Domestic Power Consumption
The average UK home consumes approximately 5000 kWhrs per year or more accurately around 12 kWhrs per day. This figure averaged over 24 hours is 500W which is not a vast amount of power. It is almost feasible for a solar PV panel to produce this from a roof area not much greater than that available on the average 3-bedroom semi. The problem however is that we don’t always get sufficient light and obviously we get none at night.
The average UK home consumption figure quoted above will include those more extravagant homes where an electric shower is being used and/or electric cooker hobs. In order to get anywhere close to being self-sufficient/utility-free, these products must not be used. An electric cooker hob/ring can consume anywhere from 1 to 3kW and there can be four of them in used when cooking a Sunday dinner. This can easily consume 2 kW hours in a day.
Electric showers are rated between 7 and 10 kW, so three 15 minute showers could consume 7.5 kWhrs.
The average UK home consumption figure also includes those homes with incandescent lighting and appliances with poor efficiency ratings. Obviously any attempt to become utility free must employ the latest LED lighting and use appliances with the maximum efficiency ratings. This will have a cost implication but surprisingly not as bad as one would first think.
Power in the utility free home
There are three main technologies used for generating electricity in the utility free home:-
- Photo voltaic panels
- Wind turbines
- Internal combustion engine-driven generators
Wind turbines and Solar PV technologies have one important characteristic in common – they are not available ‘on demand’. This means that in order to have the ability to consume electricity at any time you want, in any quantity you want (a correctly-sized generator will not be able to meet your peak demand), an electricity storage system needs to be incorporated into the utility free home.
This electricity storage system will almost always consist of a bank of lead acid batteries, and this means that there is a fundamental change in the power system of the utility free home from that of a grid connected home.
The grid-connected home electricity system is entirely AC, whereas the utility free home has a DC battery bank, a DC photovoltaic supply and a DC wind (and on rare occasions a DC water) turbine.
Coincidentally, the most energy efficient light technology – LEDs, are also fundamentally DC. Each LED light you buy to use on your AC mains circuits has AC to DC conversion built in, and this conversion has an efficiency cost. It therefore makes good sense to build a DC lighting circuit into your utility free home.
Unfortunately, the vast majority of household appliances are AC so can not be directly supplied with power from the battery bank nor from the photovoltaic panel nor wind turbines. Some appliances usually used within the mobile home, caravan and boating markets are available in DC versions, but for the majority of appliances an inverter or collection of inverters will be required.
The traditional way of providing electricity in the off-grid home is to use an AC generator. Using a device called an inverter/charger, AC power from the generator can be converted into DC power to charge a battery bank and DC power from the batteries can be converted to AC to supply AC appliances within the home.
At Utility Free Living, we believe this is not the best way to design your utility free power system.
Inverter/charger devices are very expensive, especially if you want to take power from the generator and the battery bank at the same time. If you can’t take power from your generator and battery bank at the same time, then you either need a much larger generator, or a much larger battery bank. Both these options are expensive.
Inverter/chargers are also very complicated devices and are a ‘single point of failure’. They also add an unnecessary component of inefficiency when they convert to and from AC and DC.
At Utility Free Living, we believe your generator should supply DC power not AC. This means that without any further conversion inefficiencies it can directly charge your battery bank and share the load with your battery bank when your inverters need to supply your home’s appliances at peak usage. This means that the size of your generator and the size of your battery bank can be substantially smaller than what would be required with a traditional AC generator and inverter/charger system.
DC generators are very rare devices though the technology is far from challenging. UFL has a range of domestic scale DC generators.
What is important to note is that any internal combustion engine driven generator is relatively inefficient. At the domestic scale, getting 30% of the calorific content of the fuel converted into electricity, is considered ‘good’. This means that there is a possible 70% of wasted energy. However, this energy is given off by the engine in the form of heat and this heat can be recovered and used within the home. This heat recovery technology, when added to your generator results in your system being termed a ‘Combined Heat and Power’ (CHP) system.
At Utility Free Living, we believe that no generator should exist without having heat recovery built in (unless it is a portable backup type). An internal combustion engine driven generator must always be considered as a boiler replacement as well as an electricity generating device. All the UFL DC generators come with the option of heat recovery.
The battery bank
The battery bank is the heart of the utility free home power system. There will be several sources of electricity feeding into the battery bank (generator, photovoltaic and probably some sort of turbine) and coming out will be a direct DC supply and an indirect AC supply going through an array of inverters.
Lead-acid battery technology has been with us now for 150 years. There are many developments on the basic technology currently available and there are many claims coupled to higher costs for these technologies.
At Utility Free Living, we believe that the old traditional traction type battery technology is the best lead/acid battery for the utility free home. Avoid the new expensive lead/acid technologies.
However, if you want something really special, take a look at our UFL battery range.
Due to the fact that the utility free home will have to use AC appliances yet it makes so much sense for so many reasons for the basis of the electricity system to be DC, inverters will be required. It is our belief that the best way to use inverter technology is to have multiple small inverters all being powered from the battery bank rather than having one large inverter. There are three main reasons for this:-
- large inverters are inefficient when not being used close to their full capacity
- large inverters are disproportionately more expensive than smaller inverters
- the impact of having an inverter fail is lower when using multiple small inverters
There are two types of inverter:- pure sine wave and modified sine wave. The former is much more expensive than the latter. The vast majority of AC appliances work perfectly well on modified sine wave power, so it makes good economical sense to have dedicated pure sine wave inverters for those appliances that need them. The rest can use the cheaper modified sine wave type.