What Size Solar System Do I Need? How To Size A Solar System Step By Step

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What Size Solar System Do I Need

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How To Size A Solar System (1)

How Many Solar Panels Are Needed To Power An Average House?

This is one of the most common solar energy questions. We’re so used to getting quick answers to our questions – we don’t want to be frustrated with an answer like ‘it depends’. That’s why most sites answer the question by averaging everything out, or simply collecting data from existing installations and using the data. The stock answer goes like this:

The average house consumes 30kWh of electricity per day and needs 25 solar panels each rated at 300 watts. 

But what if you’re proficient in DIY and want to install you own solar panels? How do you find out how to size a solar panel system? As an ex-installer I have to admit I did very little design work. I simply sent the customer’s energy consumption to the solar system supplier and they did all the design work.

When I retired I became more and more interested in calculations behind it all and how to apply solar energy technology to a whole range of DIY solar projects for the home.

Grid-Tie or Off Grid?

Solar panel installations come in two flavours, or three, if you count hybrid systems. Strictly speaking, hybrid installations combine the other two. I’ll be covering a hypothetical hybrid installation later in this post and explain the calculations made.

What is grid tied solar?

Most countries have government schemes whereby homeowners can generate electricity with a roof solar panel installation and sell the excess back to the utility company.

At one time the buy-back rates were so good that some private companies would install a solar system for free and take a cut of the buy-back profit for a certain number of years. Meanwhile the homeowner has free electricity (when the sun shines.)

This kind of system is connected to the electricity grid through special meters that monitor the flow of electricity – into the grid or out of the grid. A grid-tied solar system needs an inverter (read about it here on another of my posts)to convert DC into AC power and this device matches the converted AC volts and frequency (Hz) to that of the electricity grid.

A pure grid-tied solar system has no batteries connected, so when the sun stops shining, or at night, the panels stop generating electricity and the grid powers all home appliances.

Grid-tied solar system diagram

How To Size A Solar System Step By Step - Grid Connected Solar System Diagram

What is off grid solar system?


An off-grid is simpler in construction in that the converted AC power doesn’t have to be matched to the grid frequency – it isn’t connected to it! However, voltage and frequency still have to conform to strict values so that home appliances operate normally without damage.

Preppers and others who don’t want to connect to the utility grid have these systems installed but they have one major drawback; on cloudy days and at night-time no electricity is produced. A battery bank needs to be installed to supply a home’s needs during these periods and this increases the cost of the solar installation considerably.

In a working off-grid system the solar panels charge the batteries and also supply the inverter at the same time. If the home’s demand for power is high, then the power available for battery charging is lower. Solar panel and battery bank sizing calculations ensure that both are catered for.

In recent years electricity outages are becoming quite common in some regions and so hybrid systems are appearing on the scene, particularly with the rise of lithium battery technology. The hybrid installation has a battery bank connected to cover for a certain number of hours without grid supply. Switches automatically change over from grid to battery and back again when the supply returns.

Off grid solar system diagram

Off-grid solar panel basic drawing

What Size Solar System Do You Need?

  • Find out your home’s average kWh usage per day

A home’s energy consumption is measured in units of electricity called kilowatt-hours (kWh). If you had a kettle rated at 1kW (found on the product label) and it ran for 1 hour, then it would have used 1kWh of energy – pretty simple stuff.

You need to know how much energy your homes uses each day before sizing the solar panel installation and other equipment. The easiest way to do this is by checking a utility bill, which normally gives a kWh reading over a month or another period of time.

Sizing solar panels - Pepco utility bill sample - kWh

Courtesy Pepco

Divide the total kWh by the number of days and you have the daily energy usage. The average for the U.S. is about 10000kWh per year, 833lWh per month or 27kWh per day.

This value will be used to determine how many solar panels are needed.

  • How many solar panels do I need to power my house?

Many sites and installers have ‘rule-of-thumb‘ guidelines for all things relating to solar and this approach can work quite well, but it can lead to the installation of too much, or even worse, not enough solar panels.

As an earnest DIY enthusiast, I want to know more. Not incredibly technical information but basic things like; If a panel is rated at 300 watts, does that mean I definitely get 300 watts of power when I put it on my roof? If not, why not? What affects solar panel output?

How are solar panels rated?

What affects solar panel wattage output

Confused yet? Let’s dive in.

Solar panels are marketed by suppliers with ‘100 watt’, ‘300 watt’ or some other power rating, but is that the power we can realistically expect from a panel?

The most common rating for the public is the STC (Standard Test Conditions) set of testing conditions. Basically it reflects a solar panel’s output in ideal laboratory conditions. In reality, the output in watts from any solar panel is far less.

  • STC solar panel rating: 25 degrees C, 1000W/m2 and  1.5 air mass spectra (atmospheric conditions)

Another set of test conditions Nominal Operating Cell Temperature (NOCT) lays down more realistic conditions, namely lower operating temperature and irradiance levels.

  • NOCT solar panel rating: 20 degrees C, 800W/m2 and all-round breeze of 1m/sec

A further set of conditions were defined in the U.S. called PTC or Performance test conditions which adjust previous values and adds altitude in order to more realistically reflect power output in real conditions.

  • PTC solar panel rating: 20 degrees C, 1000W/m2, breeze of 1m/sec and altitude above ground 10m

It’s worth noting that the operating temperature of solar panel mounted on a roof can be much higher than these standards – 40 degrees and above is not uncommon. Power output reduces by about 0.5W for each degree centigrade.

The table below compares some well-known high quality panels and their STC/NOCT ratings:

Brand

Panel

Efficiency %

Life in Years

STC Rating Watts

NOCT Ratng Watts

LG

LG335N1C-A5

19.6

25

335

247

Panasonic

VBMS380!JO9

19.6

25

380

283

Canadian Solar

CS3U-355PB-AG

20

30

345

256

Solar World

SW 255 mono

17.9

25

255

184

Yingli Solar

Panda 60 Cell

17

10

280

202

All of the methods for rating solar panels assume one thing – that the panel is perpendicular to the sun’s rays in all four planes. This almost never happens in real life and this is the first place we start to lose power output.

The effect of tilting solar panels

Solar panel are most often installed in a fixed location such as the roof of a building which is fixed in place. The roof title is fixed at a certain angle to the horizontal and always remains so. Each degree of angle reduces the power output from the cells by a certain amount, but this is not as much as you would think.

If we say that in the Northern hemisphere the optimum angle is 45 degrees (this varies according to latitude) then the power lost if the panels were at less than 10 degrees or flat is only 10% to 15%. If you do have the possibility of tilting the panel framework then an angle of your latitude +15 degrees would give you the best overall energy production in Summer and your latitude -15 degrees is best for the Winter months.

The very worst angle to mount panels is at 90 degrees, or upright. In this case you’ll lose 30% of the output. The very best direction in the Northern hemisphere is South but small variations are not too serious – South-East and South-West are good.

Impact of tile and direction on solar panel output

Best direction for solar panels – the effect of panel orientation

The question of orientation has a much more marked effect, as the distance travelled by sun as it moves across the sky from East to West is much greater than the difference in it’s height across the seasons.

If the panels face due East or West then you would stand to lose something like 20% of your electricity production over the year; not an ideal way to maximise your investment. North is the worst case scenario but loss of output isn’t as great as you would imagine either, due to the effect of diffuse sunlight reacting with the solar cells.

When figuring out the number of solar panels you would need for your home you need these key pieces of information:

  • daily energy consumption in kWh
  • peak power (the greatest power that might be taken at any time, when a fridge motor start for example)
  • roof tilt angle (if fixed installation envisaged)
  • roof orientation (direction)
  • insolation value (amount of sunshine energy falling over a period of time – daily, in kWh)
  • solar panel output (real NOCT)

We discussed energy consumption and how to get it from your utility bills. Peak power draw might be a little tricky. The best way to determine this is by simply asking your local electrical contractor or solar installer.

Roof title and orientation with respect to the sun are simple measurements and the realistic solar panel rating in watts can be found on the solar panel spec sheet.

STC rating of solar panel vs NOCT rating in watts

Courtesy Unboundsolar

Solar maps are a great way to find the insolation for your exact location. Just enter your latitude and longitude, and the value returned can be used with confidence in your calculations as it’s based on real historical data. A solar power meter would give you an instant reading (irradiance) but you need to know how much energy shines down over the whole day.

Typical Solar System Design Calculation

  • Location: Goshen, Indiana, USA: 41° 34′ 56″ N / 85° 50′ 3″ W
  • Energy Consumption: Average 28kWh/day
  • Battery Backup: 24 hours
  • Roof angle: 35 degrees
  • Roof Orientation: South
  • Panels to be used: Canadian Solar 335W (STC)

Average yearly consumption of a domestic consumer is around 10,000 kWh. So, the average load for a specific day is calculated around 28kWh. Considering 28kWh load for the whole day, the average ‘spot load’ could be estimated 1.17 kW.

Design Calculations – The Solar Panels

As detailed and component wise consumption is not available, so we can considered 2 KW spot power for system design.

Use a solar ‘sun hours map’) to estimate available PV resources. The system should be sized based on the month with the highest power consumption and/or lowest solar resource, typically December or January in USA.

As per NREL solar map, Indiana state has low solar insolation in January. Generally, 2.5 sun hours is a good estimate, but it could be a little lower or higher depending on your location. We will use 2.5 minimum sun hours for our design.

How many solar panels for your home calculation

  • Projected Load:  2000 watts (W)
  • Daytime Usage:  6 hours
  • Total Load: 12000 watt-hours (Wh)
  • Panel Design Factor: (to account for system losses) 1.3
  • Panel energy required: 12000 x 1.3 = 15880Wh
  • Effective sunlight time: 2.5 hours
  • Total Modul Power Required: 15880/2.5 = 6345W
  • Inverter Required: 6000 watts
  • Number Of PV Panels Required: 6352 watts/335 = 19 (assume STC panel rating of 335 watts/per panel.)

This number of panels covers the day-time , but energy consumption of 12000 watt-hours but what about night and those times in the day when production is very low?

This load will be fed by the batteries which need to be charge during the day:

  • Load supplied by batteries = 28kWh/day – 12kWh = 14kWh
  • Apply factor for system losses= 14kWh x 1.3 = 18.2kWh
  • Solar panel power required = 18200/2.5 peak sun hours = 7280W
  • Number of panels required to charge batteries = 7280/335 watts = 22 panels of 335 watts

The total number of panels required for an off-grid solar system in this average American home is 41 panels of 335 watts rating. (13635W)

Solar battery bank calculator


The following sections detail how to calculate solar panel battery and inverter for this fictitious homeowner’s system to determine how many batteries to power a house in Goshen, Indiana.

Battery Size Calculation:

The following calculations were conducted considering 28kWh consumption for 24 Hours and a 24-hour battery backup.

Backup For 24 Hours

Rating

Measurement Units

Total load for 24 hours backup

28000 

Watt-hours (Wh)

Battery bank voltage

48

volts

Total amp-hour rating

28000/48=583

amp-hours (Ah)

Required Batteries (60% DOD) Lead-Acid

583/0.6 = 972 (say 1000)

Ah

Number of batteries @250Ah each

4

The calculated battery backup is 1000Ah (Including DOD at 60%) for 24 hours uninterrupted power supply in case of grid failure. As per inverter DC input (48VDC input) requirement, 4 batteries with rating 12V each (Total will become 48VDC) should be connected in series.

The above calculation takes into account the use of lead-acid cells whose life varies drastically according to DOD (Depth of Discharge.) The lead acid battery in your car has a very low DOD, as it needs to supply power for just a few seconds before the engine tuns and charges it back up.

A deep-cycle lead acid battery can theoretically be discharge up to 80% but this drastically reduces it’s efficiency and life. If discharged to only 50% it would have 3 times the useful life. If more batteries were installed then perhaps a DOD level of 20 to 30% could be achieved and the batteries would last even longer.

Even so, the realistic life of any battery is tied to charge/discharge cycles and a lead acid cell’s life is around 7 to 10 years IF treated well with low DOD. IMO the best technology for modern solar energy system backup is lithium phosphate.

lithium phosphate battery best for solar energy storage

Known as LiFePO4 batteries, they are considered to be ideal for solar energy storage. Some types of lithium cell (li-ion) similar to the ones used in Tesla cars have been known to overheat and even explode, but lithium phosphate is the safest of them all.

Although not normally a design consideration in fixed installations, LiFePO4 batteries are lighter than lead-acid; just a third of the weight for the same amp-hour capacity. I use these in applications such as my DIY motorized solar inflatable kayak. Before this I had a series of batteries starting with a 90Ah lead-acid weighing 30kg!

The big advantage of lithium over lead-acid is the Depth of Discharge and length of life. Lead acid cells have to be designed specifically to give deep discharge of between 50 to 80%. LiFePO4 batteries are inherently deep discharge, lasting many years. Used with a well-sized solar panel array lithium batteries can be a life-time purchase!

With a discharge  of 80% a battery will last around 2000 cycles and up to 5000 cycles at 60% discharge. If a system is designed well, these batteries will last for many years.

How to choose inverter for solar system – Solar Inverter Calculation

The selection of the solar inverter depends upon the selection of the specific solar system solution. As per system design, a 4KW solar Inverter must be install to control the above-calculated load and the inverter must have a minimum 60A charging current.

For the best power quality and sustainable solution a hybrid inverter with MPPT charge control and pure sine wave is recommended. The ideal inverter can be connected with 120 or 220VAC input voltages and has output voltage options for 120V and 220V loads.

It needs to have the capacity to run air conditioners and similar equipment incorporating compressors that have inherent surge currents associated with motor-startup. To run 110V appliances on this inverter you need to connect to a 120V breaker.

A generic connection diagram of the system is shown below.

how many solar panels design calculations hybrid schematic

Electrical Wiring Schematic

The table below shows the list of components that will be used for this hybrid Solar system installation.

Component Description

Quantity

Solar Inverter With Controller - 4kW, MPPT, split-phase, pure sine wave

1

Solar panels - 335 watts each. The string voltage range should be in between 60~ 145VDC.

12

PV Combiner Box - Midnite Solar MNPV6 to connect 4 PV arrays.

1

Circuit Breakers - Midnite Solar MNEPV-20

4

Solar Panel Mounting - IronRidge XR100 Roof Mount Racking Kit

12

Batteries - (12 V, 250 AH lead-acid),
The batteries must be connected in series for Inverter DC input at 48V. 

4

DC cable for Solar Panels - MC4 connectors with 10 AWG Cable Extension 

(Length as per requirements)

Here the percentage costs of the installation equipment without labour costs (complete price including labour and permits approximately $3.5 per watt by a professional installer.)

  • Solar panels: 37%
  • Inverter: 10%
  • Batteries: 35%
  • Rest of the system (mounting, cables, breaker, junction boxes): 18%

The approximate cost of equipment only is about $8200. Note: Cost of professional installation complete (without batteries) would be about $18000.

Payback Calculations Solar Panel Payback Period For This Installation

  • Assume that Unit elec. cost Indiana = 13.5 cents/kWh
  • Considering 10,000 kWh bill for a year = 13.5 * 10,000 = 135,000 USD = 1350 USD
  • Monthly Average Utility Payment = 113 USD
  • Total Cost of designed solar system (DIY installation) = 8,200 USD
  • Recovery Period= 8200/1350 = 6 years

The above calculations are based on estimated load consumption; the actual design parameters would be finalized after detailed discussion with the customer.

Tracking solar panels vs fixed

Dual axis solar tracker panel output gains

So far we’ve assumed that panels will be fixed onto a roof more or less permanently, which is the most common situation for homeowners. A system called auto-tracking can significantly improve the output in watts from a solar panel array, thereby reducing the number of panels and the size of inverter required.

There are two main types of system; single axis and dual axis. The sun’s trajectory varies according the time of day and also the season. The effect of the sun’s movement across the sky in daytime is far greater than the vertical changes and so a single-axis tracker follows this movement from East to West. A single axis tracker can improve solar output by up to 40%.

The sun also moves vertically during the day and a dual-axis tracker follows this movement as well – it’s the best possible solution to gathering the maximum amount of energy from the sun. A dual axis tracker can improve solar panel output by up to 65%.

Web Story – How Many Solar Panels To Power A House

(Click image to advance to next page)

Related Questions

How much does angle affect solar panels?

Flat mounted solar panels can lose 15% of their output capacity compared to the optimum angle for the installation latitude. the worst angle is 90 degrees, when the panel is upright. The output may fall by 25 to 30% at this angle.

How do you determine the best angle for solar panels?

The general rule of thumb is to add 15 degrees to your latitude in Summer and subtract 15 degrees in Winter. Seasonal adjustment for tilt angle is recommended to maximize solar panel output.

What is the best direction for solar panels to face?

The best direction to point solar panels in the Northern hemisphere is due South. South-West and South-East are also reasonable orientations but never point them North. However, in the Southern hemisphere the best direction fo a solar system is North.


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