’t feed back into the panels and damage them.

The convention for connecting solar panels to the boxes is by MC4 connectors.

Domestic solar system junction box and MC4 connectors

Junction box with by-pass diodes for solar panel protection

  • DC isolator/MCB (Miniature Circuit Breaker)

DC solar system miniature circuit breakerThis is a safety device installed just before the inverter. In case of repair or maintenance of the solar system this switch is used to isolate the solar array so that any energy produced isn’t fed into the rest of the system.

Normally MCBs are mounted on a DIN rail inside a large junctions box, perhaps with other components and are generally rated 30% to 50% more than the rated current output of the array.

  • Solar inverter

This is probably the second most important component after the solar panels. It’s main purpose of an inverter is to convert direct current from the solar panels to alternating current for use with domestic appliances or to feed into the electricity grid.

The solar inverter also matches the voltage and frequency of the power from the solar panel to that of the grid.

The solar inverter employs MPPT or Maximum Power Point Tracker technology to maximize output from solar system.

Every solar panel has a certain relationship between its voltage and current when it delivers maximum power, hence the name ‘Maximum Power Point’.

The inverter tracks and adjusts its parameters to ensure that the panels deliver the maximum power possible for the conditions.

  • PV power meter

Monitors and analyzes the power generated by the solar array.

  • Single phase or three phase fused switch or AC isolator/MCB

This device can be placed between the PV power meter and import/export meter. Its purpose is to disconnect in case of  short circuit or can be manually disconnected to isolate the a.c. side of the system.

  • Import and Export meter

Solar import export meterThe export meter reads the power going out of the solar system to the electricity grid and the import meter reads the power coming in to the consumer load.

  • Earthing line

Solar panels, consumer junction and grid connection are all connected together with a suitably sized earthing cable which is terminated and Earthed at a suitable location in the ground.

 

What Is The Difference Between A Grid-tied Inverter And A Regular Inverter?

The basic purpose of any inverter is to convert direct current or DC to alternating current but not too many people know that it can also convert AC to DC. This is done by transformer, control circuits and switching in the inverter.

Alternating current is used in our home appliances because it is easier to generate at the power station, safer to transmit over long distances and efficient to transform into lower voltages for the home.

A regular inverter converts AC from the grid and converts it to DC to store it in a battery.

If there’s a power cut the inverter switches its operation and starts converting DC to AC, which is supplied to the connected load like home appliances.

When the grid electricity is back, the inverter reverses its function and starts filling up the battery again till fully charged.

A grid tied inverter is similar to a regular inverter for its basic operation of converting DC to AC and if it’s a hybrid system with batteries it can convert AC to DC too. Solar power generated as DC is converted to AC to be supplied to the grid or to the home.

A grid-tied inverter is different from a regular inverter with its ability to send power to the grid.

The solar power output, depending on the amount of sunshine available, has a varying generation profile. The AC power in the grid is flowing within strict limits of voltage and frequency.

The power from the solar panels should match these values and this is done by maximum power point technology or MPPT in the grid-tied inverter.

The MPPT in a grid-tied inverter tracks the frequency and voltage of the grid electricity and regulates solar electricity conversion to match them.

Another function of a grid-tied inverter to regular inverter is anti-islanding. In case of grid failure, the inverter shuts down the power production from the solar panels and basically it is disconnected.

It is done for the safety of the grid when it’s not transmitting current and for men working on grid repairs.

Two types of inverter can be used for grid-connected solar system; the string-inverter and micro inverter .

A string inverter is connected to an array of panels giving specific voltage and current which is fed to a string inverter for conversion from DC to AC. This technology is mature and is cheap too but has less life.

A more efficient method is to connect a micro inverter to each solar panel to convert DC to AC and perform MPPT functions at the panel itself, optimising individual solar panel output.

String Inverters (DC to AC in one location)

Micro-inverters (DC to AC at each panel)

High DC voltage transmitted across roof

Normal AC voltage easier to transmit

Single point of failure stops system

Micro-inverter failure affects only one panel

Bad/shaded panel affects system performance

Bad/shaded panel has minimal effect overall

How To Wire Grid-tie Inverter?

The most common inverter or string inverter is wired near to the solar array (of panels).

Between the inverter and the solar array is a DC junction box and a DC isolator or DC MCB. + (positive) and – (negative) wires from all solar panels of an array is brought together and connected to a junction box (it’s like the first collector of current).

From this junction box the + and – wires are connected to + and – terminals of DC isolator or DC MCB from where it can be isolated or the DC current coming from the junction box can be stopped.

Grid-tie solar string inverter basic connection

From the DC isolator the + and – wires are connected to + and – terminals of the inverter.

The AC current from the inverter can be first connected to a PV meter to monitor power generated by solar panel system.

From the PV meter, the AC wire is connected to the AC isolator and ahead to Export meter (to grid) or to connected load.

The micro inverter wiring is bit different. The DC MCB is absent in such installation and micro inverter being at the solar panel location sends AC to the AC MCB.

The DC junction box is located at each solar panel and transmits current directly to the micro inverter at very short distance.

The current from micro inverter is connected to an AC MCB from where it is further connected to the grid and connected load.

What Is Grid-tied Inverter With Limiter?

If exporting electricity to the grid is not allowed in your region or you don’t want to export electricity to the grid then grid tied inverter with limiter is useful.

The limiter checks the power requirement of the load of the house and only signals the inverter to supply only that amount of electricity which is required by the load.

A normal grid-tied inverter supplies electricity generated by the solar system to the load and if in excess it is sent to the grid.

As the system is grid tied, electricity is available from the grid when solar panels are not generating electricity like at night or due to exceptional weather conditions.

For both inverters, without limiter and with limiter, grid electricity is available to be used. In case of grid-tied inverter with limiter, excess energy is not sent back to the grid.

There can be any reason for you to not send electricity to the grid but you need grid electricity too to fulfill your electricity needs.

A grid-tied limiter will be of good use in this case and if you consume full power generated by solar system then the excess electricity can be stored in a battery.

Such a system can reduce grid electricity dependence and make full use of solar energy.

The day time load can be managed by solar panels and inverter alone, while at night batteries can be used to manage electricity and differences can be compensated by the electricity grid.

Grid-tied Solar System Diagram

Grid Connected Solar System Design Connection Diagram

Basic Grid-Tied Solar Panel Installation Schematic Showing Main Components

Grid-tie Solar System Design Calculations

Sizing and system designing of solar PV systems

What will be the number and wattage of solar panels needed for a fixed solar installation to cover consumption of an average U.S. home? (Assume 100% Ratings of all equipment used in installation.)

Let us assume the house to be in Burns, Oregon USA, with an average electricity consumption for a month in the year as 1000 kWh and per day it’ll be around 33 kWh.

As per Global Solar Atlas, per day average direct normal irradiation at this location (Burns, Oregon USA) is 5.81kWh/m2.

  • Location: Burns, Oregon, USA
  • Average energy consumption: 100kWh/month or 33kWh/day
  • Irradiation for Burns, Oregon (Global Solar Atlas): 5.81kWh/m2 per day

We need number of solar panels to supply 33kWh per day of electricity to house when it gets average 5.81kWh/m2 of solar radiation.

Now 33kWh/day supply to house is through a PCU or power conditioning unit which has inverter and MPPT function. Adding 30% to the electricity consumption which will be supplied by the PCU, we can use line loss factor 1.44.

PV System Losses

A figure of 30% is used to compensate for the overall system losses, which is on the high end to make sure the number of panels isn’t underestimated. Individual losses are small but accumulate in large array systems – see table below:

Cause of Energy Loss

Percentage Loss

Total Losses

Shading

7%

23.8% lost

Dust and Dirt

2%

Reflection

2.5%

Spectral Losses

1%

Irradiation

1.5%

Thermal Losses

4.6%

Array Mismatch

0.7%

DC Cable Losses

1%

Inverter Losses

3%

AC Cable Losses

0.5%

With these losses in mind, the energy requirements become:

  • 33kWh/day x 1.44 = 47.5 kWh/day is to be supplied by the PCU to the load.

Considering efficiency of inverter to be 96% the amount of electricity to be supplied by the solar panels to the inverter = 47.5/0.96 = 49.5kWh/day.

Average normal irradiation is 5.81kWh/m2.

  • 49.5kWh/day of electricity can be generated from 49.5/5.81= 8.5kW or 9kW of solar panels working at 100% capacity rating.

To calculate no. of solar panels we can divide wattage required by wattage of desired solar panel.

  • Number of panels required = 9kW/300w per panel = 30 solar panels.

Number of panels required will be 30 if 300W panels are used at your house in Burns, Oregon. This will satisfy 33kWh or less electricity consumption of any day of the year.

2. The same design calculation as above but the homeowner wants to have 20% spare capacity to sell back to the utility company.

If 20% spare capacity is to be added to the same system, then 11kW of panels will be required to satisfy consumption of the house and sell excess to the utility company.

Number of solar panels will be 37 if 300W panels are installed and capacity of solar system is increased to 11kW.

3. The effect of dual-axis tracking

There is a wide variation of efficiency of solar systems with dual axis tracking – it can vary between 20% to 80% more than fixed solar panels.

Let’s assume the dual axis tracker can increase energy yield by 40% as compared to a fixed solar panel array, then the number of solar panels can be reduced.

In this case 6kW of solar panel system will be able to cater for 33kWh or less of electricity consumption.

Only 20 solar panels of 300W each with dual tracker will be required to cater the electricity consumption of 33kWh or less per day.

How Many Solar Panels Do I Need To Power My House Calculator

 

Related Questions

Can a house run on solar power alone?

A big enough solar panel array may run all house appliances during a sunny day, but in cloudy periods or at night batteries would be needed as solar panel produce no energy without sunlight.

What happens to solar panels after 25 years?

Solar panels have life of 25 to 30 years. After this time the output may reduce to 80% of their original power. Solar panels will continue to generate electricity for many years at a gradually reducing rate.

Will a 5kW solar system run a house?

The average US home energy consumption is 30kWh/day, which requires a solar panel array of 9kw or 30 solar panels each rated at 300 watts. A 5kW solar system will run a house if its energy consumption is 17kWh or less.

What is a grid-tie solar system?

 


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