How To Calculate Solar Panel Battery And Inverter Size

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Sizing solar panels, battery energy storage and inverter

Sizing solar panels, batteries and inverter for a solar system

A true off-grid solar power system includes solar panels, a bank of batteries for energy storage and one or more inverters. This kind of system has no connection to the utility grid.

It is possible to have home battery storage, even when normally using the utility company’s grid connection. The batteries automatically come on line if or when the normal electricity supply fails.

The usual procedure for sizing any kind of system is in this order:

  1. Size the solar panels according to energy consumption
  2. Size the inverter according to the solar panel system power rating
  3. Size the battery bank according to how many hours you need it to run i.e. autonomy

Solar panel size is found by dividing daily load kWh by the location’s irradiance to give solar kW rating. Inverter size is equal to solar panel rating. Battery size is found by multiplying the daily load by the number of days autonomy required, and dividing by system volts to give amp-hours.

Video – How to size solar system and battery size

 

How do you calculate battery and solar panel size?

Although closely related, battery and solar panel size are not the same.

The solar panel array size is determined by the energy consumption of your home, while the battery size depends on the amount of autonomy required.

How long do you want batteries to supply power in the event of grid failure and/or cloud cover?

I created a resource post that goes into great detail about buolding your own DIY off-grid solar system – read it here.

How to calculate solar panel system size

Solar panel sizing starts with the load to be supplied. In the case of a home, then the annual, monthly or daily kilowatt-hours can be used.

The fastest and easiest way to get this is to take it from last year’s utility bill.

Once you have the home’s kWh, then it can be translated into solar panel generated energy, system rated watts and work out how many panels are needed.

How much energy does the average home use per day?

According to US government, the average American consumer uses about 30kWh per day or 900kWh/month. Using this as the goal for solar panel sizing, we can work out the solar power needed.

How does irradiance affect solar panels?

Although there are several factors impacting solar panel power output, irradiance or the sun’s energy, is by far the biggest.

Read about solar panel efficiency on one of my blog posts.

Irradiance is measured in kilowatt-hours per square meter per day or year (kWh/m2/day) and varies considerably depending on your geographic location.

Irradiance values in kWh/m2/day are also known as peak-sun-hours and this is used by solar installers to work out how much energy a solar installation can generate.

It can be found from historical data using database lookups on sites like GlobalSolarAtlas. Simply enter your city and note your local peak-sun-hours – see image below:

Use peak-sun-hours to size solar panels array

Chicago peak-sun-hours is almost the US average

The table below shows the irradiance values for 5 cities, both in the US and abroad:

Table – Irradiance in 5 different locations

City

Chicago, Il

London, UK

Glasgow, UK

Houston, Tx

San Fran., Ca

Irradiance (peak-sun-hours)

4.043

2.374

1.860

4.253

5.699

Incidentally, the peak-sun-hours for Chicago is about the US average (4), so I’ll use that in my calculations.

Solar panel sizing calculator

Daily energy required = 30kWh

Solar power wattage required = 30kWh/4 peak-sun-hours = 7.7kW of solar power rating

Let’s say we uses 300 watt solar panels, then:

Number of solar panels = 7700 watts/300 watts = 25 solar panels (each 300 watts)

Average pv system losses

Unfortunately, the above calculation assumes that we get all the power indicated by the solar panel power ratings – we don’t!

PV system losses amount to almost 25%, so we need to add at least 25% to the number of panels needed:

Number of panels to account for losses = 25 + (25 x .25) = 38 panels (each 300 watts)

Use the solar calculator below to estimate how many solar panels your home would need:

Calculator for sizing solar panels

What size inverter do I need?

In general, it’s best to size an inverter at about the same power rating as your solar array. This is because inverters are at their most efficient when fully loaded.

Full load efficiency of a good quality inverter is often 97%. However, if very lightly loaded, losses can be significantly more – see inverter efficiency graph below:

How to size a solar inverter

Inverters shouldn’t be over-sized – aim for the same power as the solar array

How do you calculate solar battery power?

Battery energy storage capacity depends on the length of time you want the batteries to supply your home and your energy consumption.

Let’s assume a home energy consumption of 30kWh per day, over 24 hours. If you wanted the batteries to last for 24 hours, then the battery kWh would be roughly equal to the energy consumption.

If you wanted to be able to last for 2 days, then simply double the battery capacity.

Off grid system energy storage is often sized to account for 2 full days of autonomy, just in case there is extensive cloud cover and no power is produced at all for one day.

Battery capacity required for 2 days = energy consumption 30kWh x 2 = 60kWh

Battery Ah required = 60kWh/battery bank voltage = 60000/48 (say) = 1250Ah

Note: Lead-acid deep-cycle batteries have a recommended Depth of Discharge of 50%. This means that double the calculated Ah will have to be installed:

Battery Ah required = 1250Ah x 2 = 2500Ah

How do I know how many batteries I need for my solar system?

Using the result of the above calculation and assuming 200Ah deep-cycle batteries:

Number of batteries needed for 2 days autonomy = 1250Ah/200Ah = 6.25 (7) batteries, each 200Ah

Note: Lead-acid deep-cycle batteries have a recommended Depth of Discharge of 50%. This means that double the calculated Ah will have to be installed:

Number of batteries required = 2500/200Ah = 12.5 (13) each 200Ah

Use the calculator below to find out how many batteries your home would need:

Calculator – Solar Battery Sizing

Will a 5kW solar system run a house?

You need to know how many kilowatt hours of energy your house uses per day/month or year.

Using the American home average of 30kWh/day and system loss factor of 1.44, we can work out how much energy a 5kW solar system could generate.

Applying the PV system loss adjustment factor of 1.44, you would need to generate:

30kWh x 1.44 = 43.2 kWh of solar energy

With an average of 4 peak-sun-hours 1 solar watt can generate 4 watt-hours/day. Therefore, the solar power needed to supply this home is:

43200/4 = 10.8kW

A 5kWh solar system would not run the average US home with energy consumption of 30kWh/day. More than double this amount would be needed at 10.8kWh if the total energy requirements were to be generated from solar.

How many panels are in a 5kW solar system?

Large domestic solar systems generally use the larger rated panels between 200 to 300 watts. Some system even use 400 watt panels and higher!

300 watt solar panels are common, so I’ll use these as the basis for the calculation. Again, we have to assume an average 4 peak-sun-hours as we don’t know the location and can’t adjust for irradiance.

A 300 watt solar panel can generate 1200 watt-hours of energy per day with irradiance of 4 peak-sun-hours.

Number of panels required for a 5kW system =  5000/300 = 16.66 (17) 300 watt solar panels

How much will a 5kW solar system save me?

This is best answered by an example calculation showing 5kW solar system savings and payback period for a system installed in San Francisco, Ca:

  • Location: San Francisco, California
  • Solar power system size: 5kW
  • Total cost of solar system = $17000 – 26% tax credit = $12580
  • Peak-sun-hours in SF = 2089.1 Peak Sun Hours/year
  • Power produced by 5kW solar system = 2089.1 peak-sun-hours x 5kw = 10445 kWh
  • San Francisco domestic power cost = 25.7 cents/kWh
  • Annual savings = solar generation x power cost = 10445 x 25.7 = $2684
  • Time to payback solar costs  = install cost/annual savings = 12580/2684 = 4.68 years

How many solar panels does it take to charge a 200Ah battery?


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