"Can I go off-grid with solar?" is a question we are asked at least once a week. When the Big 6 hike electricity prices, make that once a day.
It's a particularly enticing proposition, especially if you are 'self-building' your dream home and aiming for PassivHaus standards and Grand Design prizes.
The short answer is "Yes you can go off-grid with solar."
A better answer is "Don't do it unless your grid connection is really expensive." Instead, make the grid work for you by taking full advantage of time-of-use tariffs (off-peak charging).
To illustrate our better answer, we've modelled an off-grid residential property with an annual electricity consumption of 5,000 kWh.
To go off-grid with any renewable energy system, you need a renewable energy source (solar, wind, or both), a battery system, and realistically, a backup generator.
Why do you need a backup generator?
Because the wind blows intermittently and the sun shines only in the day, and only for a few hours in winter, without a generator you would need significantly to over-size your renewable energy system to cover the 'worst case' weather conditions.
And this would mean that it had to be powered down during the best weather, such that some of the generation was wasted. Thus increasing your 'lifetime cost per kWh' of electricity.
Modelling an average-to-large home using 5,000kWh per year, and a maximum instantaneous demand of 10kW, we tried three different off grid solar system configurations with varying sizes of solar PV + battery, as follows:
| System 1 | System 2 | System 3 | ||||
|
Generator: |
11kW LPG | 11kW LPG | 11kW LPG | |||
|
PV system size: |
5kWp | 10kWp | 30kWp | |||
|
Battery capacity: |
10kWh | 20kWh | 90kWh | |||
|
Battery max continuous charge / discharge rate: Max peak discharge rate: |
3.4kW / 4.5kW 10kW |
6.7kW / 9kW 20kW |
20kW / 24kW 30kW |
The battery storage systems were based on combining a Victron inverter / charger with off grid storage batteries from BYD (LFP - lithium ferro phosphate).
In a single year, the modelled contribution of the generator, solar PV system (electricity used immediately) and battery (solar electricity stored and later discharged) was as follows:
| System 1 | System 2 | System 3 | ||||
| Breakdown of Annual Supply to Load (kWh) | ||||||
|
- Generator (kWh): |
1,992 | 40% | 1,022 | 20% | 0 | 0% |
|
- Solar PV direct (kWh): |
1,256 | 25% | 1,350 | 27% | 1,354 | 27% |
|
- Battery (kWh): |
1,752 | 35% | 2,629 | 53% | 3,646 | 73% |
|
- Total (kWh): |
5,000 | 5,000 | 5,000 | |||
What is clear is that in order not to need the generator at all, we really had to ramp up the size of the solar PV system and the capacity of the batteries: to 30kWp of solar PV and 90kWh of batteries.
With the smallest solar PV and battery system, the generator provided 40% of the load.
Cost considerations aside, the trouble with ramping up the size of the solar PV and batteries, is that in summer / good weather a lot of solar PV electricity has to be 'wasted' by powering down the inverters. Note that in an off-grid system, once the batteries are full, if there is insufficient load to use the solar generation, the solar PV system has to power down to stop the solar PV from over-powering the property's electrical distribution system.
This is how much potential solar electricity is lost by each system every year to 'powering down':
| System 1 | System 2 | System 3 | ||||
| Annual solar PV output (kWh): | 4,487 | 8,974 | 26,921 | |||
| Lost to 'powering down': | 1,194 | 27% | 4,567 | 51% | 21,526 | 80% |
Even with the small 5kWp solar PV and 10kWh battery storage system, 27% of the solar PV generation has to be curtailed. With the large 30kWp / 90kWh system, you get rid of the generator, but a staggering 80% of the solar generation has to be curtailed.
Note that when the system is grid-connected, this 'excess' goes back to the grid and you get paid for it. Thus the solar PV system is allowed to operate at peak output at all times.
So we've established that it makes sense to have a generator. What about system cost?
Assuming an installed generator cost of £7,500 and a running cost of 15p per kWh for the generator and annual servicing cost of £200, the up-front system costs and on-going running costs are as shown in the table below. The table also shows the annual saving of the system compared to buying 5,000 kWh off the grid at 16p per kWh:
| System 1 | System 2 | System 3 | ||||
|
System cost: |
£24,000 | £39,000 | £110,000 | |||
|
Running cost per annum: |
£500 | £350 | £200 | |||
|
Saving against grid electricity each year (16p per kWh grid cost): |
£300 |
£450 |
£600 |
The 'optimal' payback is from System 1 or System 2. But clearly if you can use the grid, you should - if your grid connection is cheap to secure, there is no point spending £24,000 to save £300 a year.
If on the other hand, you are building a property in the middle of nowhere and you have been quoted £50,000 plus to bring electricity into the property, then it will make financial sense to ditch the grid.
If you are building your dream home and the cost of running an electricity supply from the grid into the home is going to exceed £50,000, give us a call, and we will gladly help you to design and install an off-grid electricity system.
Otherwise it's probably best to stay connected for now... use solar and / or a battery system to insulate yourself from peak electricity charges, while taking advantage of cheap overnight import tariffs and the export tariff.