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 a variety of storage solutions at varying prices.  Looking at the price/kWh of storage capacity is a good level playing field to compare different systems with.  What is the right number of kWh for your property?  Well that depends on you, the more kWhs you have the more you’ll reduce your bills and be able to take advantage of the ‘Time of Use’ tariffs that are starting to emerge on the energy supply market.  With these tariffs you can charge on cheap rate and even sell back to the grid at higher rate periods.  The more storage you have, the more you can sell.

6. What about power cuts?

Most systems are designed just for storage. However, some offer back-up 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.

Tesla Powerwall 2


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