What Is Off Grid Solar System? Is Off Grid Solar Worth It?

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What is off grid solar?

Is off grid solar worth it?

An off grid solar system is an autonomous power generation system that produces electricity from the sun’s energy using solar panels. With no connection to the utility grid, the home is self-sufficient in energy. An off grid system needs solar panels, a battery bank, a battery charger and an inverter.

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Introduction to off-grid solar

Off-grid living is the dream of many home owners, whether wanting to reduce bills, achieve independence or prep for a dystopian future when energy is scarce.

It fits in very well with the American pioneering spirit and DIY off-grid solar appeals to the American ‘can do’ approach to life. How does an off-grid solar power system produce electricity and what do you need to build one?

Read on …

Do it yourself off grid solar systems

More and more people are building their own solar power systems, both off grid and on-grid (grid-tie). Grid-tied solar energy is the most popular.

Power feeds in and out of the utility grid and in most states over-production is credited and you can use it when the sun doesn’t shine – it’s a great system.

Off grid users need a couple more components than their on-grid cousins, simply because solar panels will stop generating electricity at night and the output may also be severely reduced on cloudy days.

Build your own off grid solar system

Before considering building an off grid power system yourself, it’s a great idea to fully understand how they work and most importantly how to size the components. This is crucial.

There are many resources, such as this site DIYSolarShack, to help in off grid design, but it can save a lot of time and thinking power if you simply hire a professional installer to do the calculations for you.

Video – DIY Off-grid solar design and installation

 

How do off grid solar systems work?

All solar generating systems produce electricity from the sun and use the same basic solar panels to do that. The difference between off grid and on grid solar is defined by what happens to the electricity after it’s produced.

Grid tied solar systems feed into the utility grid, which acts as a giant energy storage system. Sometimes it acts as a load and other times a supply of electricity.

Off-grid systems need an energy storage system as well and this takes the form of a bank of several batteries.

The power that passes from the solar panels to the batteries as they charge is DC. Normally, this DC voltage is converted to AC by an inverter, allowing standard home appliances to be used.

Some people may decide to use only DC appliances and this makes the whole system more efficient. You’ll see why later when I explain how inverters work.

What is needed for off-grid solar system?

Elements need to build an off grid solar system

The 4 elements needed for off grid solar are solar panels, solar battery charger, batteries and an inverter

There are 4 basic components of an off grid solar power system:

  1. Solar panels – for converting the sun’s energy into electricity
  2. Battery bank – the energy storage system
  3. Solar battery charger
  4. Inverter – conversion of DC to AC

What are the components of solar power system?

1. Solar panels

What type of solar panels for off grid solar?

There’s a range of solar panel types to choose from for off grid solar power

Solar panels may be monocrystalline or polycrystalline, and fixed or flexible.

Monoscrystalline are more efficient, and therefore more expensive, but the gap in efficiency between the two is closing all the time.

Panels range from 100 watts up to 400 watts and the higher power outputs often represent the best value for money. Most company run deals with deeper discounts the more you buy.

Flexible solar panels are becoming popular, particularly when mounting on stuctures that may not take the weight of fixed panels. Flexibles weigh 20 times less than fixed solar panels.

Characteristics

Monocrystalline

Polycrystalline

Thin Film (amorphous)

Composition

One crystal

Multiple crystals

Thin silicon layers

Color

Black

Blue

Multiple - varies

Efficiency

17 to 23%

10 to 14%

5 to 6%

Lifetime Duration

25 to 30 years

20 to 25 years

15 to 20 years

Manufacturing Process

Complicated

Easier

Complicated

Cost

Expensive

Cheaper

Expensive

Temperature Tolerance

0 to 5%

-15 to +5%

-3 to +3%

What size off grid solar system do I need?

Solar system sizing depends on you estimated annual energy usage. If your home was connected to the grid, then this is easy to find as it appears on your utility bill.

Simply read the previous year’s energy consumption in kilowatt-hours (kWh). An average value for the US is 11000kWh per year or 30kWh/day.

If it’s a new build, then you need to estimate your average energy needs from the appliance ratings. This is fairly easy, but fridges, freezers and AC need special consideration – more later.

On of the most common mistakes when designing off-grid solar systems is to undersize the installation, either by under-estimating the energy needs or over-estimating the annual energy a system will realistically generate.

The table below shows average power ratings for common household appliances:

Appliance

Rating (watts) Note: Starred items use extra power at start-up - see article text

Ceiling Fan

60

Dehumidifier

240

Electric Can Opener

170

Rice Cooker

200

Slow Cooker

160

Vacuum Cleaner

200

Desktop Computer

100

Home Sound System

95

Television

230

Electric Blanket

200

Paper Shredder

220

Projector

270

Corn Popper

275

Crockpot

250

VCR

100

Electric Fence (25 miles)

250

Laptop Charger

125

Food Processor/Blender

360

Monitor

250

Electric Hedge Trimmer

300

10000 BTU central AC

1500 (4500) *

Refrigerator

400 (800) *

Deep Freezer

500 (1500) *

Electric Oven

2150

Fridge Freezer

700 (2100) *

Washing Machine

1200 (2500) *

 

The starred items pull more power at startup so extra battery capacity should be added at the design stage to make sure these demands can be met.

Many appliances will give an average annual energy consumption on their labels, which is mostly the case for appliances with compressors on board, and also washing machines.

For the rest of your appliances, add up the power ratings in watts and estimate how many hours per day/month/year they will be used. Multiply the total watts by annual hours to find the estimated energy needs of your home.

It’s a good idea to add 20% to account for any future expansion.

How to calculate off grid solar system

Let’s say the energy required is the same as the US average at 30kWh/day.

A typical Solar System Design Calculation needs these values:

  • Location: This is needed because different locations have different irradiance values (sun’s energy)
  • Energy Consumption: We’ll use 30kWh/day
  • Battery Backup: Assume we want to have enough battery power for 24 hours
  • Roof angle: Optimum for latitude (use 45 degrees average)
  • Roof Orientation: As due South as possible
  • Panels to be used: 300 watts

I’ll choose the location as Houston, Texas. Irradiance for any location can be found using a site such as Global Solar Atlas and others, which have a database of global irradiance per day or year.

The image below shows the daily irradiance for Houston Texas:

Off grid panel sizing - irradiance houston tx

Irradiance is used to size solar panels for off grid systems

Irradiance for Houston Texas is 4.67kWh/m2/day, also known as peak-sun-hours.

How many solar panels do I need to go off grid?

Dividing 30kW by 24 hours we get a ‘spot’ load of 1.26 kW – this is the average load we can expect at any instant in time over 24 hours.

However, most load is drawn during the day, so I’ll assume that spot load in daylight hours is 2kW and that load will be drawn for an 8 hour period.

  • Total load daylight = 2kW x 8 hours = 16kWh

Multiply this load by 1.3 to account for solar system losses (up to 23%):

  • Total daylight panel energy required = 16kWh x 1.3 = 20 kWh
  • Total Module Power Required: 2000/4.67 = 4282 watts

Number of solar panels required = 4282/300 watt per panel = 14 x 300 watt solar panels

That accounts for energy usage (16kWh/day) during the day, but what about night? I estimated that 12kWh may be used during the times when solar panel generation is very low or non-existent.

This means that the solar panel array has to provide this amount also to charge the batteries ready for night-time:

  • Total charging load = 12kWh x 1.3 (solar system losses) = 15.6kWh
  • Solar module power required = 15.6kWh/4.67 peak-sun-hours = 3340 watts
  • Number of solar panels required = 3340/300 watt per panel = 11 x 300 watt solar panels

The total number of solar panels for this off-grid system in Houston Texas is 25 x 300 watts (7.5kW).

(Disclaimer: These figures are for guidance only – I strongly recommend you get an estimate from a professional solar installer.)

2. What are the best batteries for an off grid solar system?

Can off grid solar system work without batteries?

An off-grid system can work without batteries but would be unreliable. There will be no power generated when the sun doesn’t shine at night and very little produced on cloudy days. A battery bank is essential if an off-grid solar system is to supply a home over 24 hours.

How many batteries do I need for my off grid solar system?

The table below shows the calculation for the example installation in Houston Texas. Note that less lithium phosphate (LiFeP04) batteries are needed due to the greater allowable depth of discharge allowed.

Backup For 24 Hours

Rating

Measurement Units

Total load for 24 hours backup

30000 

Watt-hours (Wh)

Battery bank voltage

48

volts

Total amp-hour rating

30000/48=625

amp-hours (Ah)

Required Batteries (60% DOD) Lead-Acid

583/0.6 = 1041

Ah

Number of lead-acid batteries @250Ah each

4

Required Batteries (80% DOD) Lithium Iron Phosphate

583/0.8 = 729

Ah

Number of Lithium batteries batteries @250Ah each

3


How long do off grid solar batteries last?

Lithium are more expensive to buy but are cheaper in the long run as they last a lot longer (5000 charge/discharge cycles compared to 500).

Most days in the summer the lithium batteries wouldn’t be discharged down to their maximum allowable discharge depth of 80%, so let’s say an average DoD (Depth of Discharge) of 60%.

At this DoD lithium batteries are likely to last for 10000 cycles.

Expected life estimate:

10000 cycles/365 days (1 cycle per day) = 27 years

The best number of cycles for a good quality lead-acid deep-cycle battery is about 1000 cycles at DoD of 50%.

Expected life estimate for deep-cycle lead-acid:

1000 cycles/365 = 3 years (approximately)

If the DoD is less than this then the batteries will last progressively longer. Expect 5 or 6 years with a variable DoD under 50%.

3. Off grid solar battery charger

A true autonomous solar power system needs a substantial bank of batteries to be completely independent of the utility grid and these need to be charged daily.

The solar charger size is determined by the number of batteries which in turn depends on the energy they need to supply over any given time (next section).

What size battery charger do I need?

As a general rule, lead acid batteries need a solar charger with output in amps 20% of the battery bank amp-hour capacity.

For my example off grid installation that would be:

Battery capacity 1041 x 20% = charger output size = 208 amps

The input volts should be matched to the solar panel output volts and output volts to the battery volts.

For lithium batteries output current of 30% battery capacity is recommended.

Important: All batteries have a Maximum Charge Voltage and Maximum Charge Current, which can be found in the specifications. These should not be exceeded ot the battery life may be significantly reduced.

Most good quality chargers used in off grid solar power installations can be set for voltage and also have variable current output.

4. Inverters – What is an off grid solar inverter?

What inverter do you need to go off-grid?

Sizing your off-grid inverter

An inverter converts the DC power generated by solar panels and used by all kinds of batteries. It’s essential if using AC appliances in your home.

The only alternative is to use all DC appliances. This is possible but they tend to be expensive.

What size inverter do I need for off grid solar?

In general, it’s advised to use an inverter about the same size as the solar panels output in watts, or a little higher. For this example you would need a 8000 watt inverter with a charging current of 60 amps.

It’s best not to oversize the inverter because the losses become heavy under a certain load. A good quality solar inverter may have an efficiency between 95% and 97%, which is good but the power lost is not insignificant in a large system

A 5000 watt off grid solar power system would lose 250 watts at full load, and even more if half-loaded or less. Most inverters will overload up to 50% for short-periods.

Frequently asked questions about off grid solar power:

How much does an off grid solar system cost?

The average off-grid solar system professionally installed in the US costs about $2.90 per kW. A 5000 watt solar power system would cost be in the region of $15000.

The cost breakdown is as follows:

  • Solar panels: 37%
  • Solar Inverter: 10%
  • Battery storage: 35%
  • Rest of the installation (panel mounting rails , cables, breakers, junction boxes): 18%

If you’re a DIY enthusiast, the equipment cost is less than 50% of an installer’s price.

Conclusion – Is off grid solar worth it?

This post outlines the basic design procedure for building an off-grid solar power system. It all starts with knowing the home’s energy needs and then  working forward to size the solar panel array required and the battery capacity.

Battery capacity depends on the level of autonomy – 24hrs to 36hrs is a good guide.

The final test to see if off grid solar is worth the trouble and expense is to calculate how much money it saves in electricity costs and how long the system takes to pay back installation costs.

Off Grid Solar System Payback Period For 5kW Installation

  • Average Unit electricity cost Houston Texas = 11.5 cents/kWh
  • Considering 10,000 kWh bill for a year = 11.5 * 11,000 = 135,000 USD = $1265
  • Average Utility Bill = $105.4/month
  • Total Cost of designed solar system = $15000
  • Recovery Period= 15000/1265 = 11.85 years

Pages relating to off-grid solar systems on DIYSolarShack:

What Size Solar System Do You Need?

Grid-Tied Solar Panels – System Design

Questions related to ‘What is off grid solar system‘:

How many solar panels do I need for 5000 watts?

There are two ways of looking at this. You would need 17 solar panels rated at 300 watts, 25 solar panels rated at 200 watts and 50 panels rated at 100 watts to build a 5000 watt solar power system.

However, this would only give you the solar system rated power, not the actual output in watts.

Due to PV system losses of about 23%, the number of solar panels required should be multipled by 1.44 when sizing solar panels. In this case you would need 24 solar panels rated at 300 watts.

What can a 5000 watt solar system run?

A 5000 watt solar system with an average irradiance of 4 peak-sun-hours will generate 20kWh per day.

The average energy consumption for an American home is about 30kWh/day, so a 5000 watt solar system wouldn’t be enough to run all the appliances.

That said, such a system could run a lot, including washing machines, microwave, smaller kitchen appliances and TVs. Just don’t include AC or heating.

How many solar panels do I need for a 2kW system?

Using 200 watt solar panels you would need just 10 to get the overall 2kW system rating.

A 2kW solar system would generate 8kWh per day when irradiated at the average 4 peak-sun-hours per day.

What can a 2kW solar system run?

A 2kW solar system can run a fridge, a freezer, general small kitchen appliances and a TV.

A 2Kw solar system isn’t big enough to power any kind of heating or AC unit.

Can solar panels run 24 7?

Although research is being done with special solar panels that are able to store a little electricity for night-time use, standard solar panels cannot operate when there is no light.

Solar panels can only convert sunlight into electricity and they are quite inefficient at about 20%. When there is no light energy solar panels stop working completely.

Even in daytime most of the power produced is concentrated in the 4 to 6 hours around mid-day. Electricity generation from solar panels drops off drastically in the early morning and late evening.

How many solar panels do I need for 2500W?

A 2.5kW solar system would need 8.33 solar panels at 300 watts each, so 9 would be required.

In theory, the energy produced by a 2500 watt system would be 2500 x 4 peak-sun-hours = 10kWh/day.

In practice, PV system losses of 23% should be deducted , this solar power system would generate about 7.7kWh/day.

What can a 30kW solar system power?

A 30kW solar system is a a very big installation. With an irradiance of 4 peak-sun-hours it could generate:

30000W solar x 4 kWh/day = 120kWh/day

However, PV losses would reduce this to about 23% less, so it would be around 93kWh.

For comparison, the average energy consumed by an Americam household is 30kWh, so a 30kW solar system could power 3 US homes.

How much electricity will a 30kW solar system produce?

Electricity produced by a solar system can be measured in watts (instantaneous value) or a more useful measurement is watts over time.

In theory a 30kW solar system could produce:

3kWW solar x 4 kWh/day = 120kWh/day

But this isn’t going to happen! PV losses of at least 23% will bring this down to 93kWh.

How much power can a 5kW inverter produce?

A 5kW inverter can only convert DC power from solar panels or batteries into the AC power need to run home appliances. An inverter cannot produce anything – it can only convert.

Good quality inverters are very efficient (up to 97%) if fully loaded and become very inefficient when lightly loaded.

This means that a fully loaded 5kW inverter would deliver an output power of about 4.85kW.

How many solar panels do I need for a 5kW inverter?

It’s normally sized the other way round i.e. the load required determines the solar panel array size, which determines the inverter size.

Inverters should be fully loaded for maximum efficiency, so 5kW of solar panels will be needed.

For example, 5kW equates to 17 solar panels rated at 300 watts, or 25 solar panels rated at 200 watts.

Resources: 

How Many Solar Panels Do I Need?

5000 Watt (5kW) DIY Solar Install Kit w/Microinverters

2kW Solar System – Solar Power Quotes & Information

30000 Watt (30kW) DIY Solar Install Kit w/SolarEdge Inverter

5kW Solar System Information And Pricing – SolarQuotes


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