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Off-Grid Power: Is It Time to Ditch the Grid?

Erica Charles 23 Nov 2018

"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.

First things first, what does an off-grid solar system look like?

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 back-up generator.

Why do you need a back-up 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.

For example...

Modelling an average-to-large home using 5000kWh 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.

Rough Costs

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 5000 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 £20,000 plus to bring electricity into the property, then it will make financial sense to Ditch the Grid.

Conclusion

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 £20,000, 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, whilst taking advantage of cheap overnight import tariffs and the export tariff. 

Topics: Off Grid Power

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