No one installs batteries because they look nice. Well, maybe some of you do. The rest use them to make their home that bit more smart.
In the following few hundred words I’m going to run through my approach to batteries. They have risen like a storm as they have transitioned from an expensive solar bolt on, to cheap(er) heat pump partner. Deciding on what battery, how big, and exactly when to install is hard. Please use the following as a set of pointers and things to think about.
Why batteries
Before jumping into sizing, why would one install a battery? They act as shock absorbers for your building, smoothening peaks of generation and consumption. What does this mean?
1) When you’re generating more electricity from your PV panels than your building is using, a battery will store that electricity rather than it flowing back into the grid for pretty mediocre returns. This means you can use it later in the evening when your panels aren’t producing anything.
2) When your house and heat pump are calling for lots of electricity (e.g a cold winter day when you’re boiling the kettle for a tea before the latest episode of the Night Manager), your battery allows you to use electricity it stored earlier in the day when electricity was cheaper, rather than buy from the grid where it is expensive because everyone is in the same boat.
Picking the right battery
To pick the ‘right’ battery for your building, you need to decide what right means.
You could optimise for solar self-consumption – you want to use every bit of electricity you produce with your solar panels. You want to do this to maximise your solar panel investment. I think to define your approach because of this is whacky – your electricity usage in the summer is low, notwithstanding your heating. Lights are off and you’re outside more. Sizing a battery system for this reason only is not good design.
You could optimise for back up – when there is a powercut, you want your building to stay connected. Again, I question this – powercuts are rare in occurrence (an average UK house will experience 4 powercuts in 10 years with most unnoticeable). Sizing a battery to keep your whole house online in these rare situations is a massive waste of money for most. However, if you live in a remote area where the grid is fragile, backup becomes a much higher priority. For everyone else, there are options to keep critical circuits online (usually freezer and phone charger for contacting relatives or keeping up to date on TikTok) which don’t require sizing a battery for whole house back up.
You could optimise for Time of Use tariffs – buying electricity when it is cheap and using it when it is expensive. This is a good way to size a battery. It allows you to save money on your bills every day of the year, and most importantly is optimised to save you money on the most expensive days! This is a good focus for sizing, so how do you do it?
Daily consumption approach
To size the battery, we size it to be most useful using Time of Use tariffs to reduce your bills. The benefit will be most felt when the building is using the most electricity.
The first step is to model your daily electricity consumption over a year. To avoid buying a massive, expensive battery that sits half-empty most of the year, we don’t size it for your absolute busiest day (like Christmas). Instead, we look at your ’80th percentile’ day – a busy winter day that represents a high, but realistic, amount of usage. You want a battery capacity that can cover most of that daily ‘peak’ load using cheap overnight electricity.
The next consideration is charge and discharge rates – you need to make sure that the battery is able to charge fully within the period of cheap rates. You also need to confirm the battery is able to discharge during the time of use to benefit your bills.
Is now the time?
Batteries have been a revelation since they were introduced to the mass market about ten years ago, but, they are expensive and can often drive upfront costs above budgets. It might not be the end of the world to delay your battery storage installation:
- 1) Prices are coming down, and capacity increasing. If you were to delay you may end up with a cheaper and better product
- 2) Sizing a battery is hard, and calculations on consumption and generation are difficult. If you wait twelve months you will have real data to hand to make a more informed sizing decision.
Having said that, batteries are effective in reducing bills from the moment they’re installed, and most now can be updated remotely meaning their hardware can be run more efficiently over time. It’s always worth running through a sizing exercise to understand the numbers involved – upfront cost, ongoing running cost savings.
So, batteries
As I mentioned at the start, no one installs batteries because they look nice. They are a financial investment, and as with any investment you need to consider the upfront cost, the ongoing saving, and how long it’ll take to recoup your initial spend.
In order to assess upfront cost you need to size it effectively – too little and ongoing savings will be lower than they can be, too large and you’ll spend loads with lower relative savings.
At Better Planet, we’ve been installing batteries for ten years, specifically to benefit from ToU tariffs and solar PV. We’re on hand to help – give us a call!
