Lithium (LiFePO4) Batteries designed to capture electricity generated by your solar PV system allow you to store more solar electricity for use later in the day. This blog post sets out the main features you might need to take into account when deciding if a battery storage system is suitable for you and your solar PV system. Potentially batteries can help you use more of the electricity generated by your PV solar system, saving you money on your electricity bill. It is also important to note that, contrary to expectations, some battery storage systems are not designed to work for solar application. In addition there are many poorly designed battery systems and that is why we at EnGoPlanet decided to write more about this topic. Battery prices continue to fall and the technology is also improving, meaning that battery storage is becoming a viable economic option for some households and businesses.
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The solar PV system on your roof will generate electricity during the day that you can use in your home. Without a means of storing that solar electricity, any surplus energy that you don’t use is ‘exported’ to the local electricity grid. During times when the panels are no longer generating (or not generating enough for your needs), you need to buy electricity from your electricity supplier.
‘Energy storage’ lets you store the surplus solar electricity, instead of exporting it. Battery storage lets you use more of your solar PV system’s output (in the jargon, it ‘increases ‘self-consumption’).
This reduces the amount of grid electricity you need to buy, saving you money on your electricity bill.
During the day : • the solar PV system generates solar electricity • the battery storage system will check if all the generation is being used to power your lights and appliances • if you’re not using all the electricity that the solar PV system is generating, then the system will ensure that any surplus energy is used to charge the battery • once the battery is fully charged, if there is still more solar electricity being generated, this will be exported to the grid (or in some systems, will be diverted to other uses e.g. to an immersion heater) In the evening or at time of low solar generation: • the solar PV panels have a reduced or zero output • the battery system can discharge the stored electricity, providing you with renewable-generated electricity at no additional cost • once the battery is discharged, if you need to use more electricity, you buy it from your electricity supplier
Battery Types for Solar Application
The two types of batteries most commonly offered for solar PV storage in the home are lithium-iron phosphate LiFePO4 and lead-acid batteries. Some of their key features and differences are set out here:
Lithium (LiFePO4) batteries:
• Increasingly common in domestic grid-connected or off the gird solar PV storage systems • Lighter • Require integrated battery management system (BMS), that manages charge / discharge (EnGoPlanet Lithhium LiFePO4 battery has integrated BMS) • More efficient • Can discharge more stored energy • Longer expected lifetime
Lead-acid batteries • Cheaper • Heavier and larger • Need good charging and discharging routine to maintain battery health • Less efficient • Shorter expected lifetime
Battery storage systems are often provided with a power rating in kiloWatts (kW). Storage batteries for a grid connected solar PV storage system are typically around 1kW to 7kW. This is the capability of the battery to provide power. A battery’s stated electricity capacity, as expressed in kilowatt-hours (kWh)1 is generally larger than the battery’s actual usable capacity, because:
• all batteries lose some energy in charging and discharging, though some have better ‘charge-discharge efficiency’ than others.
• most batteries are not designed to be routinely fully discharged (can reduce battery life). Some have deeper discharge capability than others. Typical Lead-acid battery systems may be setup to limit the ‘depth of discharge’ to around 50%, Lithium-ion systems to 75% or more.
What a battery storage system could power?
A fully-charged medium-sized system could store sufficient energy to power during the evening your lights and lower-powered items like your fridge-freezer, TV and laptop. Over four or five hours, all of these together will use at most a few “units” or kiloWatt-hours (kWh), of electricity. However, the battery will quickly run out if you put on heavy energy users like the washing-machine or tumble-dryer: these can consume 2 - 3kWh in a single use. And in winter, the battery might not store enough to provide for even the lower-powered items for many hours.
A battery’s efficient lifetime depends on the technology and the way the battery is used - significantly on the number of ‘cycles’ (complete full battery charge and discharge) that they undergo. Manufacturers generally give an expected lifetime in years and/or in ‘charge-discharge cycles’.
For example: • ‘Life expectancy = 10 years or 10,000 cycles, whichever is the sooner’ Lithium batteries last longer than lead-acid: you may see a 10-year lifetime expectancy claimed and this is improving all the time. Normally the battery storage system will monitor the battery performance and should give you an indication when your batteries need replacing. Some battery system manufacturers operate a battery leasing and/or replacement scheme for worn-out batteries and arrange for the safe disposal/recycling of the battery.
The batteries in a solar PV storage system work like any rechargeable battery: they charge direct current (DC) from an external source (e.g. your solar PV system) and discharge DC when energy is required.
Solar PV panels generate far less energy in winter, so the system may not generate enough surplus solar electricity to fully charge the battery during the winter months. Leaving a battery sat partially discharged for long periods can reduce its lifetime. This is particularly the case for lead-acid batteries. To maintain battery health, the system may have a ‘winter mode’ setting that during the winter puts the battery to sleep, reduces the discharge from it or charges it from the mains.
There are two main ways of linking a battery storage system into such a system:
• DC Coupled: the batteries are installed on the same side of the solar inverter as the solar PV panels, they charge from the panels, and their DC is only converted to AC when it’s used (‘DC-coupled’)
• AC Coupled: the batteries are installed on the grid-side, where the solar PV’s DC has already been converted to AC (‘AC-coupled’). A separate inverter converts the AC back to DC for storing in the battery. When the battery discharges, the same separate inverter converts the DC back to AC.
You’re more likely to be offered an AC-coupled system if you’re looking to add a battery storage system to an existing solar PV system (they’re more suited to such ‘retrofit’ applications). For retro-fit applications, the installer will need to verify that the new equipment being installed is compatible with the existing equipment. DC coupled systems can be installed as a retrofit but more equipment will need to be added or replaced. You’re more likely to be offered a DC-coupled system if you’re installing a solar PV system and a battery storage system from scratch. Many DCcoupled systems will not operate in a power-cut (see below) and it may affect your Feed-In tariff income (see Section 5.2 ‘The impact on your Feed-in tariff income from solar PV’).
If having read this far, you think storage may be for you, then there are lots of questions you should ask any prospective installer to ensure you have the correct information. There’s a list of suggested questions at the end of this Guide. But before you get to that point, you might want to weigh up a few things. 5.1 Your ‘load profile’: how much of your solar PV generation you use/are likely to use You may use (or already use) most or all of the electricity generated by your solar PV system during the day, for instance if:
• you are at home during the day and/or
• you set energy-intensive appliances on timers to run during the day and/or
• you divert some of the electricity, for example to an immersion heater to heat hot water. In this case, your ‘self-consumption’ of the solar PV electricity is already high.
And your ‘load profile’ – when you use electricity and how much you use – means you won’t have much (or any) surplus electricity from your solar PV system to store in a battery. Information on how much solar electricity you export to the grid at different times of the year is useful to have when considering these points.
Costs vs benefits
You can weigh up the substantial costs of the battery storage system against the potential benefits. In purely financial terms, you can compare the cost of the battery system with your estimated benefit in savings. On the cost side, you need to take into account:
• the full cost of the system, including any running costs
• the cost of replacing the battery at least once in the lifetime of your PV panels. On the benefits side, every kWh you use from the battery is a kWh you don’t have to import from the grid and pay your electricity supplier for. How much YOU will save over the course of a year depends on your circumstances, including: your solar PV system, your ‘load profile’, the battery concerned, your electricity tariff per kWh.
In working out savings, remember to factor in:
• The cost of any mains-charging
• Whether your FIT generation or export tariffs are affected
• The savings won’t be the same in the winter months as in the summer. Comparing total costs with net benefits per year will show you how many years it would take for the system to ‘pay for itself’ or ‘payback’. This is an approximate calculation only. It assumes a constant level of annual benefit over the years and does not take into account any inflation, or changes in usage, electricity prices or export tariffs.
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