Sizing a battery bank for an off-grid home

Sizing a home bank starts with knowing your winter daily consumption in kilowatt-hours, and then it gets harder, because the bank is a buffer between what you generate and what you use, and its size falls out of how both of those behave. This is worth stating again. Think of the role of the battery as a temporary buffer between what you generate and what you consume.

Anyone who hands you a formula with a simple days of autonomy figure in it has skipped the part that matters. What follows is what we actually work through: how to get your winter number, why the generation and consumption patterns move the answer more than the headline figure does, how chemistry changes the usable fraction, and where system voltage stops being a choice.

Start with a number. What's going to be the most demanding time on your system? Most commonly, that's going to be during winter, as solar generation is at its lowest and consumption tends to be higher. Estimate the exact figures for that period of time. What is the generation going to be over the worst weeks of that period? What is the energy consumption going to be during that time? Then get some results. What's it going to take for you to generate enough during those periods? And do you have the money and space for the equipment, and is that acceptable? For some people, that is an acceptable solution and value for money, for others it's not acceptable. If that's not acceptable, then we need to look at alternatives and flexibility in ways we can have the solution work but cost less or take up less space if space is the constraint. We need to look at specific things, such as reducing consumption and adding additional generation, such as using a fuel power generator. And implementing an amount of those which is acceptable to balance with reducing the size of the system. There's no one set figure that suits everyone. This part is subjective. Different people have different tolerances for things, different budgets, and they assess value for money differently. In this part, it is quite personal and subjective. No two people will come up with the same result. This work covers two aspects:

• Very technical
• Subjective, that's the human being's experience of it

You need to cover both sides to get the solution that suits you as far as the performance, the reliability, and the cost of the system.

You also need to consider the time of day that most of your consumption is occurring. For example, if you're a very heavy night consumer (not much is happening during the day, but people come home at 5:00, start turning on all the heaters, all the cooking appliances, and start running washing machines), they have other activities that go late into the night or overnight. You're going to need a much larger battery bank. This is because, if we go back to the earlier point, what is the battery? What is its role? Remember, it's a temporary buffer between generation and consumption. And if you have a big lag (as in, a difference in timing between generation and consumption), you're going to need to buffer more energy and hold it in that buffer until such time as you want to take it out. If your consumption is correlated (as in, happening at the same time as your generation), you have little to no need for the buffer, so your battery bank can be a lot smaller. You can do this on purpose to minimise the cost of the system and maximise its efficiency by running more of your loads during the day when your generation is happening. If you're generating exactly the same time as you're consuming, you don't have a requirement for a large battery. So, this creates the question for our customers: what consumption can we move to happen during the day? Can we set cookers, appliances, washing machines, refrigerators, freezers, and other loading such as pumps to occur during the day? That reduces our battery requirement and also wear and tear on the battery.

We can see there is quite a degree of flexibility in shaping the system to what works for you. We can have different generation sources, supplementary sources, and times of day that we can run different loads. A selection of those can be chosen on the basis of what's best for you.