Batteries for your PV System: How much do you need? | RenewEconomy

Batteries for your PV System: How much do you need?

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Just as important as how cheap batteries are getting is another crucial question: how much battery do you need?

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bartery storageWith Tesla’s big stationary storage announcement last week, much attention was paid to how battery prices have fallen faster than expected. The $350 per kWh price for the 10 kWh battery was ahead of most analysts’ forecasts.

Not surprisingly, the emphasis has been on using those batteries—at least initially—for backup power and demand charge reductions, but as SolarCity CTO Peter Rive recently noted, they’ll also soon be used for grid-connected solar customers. That grid-connected solar-plus-battery combo is exactly what Rocky Mountain Institute and HOMER Energy analyzed in The Economics of Load Defection released last month.

But just as important as how cheap batteries are getting is another crucial question: how much battery do you need?

Understanding the role of storage and how to size a battery bank is not simple. It is one of the reasons we developed the HOMER software for modeling hybrid renewable systems, including integration of battery banks.

Are you using batteries for critical loads or all loads? Do you need battery backup power for hours, or days, or weeks? Is the system grid-connected or off-grid?

These merely scratch the surface of the many considerations that go into modeling batteries. And how you answer them can result in vastly different battery estimates. For example, the oft-cited Konterra microgrid in Maryland has enough battery backup to power critical loads for about four hours if the broader grid goes down. By contrast, a recent report from Moody’s estimates that off-grid solar-plus-battery systems would need a whopping (and absurdly unrealistic) 60 days of storage.


Consider how batteries might interface with a grid-connected, self-consuming rooftop solar PV system, such as you might find in places without net metering. Without net metering or batteries, the solar system would be sized based on the mid-day loads when solar generation is greatest, because any solar energy in excess of those loads would go to waste. Since most people aren’t home during the day, their mid-day loads would be smaller and thus the optimal size of the solar array would be quite small.
Cost-effective batteries—like those modeled in The Economics of Load Defection—would allow much larger solar arrays. Still, it is not an obvious sizing decision because energy consumption varies from day to day and month to month and the solar production varies even more. The scenario requires sophisticated modeling to optimize based on chronological simulations.


Backup power for reliability and resilience is another rationale for batteries. Solar PV systems without batteries do not protect consumers from utility outages. This also presents a challenging sizing problem depending on how much reliability is desired. For this application the size of the battery bank depends on how long an outage you want to protect yourself from. For short outages, the sizing problem is not too difficult, but the same system that could provide power indefinitely in the summer (short of running the air conditioner) may only be good for a couple of days in the winter (thanks to longer stretches of cloudy, stormy days that inhibit solar production).


Most off-grid systems larger than 1 kW have a backup generator. HOMER analyses have consistently shown a great reduction in battery sizing with a backup generator, even if it is only used very occasionally. Backup generators have numerous drawbacks, especially if they are used more than occasionally, but consumers can successfully avoid the use of a backup generator if they are able to carefully manage their loads.

Understanding what electricity is used for and how much electricity different appliances require is crucial for anyone trying to live off-grid, even if it is just for an extended utility outage. Most high-tech appliances use very little power—LED lighting, portable electronics, and efficient televisions use less than a total of 2 kWh per day. Refrigerators, furnace fans, and pumps are larger loads that can use up to 2 kWh per day each. Heating, hot water, clothes drying, and cooking use a lot energy, but they can be served with natural gas or propane (though they do run on electricity in many homes and thus can be a significant consideration for some). Air conditioning is the one appliance that requires electricity and in large amounts. In dry regions of the West, evaporative coolers can be used that don’t require a lot of power, but they are useless in humid climates.


So, how big do battery banks need to be? It depends. Can you use the utility as a “battery?” Is your goal to reduce your consumption to zero or to find the best return on your investment? Are you worried about outages? If so, are they likely to be short or long? Can you be flexible about how much energy you use during an outage, especially air conditioning? Are you willing to occasionally use a backup generator? The answers to those questions affect your sizing decision in a big way.

If you only use gas appliances for the large loads during an outage and have a typical-sized PV array, then 7–10 kWh of batteries—like that of the Tesla Powerwall—is sufficient to get through most outages of most any length. If you are worried about blizzards, you might want a backup generator even though you might only use it a couple of hours per year. If you can’t imagine living without air conditioning, you will need a much larger solar array, and the trade-off between sizing the solar array, the battery bank, and your actual cooling needs becomes critical.

Although storage prices are dropping rapidly, correctly sizing the storage for the location, usage pattern, and other design constraints, such as grid reliability and rate structure, is essential to creating systems that make economic sense. We strongly caution against “back of the envelope” or “rule of thumb” estimates for such an important decision.

Peter Lilienthal, Ph.D., is CEO and founder of HOMER Energy LLC and original developer of the HOMER modeling software. HOMER Energy provides the HOMER software, training, analytical services, and community market access tools to professionals in the energy industry who desire to analyze and optimize distributed power systems, microgrids, and systems that incorporate high penetrations of renewable energy sources. Images courtesy of Shuttlestock.

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  1. PaulW 5 years ago

    This topic, along with sizing of output inverters, will be the most important factors in framing the 2 or 3 value propositions put in front of customers. As you suggested, the answers to; Do you want to be off-grid, or smooth out your peaks, or redirect exported energy into self-storage? need to be understood and explained. There’s a huge difference in outcomes/expectations and cost depending on how you answer.
    The big gap to fill, an absolutely necessary part of the pre-sale assessment, is an ability to baseline a user with the kind of highly granular information need to yield parts of those answers.
    The industry and its marketing need to be careful not to over do expectations and green-wash the technology and products.

  2. disqus_3PLIicDhUu 5 years ago

    The big difference between the old lead clunkers and lithium is the DOD level means for a much smaller Ah system,, so instead of 1 tons of lead it would be replaced with 100kg of lithium, or with a Redflow system you could go to 100% DOD if necessary.

    • john 5 years ago

      I tend to agree strongly with you re the Flow Battery, because of its advantage of being able to be charged of discharged to any state of DoD, plus the cost to refit is going to be considerably less than buying another lithium battery.

      • disqus_3PLIicDhUu 5 years ago

        Yes just some replacing of cell stacks and some servicing, would expect.
        Look forward to seeing them hitting the mass market soon.

        • john 5 years ago

          As you are aware there are several who are quietly working away trying to get air and get the message out however they are totally drowned out by the Li people.
          Redflow is a case in point it should be spelling out the advantage of using a flow battery that can be at any state of charge and cheap to refit; however I can not remember 1 item from them in 2 years other than a cover by some outlet.

          • disqus_3PLIicDhUu 5 years ago

            Personally I’m agnostic as to what type of these is used.
            Both have their +& – issues.
            Li is very versatile and maintenance free with a good BMS. The flow battery, while good in theory, I would think not so versatile for all installs.

          • suthnsun 5 years ago

            I think you need to read Sandia labs report on Redflow batteries, there can be serious consequences for 100 % discharge – unless they have since solved those problems.
            Also they are really only designed for daily cycling as I understand, relatively low power charge and discharge , so not ideal for solar.
            Ultrabattery (CSIRO) may be better for this duty.

          • disqus_3PLIicDhUu 5 years ago

            Haven’t come across the Ultrabattery.
            What’s the specific energy like, I can’t find much info.

          • suthnsun 5 years ago

            Ecoult are the marketing company, King Island has a big system. Hybrid capacitor battery, quite a bit of info on ecoult website

          • disqus_3PLIicDhUu 5 years ago

            Thanks for the info.

  3. Geoff Bragg - SEIA 5 years ago

    All excellent discussion. If you’re planning on using your typical solar PV inverter as the DC-AC inverter with a Powerwall battery as Elon Musk suggested in Giles’ other article today, it will have its own anti-islanding protection inbuilt to AS4777, so it won’t do anything but shut-down on grid failure. A multi-mode inverter will be required to isolate the grid and continue supply while islanded. Some details to be ironed out.

  4. hugh spencer 5 years ago

    Living for 20+ years with stand alone PV (in the wet tropics) – I find that installing a simple battery AH monitor (such as the Trimetrics 2020 – which has large red friendly numbers) changes people’s power use behaviour markedly – it can be mounted in a much trafficked area of a house – and is like a fuel gauge – you can see at a glance your energy use. As part of our research program, we have developed a desulphator which appears to effectively double the life of LA ‘clunkers’ – which rather changes their economics – however it only works with AGM and gel types (don’t know why) – Also LA’s should never be discharged below 30% of their AH capacity if you want them to last. I think LA technology will be around for a long time yet – Tesla notwithstanding! – and the LA technology is improving all the time.

  5. Max Hamilton 5 years ago

    Redflow say temperature range is from 10 degrees to 50 degrees. Great for hot climates but what about areas where temp drops below zero?

  6. phred01 5 years ago

    In practice what is going to happen the power providers with tooth & nail will be protecting their business models. For those that can install large arrays & fit a wind generator off grid will end up as the obvious option. As a community we will end up sharing the benefits of renewable to the full extent. I c the pole tax coming in again so instead of taxing windows bit will be solar panel/ wind generators

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