Battery Storage

Enjoy the sunshine… at night.


Now you can reduce the amount of money you put in your energy supplier’s pocket.  Adding battery storage to Solar PV systems increases your self consumption and reduces your reliance on the grid.  So what should you look out for when choosing a battery storage system?  Here’s 5 key points to get you started.

  1. What is the ‘Useable’ capacity. Capacity refers to the amount of energy in kWh (units) that the battery can store.  No battery should ever be drained completely, however some batteries are sold quoting their ‘total’ capacity while others will state their ‘useable’ capacity, check what’s being stated.  e.g. Tesla Powerwall 2 is actually a 14kWh battery,  it’s sold as 13.5kWh, it’s useable capacity.  It will never fully discharge to prevent damage.
  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% and then recharge 25%, this would be 1/4 of a cycle.  You need to check how many cycles a battery is warrantied for and then you can calculate how many kWhs (units of electric) your battery will deliver over it’s warrantied lifetime.
  3. What difference does battery chemistry make?  The main two chemistries available are Lead Acid or Lithium.  Lead Acid is very cheap to install but is ineffecient and won’t last very long.  Lithium (ion or iron phosphate) will last longer and deliver more kWhs (units) during it’s lifetime.
  4. Power output. Some battery storage systems only deliver 800w (watts) of power, not much good for a kettle that needs 2000 watts!  Check the power output before you buy, otherwise you may find yourself drawing a lot of energy from the grid even when you have energy in your battery.
  5. p/kWh. If a 5kWh battery is warrantied for 3000 cylces it will deliver 5 x 3000 = 15000kWh (units) of electricity over it’s warrantied life.  If you divide the installation price by 15000 you will have a p/kwh price.  Some battery storage systems will offer a p/kWh price lower than current grid prices, meaning you can ‘bulk buy’ your future electric at lower than today’s grid prices, fixing your costs for the life of the system.  Some systems p/kWh price is much higher than current grid prices, meaning you’re throwing your money away on expensive electric that would be cheaper to draw from the grid.  You can also consider that most battery storage systems are going to last longer than their warranty.

This table below allows you to compare some of the battery systems available on the market.  The prices are guide prices, they should be very close, but check with us for current prices.  Remember to add VAT.  Currently installing a battery with a Solar PV system attracts 5%VAT and retro fitting a battery attracts 20% VAT

Solar PV Battery Storage systemPower output in kWStorage capacity (kWh)Useable capacity (kWh)Cycles warrantiedkWh delivered under warrantyInstallation pricep/kWhs delivered under warrantyProduct warrantyBattery chemistry
Growatt 2.054300012000£338628p5 yearsLithium Ion
Powervault 4kWh Lead Acid1.28412004800£341071p5 yearsLead Acid
GoodWe 3.6 Hybrid / Hoppecke 8kWh3.6884250010000£398039pnone statedLead Acid
SolaX Hybrid 3.7T3.684.83.84400015360£424027p5 YearsLithium
BYD Mini ES3.03.33.0600018000£443024p5 yearsLithium iron phosphate
Tesla Powerwall 25.013.513.5Unlimited37800£625016p10 yearsLithium iron phosphate
Sonnen 8.62.5661000060000£739012p10 yearsLithium iron phosphate
Wattstor 5/6566250015000£814054p5 YearsLithium ion
Sonnen 8.82.5881000080000£864511p10 yearsLithium iron phosphate
Powervault 4kWh Lithium-ion1.24.44400016000£405525p5 yearsLithium ion

What about power cuts?

What you’re talking about here is ‘Backup’.  Most battery storage systems are designed just for storage, however many manufacturers have the capability to add on back up capability.  There’s a couple of things to be aware of.

  1. It’s best to rewire some of the house so that in the event of a power cut, heavier loads cannot be switched on otherwise they could damage either the battery or the appliance.
  2. You’ll need a large storage capacity as there will always need to be some in reserve.

If backup is something that is essential to you, please let us know and we’ll be able to design some solutions to choose from.

Tesla Powerwall 2 

After Elon Musk’s inspiring presentation at the launch of the Powerwall, demonstrating how the move to a fossil fuel free planet is achievable with just Solar & Storage the world went battery storage crazy.  Tesla’s Powerwall 2 is a big leap forward in storage with up to 13.5kWh of storage and capable of delivering 5.5kW of power continuously and up to 7kW for 10 second periods the Powerwall 2 is extremely good value.  Powerwall 2 is also slightly smaller than Powerwall 1 and it looks a bit cooler too.  Interest is soaring for this new development which will be available in the UK from February 2017, let us know if you want to reserve your’s.


Is the market leader in Germany with over 16,000 installations under their belt.  With a 10 year/10,000 cycle warranty and expandable up to 16kWh of usable storage in 2kWh increments, the sonnen system stands out as a flexible high quality option.  As your system will be connected to the internet you’ll also benefit from software updates, improving it’s efficiency.

Along with the system, sonnen are planning to bring their sonnenCommunity to the UK.  This allows you to share your energy, so if you have excess energy, you sell it to the sonnenCommunity and if you need more energy then you can buy if from the sonnenCommunity at very low rates.  A great step towards cleaning up the planet.



BYD (Build Your Dreams) are a large Chinese manufacturer of batteries for major mobile phone brands.  They’ve also been researching energy storage for renewables since 2006.  Their domestic offering is the MiniES which has 3kWh of useable storage and can be increased to 6kWh with an additional battery.  With a product warranty of 5 years and battery warranty for 8000 cycles, this is likely to be a market leader amongst the Chinese offerings.