How to size the
Off Grid Solar Power System

The first question an off-grid homeowner asks is what is the right size of the off grid solar power system. For that matter, the question is very relevant to off grid residential wind turbines and to microhydro power generators. The following paragraphs provide a step by step answer to these questions.

What is the right size of the Off Grid Power System

Off grid power systems provide the electricity for the household, it can be an off grid solar power system, a residential wind turbine or a microhydro system, or a combination thereof.

You need to know the following information before you can estimate the size of the off grid solar power system. In principle, try to reduce the peak electricity demand by shifting heavy consumers to the lower demand hours and to the time the generators are generating electricity. Best would be to employ a central automated monitor and control micro-grid system, unless the homeowner wants to handle this task manually.

  • First Step: Find out what is your average monthly consumption. You can prepare a list of your appliances, lighting bulbs, home electronics and all other electricity consumers. For each one, you need to write down how frequent and for what duration it works (refrigerator is on all the time but it works in cycles, washing machine is operated a few times in a week etc.). For each consumer calculate its monthly electricity consumption by multiplying its consumption in KWh by the time in hours it operates during a month. Sum the consumption up to get your average monthly consumption
  • Think off-grid. Remove from the list appliances that can operate not on electricity, a good example is to replace the electric water heater with a solar hot water heater, in many cases you can still use components from the existing system. Use washing machines, drying machines, dish washers and ovens during daylight as much as possible and with full load. Use CFL bulbs or LED lighting. (the purpose is to scale down the solar panels size and the batteries size as much as possible)
  • Find out what is the daylight consumption and what is the after sunset consumption, OR if you intend to use a residential wind power system, find out what is the consumption during the windy hours and what is the consumption during the hours there is no wind or almost no wind. You will eventually need to install enough deep cycle batteries to store electricity that was generated when the off grid solar power generator and/or the off grid residential wind power system are generating electricity for those hours when these generators do not generate.

Calculate the off grid power generator components' size

After that you can calculate the size of the components

  1. calculate your average daily consumption (divide the monthly average by 30)
  2. divide the daily consumption by the average number of useful daylight (6-8 hours, dependent on location and season) to get the size of the off grid solar power panels in watts
  3. In case the main generator is an off grid residential wind turbine, divide the number from 1. by the average number of wind hours to get the size for the wind turbine.
  4. The size of the inverter and the charge controller should fit the size of the solar panels OR the size of the residential wind turbine
  5. Think of the batteries as a pool of water that is filled during the day (or when the wind blows) and drained over nights (or the time of no wind).(You store electricity from the production hours to for the no production hours). You can’t however discharge the batteries bellow the 20% line (a good charge controller is taking care of that). The capacity in KWh of the batteries when fully charged is the Ah (Amper-hours) rating multiplied by the voltage (there are 12V, 24V and 48V batteries) multiplied by 1-0.2=0.8 (remember the 20% bottom line). This should suffice for your consumption during the no production hours. So you calculate backwards from your night consumption (divide it by the voltage rating and by 0.8) to get the batteries size in Ah (take some spare for rainy days).(if you have both a wind generator and the solar generator and if wind is expected after sunset you may reduce the size of the batteries bank)
  6. I won’t be surprised if you find out that you need many batteries, and that you need a battery rack

  7. Shopping Hints

    Although it might looks complicated, most on-line stores and off-line stores for solar products if they worth their name will provide you with assistance, calculators and other support. A good contractor, worth his reputation can give you a technical proposal and a price quotation for a turn key job. Don't ignore the energy tax credit available for clean off grid power investments. The IRS approves 30% with no limit, most states add state level incentives. long term subsidized financing is available in some of the states

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