Based on the current specification of the Tesla Powerwall battery there are two models. One gives 7kWh per day of usable energy when cycled daily: the other gives 10kWh if cycled weekly. Both come with ten year guarantees.
24 years ago I left the grid – not really by choice but because I moved to a remote rural property that had no mains power. The experience has taught me a lot about the complexities of living off-grid. For the past ten years I have lived on an off-grid community near Melbourne and have been involved in upgrading many of the 30 plus renewable energy systems here.
The problem for people who currently have grid power and wish to disconnect is being able to afford the same utility of energy supply that the grid has to offer. The grid in many ways is the perfect battery: almost infinite storage, massive amounts of instaneous power available and very reliable.
By contrast a battery storage system with photovoltaic panels as the means of charging has a limited amount of storage (in the case of the Powerwall 7-10kWh of capacity) and limited power (Powerwall only rated to 2kW of instaneous power).
A typical on-grid home in Australia uses about 20-22kWh/day of energy and power demand would peak around 5-6kW. Thus a single Powerwall wouldn’t meet those typical requirements. However, the Powerwall can be agregated into a larger system to provide more storage and presumable more power.
Next problem about leaving the grid is weather. If you’re relying on solar power as your energy source – it is highly variable. You may experience days or even weeks without a recharge from the Sun – you need extra storage to accommodate these periods. How much is mostly a balance between how much you want to spend on extra storage capacity and how much you’re prepared to change your consumption behaviour to match the lower input during overcast and rain periods.
In practice most off-grid homes are designed with 3-5 days of extra backup power in their battery system and some other form of generation (usually a petrol or diesel generator) to be used during extended bad weather.
So, lets try and build such a system using the Tesla Powerwall. You’re going to need to reduce your daily energy consumption by at least half (10kWh/day) and have enough storage for five days of bad weather so using the daily cycled Powerwall spec of 7kWh storage you’d need 10 x 5 = 50kWh total storage which is about seven Powerwalls! Even then you’d probably be well advised to have an autostart generator for those longer periods of bad weather.
This is not starting to sound like a viable option for the typical on-grid power user wanting to save money.
However, I have built some very small systems for on-grid users who have managed to reduce their power consumption to just 3-4kWh/day and turn off their grid connection over summer – running totally on solar plus storage for 3-4 months of the year.
It’s all about energy demand. Energy efficiency is always the cheapest option. Start by choosing only high efficiency appliances, install LED or CFL lighting, downsize your refrigerator to one that is big enough, and tailer your energy use to the weather cycles. Don’t do hot washes when there’s no solar input.
There are many more energy saving tricks that can halve or even quarter your energy bill but they take effort and commitment. Living off-grid engenders this behaviour because often you have no choice.
Leaving the grid is no minor choice. The grid offers a level of reliability and capacity that is hard to replicate. If network operators and energy retailers redesigned their product to allow “light” users to “top up” their battery systems from time-to-time for a small service fee then this would be the ideal option.
Glen Morris is the Vice President of the Australian Solar Council, has been running his renewable energy solutions company, SolarQuip, for ten years, a member of Standards Australia committee that helps write the renewable energy standards, runs renewable energy training for hybrid and off-grid at his training facility near Melbourne and lives with his partner and two children on one of Australia’s oldest off-grid communities.
This post was published on July 9, 2015 3:01 am
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Great article. Another disadvantage of an integrated storage solution, is whether its individual components like solar controllers, Battery Management Systems, individual batteries and (where applicable) their inbuilt inverters are easy to fault find and swap out parts that become faulty with generic type ones on eBay. Otherwise we're potentially buying an integrated product that ends up in landfill or stuck with expensive proprietary overseas parts. This is the potential price people may pay for the convenience of buying a black box product to meet their needs. For many decades these parts have been installed separately, so they can be replaced separately. Replacing a solar controller or a Battery Management System could become like finding/replacing an engine management module for a car.