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 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 capacity do I need?

Like Solar PV, customer desires affect the answer.  Planning an EV?  A larger battery can keep the house running and charge your EV overnight.  Grid trading: larger battery capacity means more charging on cheap rate and more to sell back to the grid at peak rate.  For high power devices larger batteries have higher output and will not be damaged by higher demand.  Naked Solar can model everything to help you decide by using PV Sol software.

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.

  1.  To prevent damage to your battery or appliances some of your circuits may need to be removed from the backed up circuits.
  2. You may want to consider 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 systemCharge/Discharge in kWStorage capacity (kWh)Useable capacity (kWh)Cycles warrantiedInstallation price£/kWh of storage capacityProduct warrantyPowercut coverAC/DC Coupled
Tesla Powerwall5.013.513.5Unlimited£7599£56210 yearsIncludedAC
Alpha Smile 14.5kWh3.014.513.922920£5600£4025 yearsNoAC
Sonnen 9.43 - 15kWh3.31513.510000£8835£65410 yearsAdditional costAC
Solax Hybrid X1 Triplepower 6.3kWh1.76.35.676000£4238£7475 yearsNoDC
SolarEdge LG Chem RESU 1059.89.3Unlimited£6100£65510 YearsNoDC
Powervault 3 - 12.3kWh3.3/5.512.312.36000£8090£65710 yearsNoAC
Solis/BYD 7kWh3.07.05.62842£4944£88210 yearsNoDC
Powervault 3 - 8.2kWh3.3/5.58.28.26000£6360£77510 yearsNoAC
Sonnen 9.43 - 7.5kWh3.37.56.7510000£6215£92010 yearsAdditional costAC
Puredrive Energy Storage 9.6kWh3.6/4.29.68.86000£6806£77310 yearsNoAC
Puredrive Energy/Solis Hybrid 5kWh3.04.84.46000£3620£82210 yearAdditional costAC
* Hot Water Cylinder & Immersion Diverter3.01010Unlimited£2300£2303 yearN/AN/A
Moixa 4.8kWh2.44.83.846000£4450£115810 yearsNoAC

* Hot Water Cylinder & Immersion Diverter is a great way of making more of your free energy from the sun.  The price of a HWC will vary from property to property so this is a rough guide price and the storage capacity is a bit of a guide and realistically achievable.

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.

Diagram of AC coupled Solar PV with storage
Diagram of DC coupled Solar PV with storage

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 Solar PV & vice versa
  • Works with any Solar PV system
Cons
  • 1-3% less efficient power transfer than DC
  • Sometimes 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%
  • May be cheaper install as Solar & Battery share the same 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 Powerwall and Gateway

TESLA

The Powerwall has transformed Solar for good. And now, all Powerwalls come with Backup meaning you’ll have electricity even in a powercut.

sonnenbatterie_8_2_small

SONNEN

European contender with excellent efficiency & performance.

resu10h_diagonal_1000x1000

LG CHEM

Popular choice of battery from South Korean company LG Chem.

solax-lg-battery-storage

SOLAX

A cheap alternative for those on a tight budget.

BYD Solar battery storage

MORE

No, it’s not a name you haven’t heard of. Just the rest of the best.

How Solar Batteries Help You Night & Day

circuit diagram of how Solar PV works with storage
EV charging from Solar PV

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.