Solar and battery storage

Solar-plus-storage: How much battery capacity do you need?

by James Martin II on January 11, 2016

in Batteries & Energy Storage,Installation advice,NSW,Solar System Products

About one in three Solar Choice customers are asking about energy storage for solar power, but few have a clear idea of how much battery capacity they need. This article takes a look at the factors that come into play when considering how to choose the right amount of battery capacity for your solar-plus-storage system, using some specific examples examining the case for installing energy storage in Sydney.

How much energy independence do you want?

Many of the customers who approach us about energy storage options say that they want to go ‘off-grid’, but that’s not the only reason to have an energy storage system installed. While this may be a popular aspiration, taking your home or business off-grid is more often than not financially impracticable or just not possible to do with solar-plus-storage alone (especially if you have limited roof space for panels). What is often more practical is a strategy of reducing reliance on the grid instead.

The list below presents the most popular uses of energy storage, sorted in descending order from greatest degree of energy independence to lowest degree of energy independence.

  • 100% energy independence (literally ‘off-grid’): Install enough solar, energy storage and other technology (small-scale wind, generator, etc) to meet all of your electricity needs. In this case you’ll need to install enough capacity to last you several days because you won’t have the grid to back you up.
  • High energy independence (winter self-sufficiency, summertime surplus): Install enough solar and battery storage to get you through the average winter’s day without having to rely on the electricity grid (even though you’re still connected). You’ll produce excess solar power to export to the grid during the summer and will have enough to get you through the shortest winter’s day. You will likely have to rely on the grid as a backup if there is a multi-day spate of bad weather during which your solar panels haven’t managed fill up your battery bank.
  • Summertime independence (grid reliance in winter): Install enough solar and battery storage capacity to get you through the average summer’s day. During the winter (and possibly autumn and spring) you’ll have to draw power from the grid to meet deficits.
  • Peak time independence: Install a system large enough to cover your electricity usage when grid electricity is most expensive. If you’re on a time-of-use (TOU) billing agreement with your electricity retailer, you’ll pay more for electricity from the grid during peak and shoulder times (in this case we will use 4:00-7:59pm and 8:00-10:00pm), so it would be in your best financial interest to reduce your reliance on the grid during these periods. (Please also note that some homes may actually use more electricity during the winter months – particularly those with electric heating.)
  • Reduced grid reliance during peak times: If you can’t afford to install enough energy storage to cover your peak time usage, you can still install enough to at least reduce your grid reliance during these periods.
  • Energy storage as emergency backup: Install a small energy storage system to be used mainly in the event of a short power outage.

What other factors do you need to consider in sizing your energy storage system?

Choosing the right energy storage system capacity can be significantly more complicated and subjective than deciding on the right solar PV system size – mainly because of the greater number of variables involved. The factors involved include:

  • Desired degree of energy independence (as discussed above).
  • The amount of electricity you use: If your electricity consumption level is low (under 20kWh/day), it will be easier and more affordable to install a solar-plus-storage system that will meet your daily needs. If your usage is higher, the outlay for a system that will meet your daily needs will also be more substantial; you may wish to look at ways to reduce your electricity consumption, or consider a lower degree of energy independence.
  • Your electricity usage pattern: According to the analysts at Sunwiz, there are five basic residential electricity usage patterns in Australia: 1) ‘Double Peak’, 2) Evening Peak, 3) High Day & Evenings, 4) Day Focus and 5) Night Focus. Examples of these patterns can be seen in the graph below; they are also discussed in further detail in this article.

Common electricity usage patterns

Electricity consumption patternsThe most common energy usage patterns in Australia: Double Peak (blue), Evening Peak (red), High Day & Evenings (yellow), Day Focus (green) and Night Focus (purple).

  • The size of your solar PV system: If you have a preexisting solar system onto which you’re planning on retrofitting energy storage, this will weigh heavily on how much storage capacity you can or should get. If, for example, you are currently exporting lots of solar to the grid at low feed-in tariff rates, you may want a larger amount of storage capacity to capture it. If, on the other hand, you’re installing a new-build solar system, it’s usually the wisest move financially to make sure it only produces enough electricity to meet only your daytime electricity demand and have a small energy storage system to capture any excess that would otherwise be ‘wasted’ on grid export. However, if greater energy independence is your goal, oversizing the solar system and installing a large battery bank is an option – and may even stack up financially, especially if you’re on TOU metering.
  • The orientation of your roof/solar panels: North-facing solar panels will generate more power than east or west-facing panels, but a west-facing array will generally do a better job of meeting your household’s electricity demand during peak & shoulder times.

Energy storage system sizing: Examples for Sydney

Solar Choice is in the process of developing a tool that will help our customers to get an estimate of the right energy storage system size. This tool will soon be available to the public, but for now we have published some examples from Sydney below. Please note that these results are rough estimates, and should not act as a substitute for recommendations based on your individual circumstances.

Assumptions:

  • Batteries are charged with excess solar – no ‘grid charging’ with cheap retail electricity during off-peak times.
  • Amount of energy storage available at the beginning of the day depends on battery state of charge from the night before. This means that the battery will only have a charge in the morning if there was solar electricity remaining from the day before (after being used to run appliances and charge batteries).
  • 95% battery storage efficiency.
  • Battery capacity is equivalent to maximum recommended depth of discharge (DoD).
  • If it is impossible for the solar-plus-batteries system to meet 100% of demand, battery power is prioritiesed for use during peak & shoulder rate times.
  • Electricity consumption level and pattern do not change between months and seasons (admittedly a big assumption).
  • Solar output data from NREL’s PVWatts calculator tool.

Example 1: Average Sydney home with 5kW system

  • Preexisting north-facing 5kW solar system
  • Average daily household electricity consumption of 25kWh
  • ‘Double Peak’ usage (typical for households with school age children)

In this instance, the solar energy system is too small to both meet electricity demand and charge the batteries – even for peak demand. Furthermore, a battery bank larger than 10kWh would probably not ever be fully charged on solar alone – the home would need to charge it with the grid at night in order for its full capacity to ever be utilised. 7kWh is probably a good battery bank size to have because it strikes a balance between utilisation of excess solar and not oversizing the battery. Installing a larger battery bank would only be worthwhile if the home had a larger solar array.

Energy storage system size  Self-sufficient on average day in Dec (summer)?  Self-sufficient on average day in June (winter)? Peak & shoulder times covered in summer? Peak & shoulder times covered in winter? Battery capacity 100% utilised? % of energy self-sufficiency (on average day)
5kWh N (43%)
N (33%)
N (27%)
N (18%)
Y 40%
10kWh N (53%)
N (42%)
N (46%)
N (36%)
Y 51%
15kWh N (51%)
N (42%)
N (65%)
N (36%)
Y (in summer) 51%
20kWh N (64%)
N (42%)
N (65%)
N (36%)
N (68% in summer)
51%

Electricity flow profile for Example 1, using 20kWh energy storage capacity

Syd 5kW solar 20kWh storage 25kWh consumption flowBlack line indicates electricity consumption, yellow line indicates solar system power output, purple line indicates battery state of charge (right axis), blue bars indicate solar power into battery, red bars indicate solar export to grid, orange area indicates self-consumed solar, purple area indicates electricity demand met by storage, and grey area indicates electricity demand met by grid. (Click to enlarge.)

Syd 5kW Solar 20kW storage 25kWh consumption usageA breakdown of where the household’s solar goes, which sources meet household electricity demand, an indicator of the maximum battery capacity used when charging batteries with solar only, and percentage of solar directly consumed by the home.. (Higher battery capacity utilisation may be possible with grid or generator charging.)

Example 2: Average Sydney home with 10kW solar system

  • Preexisting north-facing 10kW solar system
  • Average daily household electricity consumption of 25kWh
  • ‘Double Peak’ usage (typical for households with school age children)

This example is the same as Example 1, except for a larger solar system – 10kW instead of 5kW. With the larger solar array it is possible to achieve a high degree of energy independence with 15kWh or 20kWh of battery storage. However, in order for this home to achieve full self-sufficiency on the average day, it would probably be necessary to shift more electricity consumption to daylight hours and try to reduce their overall usage as much as possible (possible through simple energy efficiency measures).

Energy storage system size Self-sufficient on average day in Dec (height of summer)? Self-sufficient on average day in June (height of winter)? Peak & shoulder times covered in summer? Peak & shoulder times covered in winter? Battery capacity 100% utilised? % of energy self-sufficiency (on average day)
5kWh N (51%)
N (38%)
N (39%)
N (25%)
Y 42%
10kWh N (61%)
N (49%)
N (57%)
N (46%)
Y 53%
15kWh N (70%)
N (59%)
N (75%) N (65%) Y 63%
20kWh N (92%) N (82%) N (92%) N (82%) Y 74%

Electricity flow profile for Example 2, using 20kWh energy storage capacity

Sydney 10kW solar 20kWh storage 25kWh consumption double peak 1

Black line indicates electricity consumption, yellow line indicates solar system power output, purple line indicates battery state of charge (right axis), blue bars indicate solar power into battery, red bars indicate solar export to grid, orange area indicates self-consumed solar, purple area indicates electricity demand met by storage, and grey area indicates electricity demand met by grid. (Click to enlarge.)

Sydney 10kW solar 20kWh storage 25kWh consumption double peak consumption

A breakdown of where the household’s solar goes, which sources meet household electricity demand, an indicator of the maximum battery capacity used when charging batteries with solar only, and percentage of solar directly consumed by the home.. (Higher battery capacity utilisation may be possible with grid or generator charging.)

Example 3: Energy-efficient Sydney home aiming for energy independence

  • New-build west-facing 7kW solar system
  • Average daily household electricity consumption of 15kWh
  • ‘High Day & Evening’ usage (typical for ousehold with infants or preschoolers)

Without an energy storage system, a typical 7kW solar system would send about 70% of the electricity it produces into the grid for this household on the average day. In this case, the goal of sizing the energy storage system would be to absorb the excess solar power for later use (without energy storage the solar capacity should be reduced to 1.5-2kW for optimal financial benefit).

The table below shows that the optimal battery size for this home would be about 15kWh, which allows for self-sufficiency and near 100% battery capacity utilisation in the peak of summer. If the battery’s size were in increased to 20kWh, however, it would not be possible to charge it using solar alone, so about 25% of its capacity would never be used unless the household charges using the grid or with a generator.

Energy storage system size Self-sufficient on average day in Dec (height of summer)? Self-sufficient on average day in June (height of winter)? Peak & shoulder times covered in summer? Peak & shoulder times covered in winter? Battery capacity 100% utilised? % of energy self-sufficiency (on average day)
5kWh N (72%)
N (46%)
N (75%) N (48%) Y 59%
10kWh N (90%)
N (56%)
Y N (72%) Y 72%
15kWh Y N (56%) Y N (72%) N (99%) 72%
20kWh Y N (56%) Y N (72%) N (74%) 72%

 

Electricity flow profile for Example 3, using 20kWh energy storage capacity

Syd 7kW solar 20kWh storage 15kWh consumption flowBlack line indicates electricity consumption, yellow line indicates solar system power output, purple line indicates battery state of charge (right axis), blue bars indicate solar power into battery, red bars indicate solar export to grid, orange area indicates self-consumed solar, purple area indicates electricity demand met by storage, and grey area indicates electricity demand met by grid. (Click to enlarge.)

Syd 7kW solar 20kWh storage 15kWh consumption usageA breakdown of where the household’s solar goes, which sources meet household electricity demand, an indicator of the maximum battery capacity used when charging batteries with solar only, and percentage of solar directly consumed by the home.. (Higher battery capacity utilisation may be possible with grid or generator charging.)

Want updates about energy storage? Fill out the form below.

On the back of the success of our informative and impartial Solar Quote Comparisons, Solar Choice is currently in the process of developing an online comparison platform for energy storage solutions – for retrofit and brand new installations. If you’d like to be informed when this new service goes live, please enter your details into the form below and we will be in touch. We will also let you know when our energy storage system sizing tool is available on our website.


© 2015 Solar Choice Pty Ltd

James Martin II

James Martin II

James has been working as analyst and online development manager for Solar Choice since 2011. He holds a master's degree in Environmental Management from UNSW, and a bachelor's degree in Philosophy from Bridgewater State University in his native Massachusetts.
James Martin II

{ 8 comments }

Tom July 20, 2015 at 7:12 am

Thanks for posting this analysis – super helpful as always. Looking forward to playing with the tool (and soon, getting some batteries)!

Salty September 20, 2015 at 10:37 am

Hi James,
Thanks for this analysis! I found it helpful, however I was wondering why with pv becoming so cheap does the model for energy storage not change to reflect this shift?

For example: I have been recording the daily output of my 5kw rooftop system. In the middle of winter, on the worst rainy day it produced 9.8kw. Therefore a 10kw pv system might put out close to 20kw during the day.
In your analysis, your third example of an energy efficient household consumed 15kw per day..
Why is it still common practise to have either multiple days worth of power stored in battery banks and or to augment pv with a gen set when increasing the PV array and adding another charge controller is a more cost effective solution?
cheers-
Salty

Solar Choice Staff September 21, 2015 at 12:30 pm

Hi Salty,

If I’m understanding you correctly, you bring up a very interesting point – why not just install more PV (which will still generate some power even on a rainy day) rather than installing pricey batteries? Correct me if I’m wrong.

If you’re aiming to be off-grid or for total energy independence (which most households won’t – and shouldn’t – bother to do), then it does make sense to oversize for worst-case scenarios. However, as you’ve pointed out, the fact that a solar system will continue to generate electricity on a bad-weather day, it could very well make sense to ‘slide the dial’ up for solar PV and down for batteries – but it would depend on the circumstances of the home in question.

For those who are on-grid with their solar-plus-batteries and simply aiming to get the most out of their solar, however, it probably makes more sense to be conservative and avoid ‘wasting’ solar power by sending it into the grid – in this case making sure that the solar array is not too large is important.

Thanks again for your thoughts – this is something that we will certainly bear in mind for future analysis – especially when our research concerns going off-grid or achieving a high level of energy independence!

Mathew Gregson November 17, 2015 at 6:22 pm

Hi James, a really interesting article. The “tool” which you mention – is there any progress on it? I live in Tasmania where our power usage pattern and metering is different to the rest of the country and none of your scenarios are that helpful for me.

I’ve had my array generating data for more than 800 days. I have 6.72kW of panels into a 10kW inverter, average 48kWh of energy consumption per day, export around 50% to grid, generate on average 27kWh (maximum 50kWh).

A mostly wild-guess would tell me than I need at least a 25kWh battery storage which would still be “wasting” a lot of generation to the grid during summer?

Solar Choice Staff November 23, 2015 at 12:59 pm

Hi Mathew,

There will be an announcement when our tool becomes live, if you want to join our mailing list (you’ll find a signup field to the left of this page).

I’ve run some of your numbers through our calculator and it appears that rather ‘wasting’ your generation into the grid during summer, the problem would actually be the reverse – your panel array would probably never fill up a 25kWh battery bank, even if you expanded it to a full 10kW of capacity. The reason for this (at least in our model) is that you’re probably already consuming a lot of the solar power during the daylight hours – and each unit of solar you ‘self-consume’ is one less unit that goes into the batteries. A 15kWh battery bank might be a bit better with your current 6.7kW array (preventing ‘wastage’ in the summer). Alternatively, if you filled out the inverter capacity to a full 10kW of solar, that would afford you roughly 50% energy independence on average and roughly 65% independence in the summer (without sending too much solar into the grid).

By our estimates, 20kW solar and 40kWh of storage would allow you daily energy independence on average and in summer. For winter, you’d still be getting about 40-50% of your electricity from the grid unless you cut your electricity consumption then.

Hope this helps – happy to try to help you out with further details if you have more questions. Best of luck!

Ruben January 29, 2016 at 11:52 am

Do you run an aluminium smelter at home?

Peter Butler January 21, 2016 at 11:04 am

I may be misreading it, but the coloured graphs for Examples 1 & 2 for different amounts of PV appear to be exactly the same?

cheers
Pete

Solar Choice Staff January 21, 2016 at 2:09 pm

Hi Peter. Thanks for pointing that out – a pure mistake on our part. I have updated the graphs with the correct ones.

Comments on this entry are closed.

Previous post:

Next post: