Solar batteries & storage
A huge step towards energy independence, solar batteries let you store up power for when you really need it.
Adding battery storage minimises your reliance on the grid, and reduces the money you put in your energy supplier’s pocket. But what should you look out for when choosing a system? Below we list the questions you need to ask before making a decision.
The 6 ‘Watts?’ of Solar Storage
1. What’s the ‘useable’ capacity?
Capacity is the amount of energy in kWh (units) that a battery can store. Batteries should never be drained completely. However, some are misleadingly sold quoting ‘total’ capacity. Check what’s being stated. ‘Useable capacity’ is the figure you need to know. Though Tesla Powerwall 2 is 14kWh, it’s sold as 13.5kWh, it’s useable capacity. It will never fully discharge to prevent damage to the battery cells.
2. What are ‘cycles’?
A cycle is one complete discharge and one complete charge. In reality it doesn’t happen like that. A battery may only discharge 25%, then recharge 25%. This would be 1/4 of a cycle. So you need to know how many cycles a battery is warrantied for. Only then can you work out how many kWhs (units of electric) your battery will deliver over its warrantied lifetime.
3. What’s the battery chemistry?
There are two main types available:
- Lead Acid
- Lithium
Lead Acid is very cheap to install but it’s inefficient and won’t last for long. Though the price of Lithium (ion or iron phosphate) is higher, it’s much better value. It’ll last far longer and deliver more kWhs (units) during its lifetime.
4. What’s Charge/Discharge power?
Some battery storage systems only deliver 800w (watts) of power. No good if you want a cup of tea (your kettle needs 2000 watts). Likewise, if you’re generating 4kW but the battery can only take on 3kW then 1kW will be heading to the grid, wasting your precious free energy.
So it’s essential that you check the power output before you buy, otherwise you may find yourself drawing a lot of energy from the grid even though you have energy in your battery.
5. What’s the price per kWh?
As you’ll see from the table below, there are many storage solutions at varying prices. Checking the price/kWh of storage capacity is fair and accurate way to compare different systems. What’s the right number of kWh for your property? That depends on you. The more kWhs you have, the more you can reduce your bills and take advantage of ‘Time of Use’ tariffs that are starting to emerge on the energy supply market. These allow charging on cheap rate and selling back to the grid at higher rate periods. The greater the storage, the more you can sell.
6. What about power cuts?
Most systems are designed just for storage. However, some offer backup capability to provide power when there’s a powercut. Two things to consider.
First: to prevent damage to your battery or appliances some of your circuits may need to be rewired.
Second: you’ll need a larger storage capacity to keep some power in reserve. If backup is essential, let us know and we’ll design some bespoke solutions for you to choose from.
At Naked, we ask these questions for our customers every day.
Compare Solar Batteries
Use the table below to compare some of the battery systems available on the market. Prices are a guide only. They should be very close, but check with us for latest info. Remember to add VAT. Currently, installing a battery with a Solar PV system attracts 5%VAT, retro fitting a battery attracts 20% VAT.
| Solar PV Battery Storage system | Charge/Discharge in kW | Storage capacity (kWh) | Useable capacity (kWh) | Cycles warrantied | Installation price | £/kWh of storage capacity | Product warranty | Battery chemistry |
|---|---|---|---|---|---|---|---|---|
| Tesla Powerwall (with Backup*) | 5.0 | 13.5 | 13.5 | Unlimited | £7450 | £551 | 10 years | Lithium |
| Alpha Smile 14.5kWh | 2.867 | 14.5 | 13.92 | 2920 | £5600 | £402 | 5 years | Lithium |
| Sonnen 9.43 - 15kWh | 3.3 | 15 | 13.5 | 10000 | £9145 | £677 | 10 years | Lithium |
| Solax Hybrid X1 Triplepower 6.3kWh | 2.0 | 6.3 | 5.67 | 6000 | £4238 | £747 | 5 years | Lithium |
| SolarEdge LG Chem RESU 10 | 5 | 9.8 | 9.3 | Unlimited | £6100 | £655 | 10 Years | Lithium |
| Powervault 3 - 12.3kWh | 3.3/5.5 | 12.3 | 12.3 | 6000 | £8090 | £657 | 10 years | Lithium |
| Solis/BYD 7kWh | 3.0 | 7.0 | 5.6 | 2842 | £4944 | £882 | 10 years | Lithium |
| Powervault 3 - 8.2kWh | 3.3/5.5 | 8.2 | 8.2 | 6000 | £6360 | £775 | 10 years | Lithium |
| Sonnen 9.43 - 7.5kWh | 3.3 | 7.5 | 6.75 | 10000 | £6465 | £957 | 10 years | Lithium |
| Puredrive Energy Storage 9.6kWh | 3.6/4.2 | 9.6 | 8.8 | 6000 | £6806 | £773 | 10 years | Lithium |
| Puredrive Energy Storage 4.8kWh | 1.8kW/2.5kW | 4.8 | 4.4 | 6000 | £4742 | £1077 | 10 year | Lithium |
| Moixa 4.8kWh | 2.4 | 4.8 | 3.84 | 6000 | £4450 | £1158 | 10 years | Lithium |
* Tesla Powerwall comes with Backup as standard. This will give you cover during a powercut allowing your solar and your battery to continue operating when the grid goes down.
AC Versus DC Coupling: Pros and Cons
When you buy a solar battery, you’ll need to consider whether to go with ‘AC coupling’ or ‘DC coupling’. Below we show you how each setup works and list the advantages and disadvantages.
AC Coupled
Requires separate inverters for your battery and panels.
Pros
- More juice: you get the combined power from battery and panel inverters
- Flexible location. As batteries are best at room temperature and inverters are best at cooler temperatures, it’s easier to optimise both because they don’t need to be near each other
- Battery faults won’t affect your panels & vice versa
Cons
- 1-3% less efficient power transfer than DC
- Can be more expensive as requires more than one inverter
DC Coupled
Battery and panels share the same inverter.
Pros
- More efficient power transfer than AC by up to 3%
- Often cheaper to install as you only need one inverter
Cons
- Limited access to power.
- If there is a fault with the inverter in the future, it may render the battery useless as a compatible inverter may no longer be available.
- Battery must be installed near the inverter. Batteries prefer temperatures around 20°C while inverters need to be in the cool, so there may be a trade off in efficiency
TESLA
The Powerwall has transformed Solar for good. And now, all Powerwalls come with Backup meaning you’ll have electricity even in a powercut.
How Solar Batteries Help You Night & Day
People often ask ‘Can solar panels work at night?’ The short answer is no. But with a battery, in effect they do. During the day, when solar panels are generating more power than your property is using, excess power is diverted to the battery for use later. When the sun goes down, or if you’re using more energy than your panels are providing, the battery will kick in to give you power and prevent drawing from the grid.
