Sheet CALC-15 · Brendatt Engineering Services · System modelling
Off-Grid Priority Hybrid System Model
Sizing a system that runs a property on solar and storage, with the utility demoted to backup and a diesel set held as last resort, is a dispatch problem rather than an arithmetic one. Dividing daily consumption by peak sun hours tells you nothing about the nights the battery runs flat, the cloudy week in July, or how many hours the genset is going to run. This tool answers those questions by simulating every hour of a year.
Enter the connected load schedule, the array and battery you are considering, and the grid and genset parameters. The model then runs 8,760 hourly steps against a fixed dispatch order:
- PV to load — generation serves demand directly.
- Surplus to battery — limited by the C-rate and inverter rating.
- Battery to load — down to a reserve state of charge held back for outages.
- Grid — imported only once PV and storage are exhausted.
- Genset — starts only when the grid is down and the battery has hit its floor.
The outputs are the numbers a client actually asks about: what percentage of the year still comes off the meter, how many hours the diesel runs, whether any load goes unserved, and what the whole arrangement costs to run against a grid-only baseline. The least-cost sizing sweep tests 225 combinations of array and battery size and ranks the ones that hit your grid-independence target.
How the model works
Load profile and load shifting
Each line in the load schedule is converted to a daily energy figure from its rated power, run hours and duty factor. Loads marked as shiftable — pool filtration, heat pumps, borehole pumps, steam generators — are re-timed into the 09:00 to 16:00 window so they draw against live generation instead of stored energy. Everything else follows a residential profile with a morning rise and an evening peak. Moving controllable plant into the solar window is usually the cheapest single intervention available: it raises direct self-consumption and reduces the battery you have to buy, because energy that never goes through the cells never pays the round-trip efficiency penalty or consumes a cycle.
Solar resource
Generation is built from a monthly peak-sun-hour profile scaled to the annual mean you enter, with a June to August minimum reflecting the cloudier season, and a daily variability sequence that introduces overcast days rather than assuming a uniform average. This matters because a system sized on annual averages will be undersized against the worst month, and the worst month is where the genset hours and the client complaints come from. Hourly output follows a clear-sky curve between roughly 06:00 and 19:00, which suits equatorial sites where day length barely varies.
Storage and the reserve state of charge
Usable capacity is nominal capacity multiplied by depth of discharge. Round-trip efficiency is split evenly between charge and discharge, and charge and discharge power are capped by both the C-rate and the hybrid inverter rating. The reserve state of charge is the share of the battery held back while the grid is healthy, so that an outage does not arrive to find the bank already empty. When the grid is up and the state of charge sits below that reserve, the model tops up from the meter, but only outside the solar window, so daytime deficits are left for the array to refill.
Grid as backup, genset as last resort
Outage hours per month are distributed as events rather than spread evenly, since a system that copes with fifteen isolated minutes will not necessarily cope with one four-hour block. During an outage the battery is permitted to discharge below its reserve, down to the genset start threshold. Only then does the set run, and when it runs it also charges the battery so it is not cycling on and off at low load. Fuel is calculated on a two-term basis — a no-load component proportional to the set rating plus a component proportional to energy delivered — which reflects the poor specific consumption of a lightly loaded diesel far better than a flat litres-per-kilowatt-hour figure.
Reading the result
Grid dependence is annual import divided by annual consumption. It is the number a client who asked to come off the grid is really buying, and it is almost always higher than a daily-average calculation suggests. Unserved energy should be zero in any configuration you are prepared to quote; if it is not, the design fails during the worst week of the year regardless of how good the annual figures look. Battery cycles per year is worth watching against the manufacturer warranty, because a bank that clears three hundred cycles annually will reach its throughput limit years before the array reaches end of life.
Applying this in Kenya
Treat the output as a planning figure pending metered load logging. Nameplate ratings and estimated run hours are a starting point, but plant such as a pool heat pump or an electric steam generator has a duty cycle that only logging will reveal, and those are exactly the loads large enough to move the answer. A system that keeps the utility connection as backup remains a grid-tied installation for regulatory purposes: interconnection, anti-islanding protection and any net-metering arrangement under the Energy (Net-Metering) Regulations, 2024 must be handled with the utility, and installation work must be carried out by an EPRA-licensed solar PV contractor. Where the array is roof-mounted, confirm the point-load capacity of the roof structure before finalising layout.
Related tools and services
Use this alongside the rest of the calculator suite for cable sizing, voltage drop and tariff modelling. If you would rather have the load logging, the modelling and the drawings done for you, our consultancy services cover the full design package, and project request and matching will put your brief in front of verified contractors in the directory. Specifications and calculation templates are in the design documents library.