The best Batteries for Solar Power: a guide for home and small commercial consumers
Batteries designed to capture surplus electricity generated by your solar PV system allow you to store solar electricity for use later in the day. This guide 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 system, saving you money on your electricity bill. But they’re not for everyone, as you generally need to be generating ‘surplus’ solar electricity to store. It is also important to note that, contrary to expectations, some battery storage systems are not designed to work during power-cuts. In addition they are not cheap: at today’s prices, poorly designed systems may not repay their initial investment (in what they save on electricity bills). But 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.
1.1 This guide This guide is designed to help you decide if a battery storage system makes sense for you. It covers: - the basics of battery storage for solar PV systems
- what is the best battery for solar storage - what you need to consider to work out if they are for you - where you can find more information and - the questions you need to ask any prospective installer.
2. Storage for solar PV systems: the basics
2.1 Your solar PV system 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.
2.2 Solar PV plus storage ‘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
3. Best Battery Storage for solar PV systems: the batteries
3.1 Battery types The two types of batteries most commonly offered for solar PV storage in the home are LiFePO4 (lithium) and lead-acid batteries.
Some of their key features and differences are set out here, for the purpose of this blog we will use EnGoPlanet Lithium LiFePO4 battery and Victron Energy GEL battery.
• More expensive (however prices have drooped significantly in last 8 years) • Increasingly common in domestic grid-connected solar PV storage systems • Lighter • More efficient • Can discharge more stored energy • Longer expected lifetime
Lead-acid batteries AGM/GEL:
• Cheaper • Typically used for off-grid properties where more storage is required • Heavier and larger • Need good charging and discharging routine to maintain battery health • Less efficient • Shorter expected lifetime
You can purchase both of these batteries online by visiting this PAGE.
Batteries and battery systems can vary considerably in shape, size and weight. The greater the battery capacity, the greater the battery size and weight. Typical domestic systems vary from being the size of a small computer to the size of a washing machine.
3.2 Battery capacity 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%, LiFePO4 systems to 85% or more.
3.3 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
3.4 Battery lifetime 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’ LiFePO4 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.
3.5 Battery Warranties As with many electrical products, a warranty for a battery is normally for a shorter period than its lifetime expectancy: for example a 5 year warranty on a battery expected to last 10 years.
4. Storage and solar PV systems: how they fit together
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.
4.1 ‘Winter mode’ Solar PV panels generate far less energy in winter (in the UK, around 4 times less in December than in June), 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.
4.2 Mains-charging Repeated incomplete charging can impact on the life of the battery. So most systems will occasionally use ‘mains-charging’ (drawing electricity from the grid) in order to complete the charge cycle.
4.3 Solar PV systems without storage Put very simply, a grid-connected solar PV system works like this:
Batteries also store and discharge DC, which similarly has to be converted to AC by an inverter.
4.4 DC and AC coupled
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’) OR • 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.
Copyright: IET, Code of Practice for Grid Connected Solar PV Systems, 2015
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’).
4.5 Power-cut operation Despite expectations, some storage systems will not provide power during a power-cut. Some battery storage systems however do have an ‘off-grid’ functionality, providing a limited amount of power to your home, or to essential equipment (such as your fridge-freezer, lighting etc). But a battery may run out of power before the power cut ends – or have already run out of power if it’s been discharging all evening and the power outage starts late at night or early in the morning.
If you require your battery storage system to provide power during power-cuts it is important to talk this through with your storage installer from the outset as the system will need to be carefully designed and set-up to your specific requirements.
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 offe