Batteries are too expensive to buy ‘a couple more just in case’. We know the way to calculate how many of them you need exactly to power your house, depending on your system type. No more words. Let’s dive into numbers!
Key takeaways
The number you see in the battery name is the maximum rated capacity under perfect conditions with 100% depth of discharge. To calculate the real battery capacity, you need to work with some basic battery characteristics, which can be found in the spec sheet.
Capacity shows how much energy a single battery can store. Usually, battery capacity is measured in Ah (ampere-hours), but, for your convenience, some manufacturers indicate capacity in Wh (watt-hours). It helps you compare your energy needs and the battery capacity to make the right choice. If the capacity is indicated in Ah, here is how to convert it to Wh:
Ampere-hours are often used with small batteries and power banks. With larger batteries, watt-hours are more common
size (Ah) × nominal voltage (V) = capacity (Wh)
Power rating shows how much electricity can be drawn from the battery to power your electrical devices, measured in kW. A battery with a high capacity and low power rating supplies a low amount of electricity for a long time. That energy would be enough to supply only a few devices. However, a low power rating is a good choice for backup generators. On the other hand, a battery with low capacity and a high power rating could run your entire home, but not for long.
Depth of discharge (DoD) shows to what extent a battery can be discharged without being harmed. For example, let’s assume you have a solar battery with a 10 kWh capacity and a recommended DoD of 80%. This means you shouldn’t use more than 8 kWh before you recharge your battery again.
Round-trip efficiency shows how much energy the battery loses while just storing it. The higher the round-trip efficiency is, the less energy you lose.
Off-grid systems aren’t connected to the grid, so they can’t work without batteries at all. When choosing and sizing batteries for an off-grid system, you should follow a simple rule: the more, the better. For your battery-powered home, they are the only source of electricity when the sun is out.
The main battery characteristics to take into account are its capacity, DoD and round-trip efficiency. When multiplied, they show a real battery capacity.
One of the most popular home batteries is Tesla Powerwall 2. Its total power capacity is 14 kilowatt-hours. The safe Depth-Of-Discharge is 95% since it’s a lithium-ion battery. Round-trip efficiency is 90%. This means that a single Powerwall gives us:
14 kWh × 95% × 90% = 11.97 kWh
An average American house requires about 30 kWh daily. Ideally, house batteries should provide those 30 kilowatt-hours to ensure a one-day emergency backup. If we take Powerwall, two units would make a 24-kilowatt-hour energy bank — close enough.
Hybrid solar systems are connected to the utility grid, but they also have some extra battery storage as a backup. It is used when the sun isn’t active or the grid is down. Another function of a battery bank in a hybrid system is peak shaving: your house consumes energy from the battery when electric rates are the highest.
Hybrid systems require a hybrid inverter that can be paired with a battery and doesn’t shut off when a power outage happens. Since your house can draw extra energy from the grid, the battery bank can be much smaller. One Powerwall or any of the alternatives would suffice.
In some cases, you don’t want to spend stored energy on non-essential things during a power outage. You can make your battery bank into an emergency backup for critical loads – medical devices, a fridge, a space heater, lightning etc. For this, have your electrician install a sub-panel and wire the essentials to it.
If you want to size your battery bank precisely to meet the needs of your critical loads, write them down in a list along with their peak power requirements – the maximum amount of energy those devices will use at one time. You can find this information on the label of the devices you’ve chosen, or check it online. You’ll need two numbers:
Maximum running wattage – how much electricity the device needs to operate.
Starting or surge wattage – additional power that items with an inductive motor require the first few seconds they are running.
For loads like a refrigerator, a washing machine, an electric stove etc., add their starting wattage to the running wattage. If there are things like light bulbs, a toaster or a microwave oven on your list, count their maximum running wattage only.
Here is how you can arrange it:
When you sum everything up, you’ll get the total peak power requirements, which are about 1.7 kW in our example. That is the most electricity you’ll need at one time and this is what your battery’s maximum discharge rate should be.
Read also:
How much electricity does your house use? Breaking down electric bill
How to make home energy efficient, save money and help the planet
Grid-tie systems fully rely on the grid and grid-tie inverters can’t be paired with batteries. Still, there are ways to ensure an energy backup for your house in this case.
Note that in the US, connecting an alternative power supply to a grid connection is illegal unless there is a transfer switch in between.
Option 1: AC-coupled battery system. Solar systems can be AC-coupled or DC-coupled — learn more in our article. You can add an AC-coupled battery system to an existing solar system with a grid-tie inverter because the battery comes with its own inverter that doesn’t shut off when a power outage happens.
Option 2: Solar generator or a power station. A power station is a battery and an inverter in one. Power stations are much smaller in capacity than home battery systems — usually, from 200 watt-hours up to 6 kilowatt-hours. A power station can be recharged at home or with solar panels — read more on how to pick solar panels for a PV generator in our article. Ask an electrician to add a solar generator for power backup just like with a standard fuel generator.
Option 3: Power banks and Uninterrupted power supply (UPS). Small batteries can be enough to keep your computer or wi-fi router running during a power outage for a couple of hours. They are also handy to have when traveling.
Note that you can’t simply wire up a standard battery to an electric panel and expect it to power your home. A battery stores direct current while your house requires alternating current. Therefore there should always be an inverter in between.
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Andrey had been a news editor and freelance writer for a number of medias before joining A1 SolarStore team. Climate change and its impact on people's lives has always been among his interests and it partially explains his degree in Philosophy and Ethics.
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